diff options
Diffstat (limited to 'Documentation/vm')
| -rw-r--r-- | Documentation/vm/hmm.rst | 96 | ||||
| -rw-r--r-- | Documentation/vm/hugetlbfs_reserv.rst | 17 | ||||
| -rw-r--r-- | Documentation/vm/index.rst | 3 | ||||
| -rw-r--r-- | Documentation/vm/memory-model.rst | 183 | ||||
| -rw-r--r-- | Documentation/vm/numa.rst | 4 | ||||
| -rw-r--r-- | Documentation/vm/slub.rst | 4 | ||||
| -rw-r--r-- | Documentation/vm/transhuge.rst | 81 |
7 files changed, 313 insertions, 75 deletions
diff --git a/Documentation/vm/hmm.rst b/Documentation/vm/hmm.rst index 44205f0b671f..7cdf7282e022 100644 --- a/Documentation/vm/hmm.rst +++ b/Documentation/vm/hmm.rst @@ -189,20 +189,10 @@ the driver callback returns. When the device driver wants to populate a range of virtual addresses, it can use either:: - int hmm_vma_get_pfns(struct vm_area_struct *vma, - struct hmm_range *range, - unsigned long start, - unsigned long end, - hmm_pfn_t *pfns); - int hmm_vma_fault(struct vm_area_struct *vma, - struct hmm_range *range, - unsigned long start, - unsigned long end, - hmm_pfn_t *pfns, - bool write, - bool block); - -The first one (hmm_vma_get_pfns()) will only fetch present CPU page table + long hmm_range_snapshot(struct hmm_range *range); + long hmm_range_fault(struct hmm_range *range, bool block); + +The first one (hmm_range_snapshot()) will only fetch present CPU page table entries and will not trigger a page fault on missing or non-present entries. The second one does trigger a page fault on missing or read-only entry if the write parameter is true. Page faults use the generic mm page fault code path @@ -220,25 +210,56 @@ respect in order to keep things properly synchronized. The usage pattern is:: { struct hmm_range range; ... + + range.start = ...; + range.end = ...; + range.pfns = ...; + range.flags = ...; + range.values = ...; + range.pfn_shift = ...; + hmm_range_register(&range); + + /* + * Just wait for range to be valid, safe to ignore return value as we + * will use the return value of hmm_range_snapshot() below under the + * mmap_sem to ascertain the validity of the range. + */ + hmm_range_wait_until_valid(&range, TIMEOUT_IN_MSEC); + again: - ret = hmm_vma_get_pfns(vma, &range, start, end, pfns); - if (ret) + down_read(&mm->mmap_sem); + ret = hmm_range_snapshot(&range); + if (ret) { + up_read(&mm->mmap_sem); + if (ret == -EAGAIN) { + /* + * No need to check hmm_range_wait_until_valid() return value + * on retry we will get proper error with hmm_range_snapshot() + */ + hmm_range_wait_until_valid(&range, TIMEOUT_IN_MSEC); + goto again; + } + hmm_mirror_unregister(&range); return ret; + } take_lock(driver->update); - if (!hmm_vma_range_done(vma, &range)) { + if (!range.valid) { release_lock(driver->update); + up_read(&mm->mmap_sem); goto again; } // Use pfns array content to update device page table + hmm_mirror_unregister(&range); release_lock(driver->update); + up_read(&mm->mmap_sem); return 0; } The driver->update lock is the same lock that the driver takes inside its -update() callback. That lock must be held before hmm_vma_range_done() to avoid -any race with a concurrent CPU page table update. +update() callback. That lock must be held before checking the range.valid +field to avoid any race with a concurrent CPU page table update. HMM implements all this on top of the mmu_notifier API because we wanted a simpler API and also to be able to perform optimizations latter on like doing @@ -255,6 +276,43 @@ report commands as executed is serialized (there is no point in doing this concurrently). +Leverage default_flags and pfn_flags_mask +========================================= + +The hmm_range struct has 2 fields default_flags and pfn_flags_mask that allows +to set fault or snapshot policy for a whole range instead of having to set them +for each entries in the range. + +For instance if the device flags for device entries are: + VALID (1 << 63) + WRITE (1 << 62) + +Now let say that device driver wants to fault with at least read a range then +it does set:: + + range->default_flags = (1 << 63); + range->pfn_flags_mask = 0; + +and calls hmm_range_fault() as described above. This will fill fault all page +in the range with at least read permission. + +Now let say driver wants to do the same except for one page in the range for +which its want to have write. Now driver set:: + + range->default_flags = (1 << 63); + range->pfn_flags_mask = (1 << 62); + range->pfns[index_of_write] = (1 << 62); + +With this HMM will fault in all page with at least read (ie valid) and for the +address == range->start + (index_of_write << PAGE_SHIFT) it will fault with +write permission ie if the CPU pte does not have write permission set then HMM +will call handle_mm_fault(). + +Note that HMM will populate the pfns array with write permission for any entry +that have write permission within the CPU pte no matter what are the values set +in default_flags or pfn_flags_mask. + + Represent and manage device memory from core kernel point of view ================================================================= diff --git a/Documentation/vm/hugetlbfs_reserv.rst b/Documentation/vm/hugetlbfs_reserv.rst index 9d200762114f..f143954e0d05 100644 --- a/Documentation/vm/hugetlbfs_reserv.rst +++ b/Documentation/vm/hugetlbfs_reserv.rst @@ -85,10 +85,10 @@ Reservation Map Location (Private or Shared) A huge page mapping or segment is either private or shared. If private, it is typically only available to a single address space (task). If shared, it can be mapped into multiple address spaces (tasks). The location and -semantics of the reservation map is significantly different for two types +semantics of the reservation map is significantly different for the two types of mappings. Location differences are: -- For private mappings, the reservation map hangs off the the VMA structure. +- For private mappings, the reservation map hangs off the VMA structure. Specifically, vma->vm_private_data. This reserve map is created at the time the mapping (mmap(MAP_PRIVATE)) is created. - For shared mappings, the reservation map hangs off the inode. Specifically, @@ -109,15 +109,15 @@ These operations result in a call to the routine hugetlb_reserve_pages():: struct vm_area_struct *vma, vm_flags_t vm_flags) -The first thing hugetlb_reserve_pages() does is check for the NORESERVE +The first thing hugetlb_reserve_pages() does is check if the NORESERVE flag was specified in either the shmget() or mmap() call. If NORESERVE -was specified, then this routine returns immediately as no reservation +was specified, then this routine returns immediately as no reservations are desired. The arguments 'from' and 'to' are huge page indices into the mapping or underlying file. For shmget(), 'from' is always 0 and 'to' corresponds to the length of the segment/mapping. For mmap(), the offset argument could -be used to specify the offset into the underlying file. In such a case +be used to specify the offset into the underlying file. In such a case, the 'from' and 'to' arguments have been adjusted by this offset. One of the big differences between PRIVATE and SHARED mappings is the way @@ -138,7 +138,8 @@ to indicate this VMA owns the reservations. The reservation map is consulted to determine how many huge page reservations are needed for the current mapping/segment. For private mappings, this is -always the value (to - from). However, for shared mappings it is possible that some reservations may already exist within the range (to - from). See the +always the value (to - from). However, for shared mappings it is possible that +some reservations may already exist within the range (to - from). See the section :ref:`Reservation Map Modifications <resv_map_modifications>` for details on how this is accomplished. @@ -165,7 +166,7 @@ these counters. If there were enough free huge pages and the global count resv_huge_pages was adjusted, then the reservation map associated with the mapping is modified to reflect the reservations. In the case of a shared mapping, a -file_region will exist that includes the range 'from' 'to'. For private +file_region will exist that includes the range 'from' - 'to'. For private mappings, no modifications are made to the reservation map as lack of an entry indicates a reservation exists. @@ -239,7 +240,7 @@ subpool accounting when the page is freed. The routine vma_commit_reservation() is then called to adjust the reserve map based on the consumption of the reservation. In general, this involves ensuring the page is represented within a file_region structure of the region -map. For shared mappings where the the reservation was present, an entry +map. For shared mappings where the reservation was present, an entry in the reserve map already existed so no change is made. However, if there was no reservation in a shared mapping or this was a private mapping a new entry must be created. diff --git a/Documentation/vm/index.rst b/Documentation/vm/index.rst index 2b3ab3a1ccf3..e8d943b21cf9 100644 --- a/Documentation/vm/index.rst +++ b/Documentation/vm/index.rst @@ -4,7 +4,7 @@ Linux Memory Management Documentation This is a collection of documents about the Linux memory management (mm) subsystem. If you are looking for advice on simply allocating memory, -see the :ref:`memory-allocation`. +see the :ref:`memory_allocation`. User guides for MM features =========================== @@ -37,6 +37,7 @@ descriptions of data structures and algorithms. hwpoison hugetlbfs_reserv ksm + memory-model mmu_notifier numa overcommit-accounting diff --git a/Documentation/vm/memory-model.rst b/Documentation/vm/memory-model.rst new file mode 100644 index 000000000000..382f72ace1fc --- /dev/null +++ b/Documentation/vm/memory-model.rst @@ -0,0 +1,183 @@ +.. SPDX-License-Identifier: GPL-2.0 + +.. _physical_memory_model: + +===================== +Physical Memory Model +===================== + +Physical memory in a system may be addressed in different ways. The +simplest case is when the physical memory starts at address 0 and +spans a contiguous range up to the maximal address. It could be, +however, that this range contains small holes that are not accessible +for the CPU. Then there could be several contiguous ranges at +completely distinct addresses. And, don't forget about NUMA, where +different memory banks are attached to different CPUs. + +Linux abstracts this diversity using one of the three memory models: +FLATMEM, DISCONTIGMEM and SPARSEMEM. Each architecture defines what +memory models it supports, what the default memory model is and +whether it is possible to manually override that default. + +.. note:: + At time of this writing, DISCONTIGMEM is considered deprecated, + although it is still in use by several architectures. + +All the memory models track the status of physical page frames using +:c:type:`struct page` arranged in one or more arrays. + +Regardless of the selected memory model, there exists one-to-one +mapping between the physical page frame number (PFN) and the +corresponding `struct page`. + +Each memory model defines :c:func:`pfn_to_page` and :c:func:`page_to_pfn` +helpers that allow the conversion from PFN to `struct page` and vice +versa. + +FLATMEM +======= + +The simplest memory model is FLATMEM. This model is suitable for +non-NUMA systems with contiguous, or mostly contiguous, physical +memory. + +In the FLATMEM memory model, there is a global `mem_map` array that +maps the entire physical memory. For most architectures, the holes +have entries in the `mem_map` array. The `struct page` objects +corresponding to the holes are never fully initialized. + +To allocate the `mem_map` array, architecture specific setup code +should call :c:func:`free_area_init_node` function or its convenience +wrapper :c:func:`free_area_init`. Yet, the mappings array is not +usable until the call to :c:func:`memblock_free_all` that hands all +the memory to the page allocator. + +If an architecture enables `CONFIG_ARCH_HAS_HOLES_MEMORYMODEL` option, +it may free parts of the `mem_map` array that do not cover the +actual physical pages. In such case, the architecture specific +:c:func:`pfn_valid` implementation should take the holes in the +`mem_map` into account. + +With FLATMEM, the conversion between a PFN and the `struct page` is +straightforward: `PFN - ARCH_PFN_OFFSET` is an index to the +`mem_map` array. + +The `ARCH_PFN_OFFSET` defines the first page frame number for +systems with physical memory starting at address different from 0. + +DISCONTIGMEM +============ + +The DISCONTIGMEM model treats the physical memory as a collection of +`nodes` similarly to how Linux NUMA support does. For each node Linux +constructs an independent memory management subsystem represented by +`struct pglist_data` (or `pg_data_t` for short). Among other +things, `pg_data_t` holds the `node_mem_map` array that maps +physical pages belonging to that node. The `node_start_pfn` field of +`pg_data_t` is the number of the first page frame belonging to that +node. + +The architecture setup code should call :c:func:`free_area_init_node` for +each node in the system to initialize the `pg_data_t` object and its +`node_mem_map`. + +Every `node_mem_map` behaves exactly as FLATMEM's `mem_map` - +every physical page frame in a node has a `struct page` entry in the +`node_mem_map` array. When DISCONTIGMEM is enabled, a portion of the +`flags` field of the `struct page` encodes the node number of the +node hosting that page. + +The conversion between a PFN and the `struct page` in the +DISCONTIGMEM model became slightly more complex as it has to determine +which node hosts the physical page and which `pg_data_t` object +holds the `struct page`. + +Architectures that support DISCONTIGMEM provide :c:func:`pfn_to_nid` +to convert PFN to the node number. The opposite conversion helper +:c:func:`page_to_nid` is generic as it uses the node number encoded in +page->flags. + +Once the node number is known, the PFN can be used to index +appropriate `node_mem_map` array to access the `struct page` and +the offset of the `struct page` from the `node_mem_map` plus +`node_start_pfn` is the PFN of that page. + +SPARSEMEM +========= + +SPARSEMEM is the most versatile memory model available in Linux and it +is the only memory model that supports several advanced features such +as hot-plug and hot-remove of the physical memory, alternative memory +maps for non-volatile memory devices and deferred initialization of +the memory map for larger systems. + +The SPARSEMEM model presents the physical memory as a collection of +sections. A section is represented with :c:type:`struct mem_section` +that contains `section_mem_map` that is, logically, a pointer to an +array of struct pages. However, it is stored with some other magic +that aids the sections management. The section size and maximal number +of section is specified using `SECTION_SIZE_BITS` and +`MAX_PHYSMEM_BITS` constants defined by each architecture that +supports SPARSEMEM. While `MAX_PHYSMEM_BITS` is an actual width of a +physical address that an architecture supports, the +`SECTION_SIZE_BITS` is an arbitrary value. + +The maximal number of sections is denoted `NR_MEM_SECTIONS` and +defined as + +.. math:: + + NR\_MEM\_SECTIONS = 2 ^ {(MAX\_PHYSMEM\_BITS - SECTION\_SIZE\_BITS)} + +The `mem_section` objects are arranged in a two-dimensional array +called `mem_sections`. The size and placement of this array depend +on `CONFIG_SPARSEMEM_EXTREME` and the maximal possible number of +sections: + +* When `CONFIG_SPARSEMEM_EXTREME` is disabled, the `mem_sections` + array is static and has `NR_MEM_SECTIONS` rows. Each row holds a + single `mem_section` object. +* When `CONFIG_SPARSEMEM_EXTREME` is enabled, the `mem_sections` + array is dynamically allocated. Each row contains PAGE_SIZE worth of + `mem_section` objects and the number of rows is calculated to fit + all the memory sections. + +The architecture setup code should call :c:func:`memory_present` for +each active memory range or use :c:func:`memblocks_present` or +:c:func:`sparse_memory_present_with_active_regions` wrappers to +initialize the memory sections. Next, the actual memory maps should be +set up using :c:func:`sparse_init`. + +With SPARSEMEM there are two possible ways to convert a PFN to the +corresponding `struct page` - a "classic sparse" and "sparse +vmemmap". The selection is made at build time and it is determined by +the value of `CONFIG_SPARSEMEM_VMEMMAP`. + +The classic sparse encodes the section number of a page in page->flags +and uses high bits of a PFN to access the section that maps that page +frame. Inside a section, the PFN is the index to the array of pages. + +The sparse vmemmap uses a virtually mapped memory map to optimize +pfn_to_page and page_to_pfn operations. There is a global `struct +page *vmemmap` pointer that points to a virtually contiguous array of +`struct page` objects. A PFN is an index to that array and the the +offset of the `struct page` from `vmemmap` is the PFN of that +page. + +To use vmemmap, an architecture has to reserve a range of virtual +addresses that will map the physical pages containing the memory +map and make sure that `vmemmap` points to that range. In addition, +the architecture should implement :c:func:`vmemmap_populate` method +that will allocate the physical memory and create page tables for the +virtual memory map. If an architecture does not have any special +requirements for the vmemmap mappings, it can use default +:c:func:`vmemmap_populate_basepages` provided by the generic memory +management. + +The virtually mapped memory map allows storing `struct page` objects +for persistent memory devices in pre-allocated storage on those +devices. This storage is represented with :c:type:`struct vmem_altmap` +that is eventually passed to vmemmap_populate() through a long chain +of function calls. The vmemmap_populate() implementation may use the +`vmem_altmap` along with :c:func:`altmap_alloc_block_buf` helper to +allocate memory map on the persistent memory device. diff --git a/Documentation/vm/numa.rst b/Documentation/vm/numa.rst index 185d8a568168..5cae13e9a08b 100644 --- a/Documentation/vm/numa.rst +++ b/Documentation/vm/numa.rst @@ -109,8 +109,8 @@ System administrators and application designers can restrict a task's migration to improve NUMA locality using various CPU affinity command line interfaces, such as taskset(1) and numactl(1), and program interfaces such as sched_setaffinity(2). Further, one can modify the kernel's default local -allocation behavior using Linux NUMA memory policy. -[see Documentation/admin-guide/mm/numa_memory_policy.rst.] +allocation behavior using Linux NUMA memory policy. [see +:ref:`Documentation/admin-guide/mm/numa_memory_policy.rst <numa_memory_policy>`]. System administrators can restrict the CPUs and nodes' memories that a non- privileged user can specify in the scheduling or NUMA commands and functions diff --git a/Documentation/vm/slub.rst b/Documentation/vm/slub.rst index 195928808bac..933ada4368ff 100644 --- a/Documentation/vm/slub.rst +++ b/Documentation/vm/slub.rst @@ -66,7 +66,7 @@ Trying to find an issue in the dentry cache? Try:: to only enable debugging on the dentry cache. You may use an asterisk at the end of the slab name, in order to cover all slabs with the same prefix. For example, here's how you can poison the dentry cache as well as all kmalloc -slabs: +slabs:: slub_debug=P,kmalloc-*,dentry @@ -141,7 +141,7 @@ can be influenced by kernel parameters: (list_lock) where contention may occur. ``slub_min_order`` - specifies a minim order of slabs. A similar effect like + specifies a minimum order of slabs. A similar effect like ``slub_min_objects``. ``slub_max_order`` diff --git a/Documentation/vm/transhuge.rst b/Documentation/vm/transhuge.rst index a8cf6809e36e..37c57ca32629 100644 --- a/Documentation/vm/transhuge.rst +++ b/Documentation/vm/transhuge.rst @@ -4,8 +4,9 @@ Transparent Hugepage Support ============================ -This document describes design principles Transparent Hugepage (THP) -Support and its interaction with other parts of the memory management. +This document describes design principles for Transparent Hugepage (THP) +support and its interaction with other parts of the memory management +system. Design principles ================= @@ -37,31 +38,25 @@ get_user_pages and follow_page get_user_pages and follow_page if run on a hugepage, will return the head or tail pages as usual (exactly as they would do on -hugetlbfs). Most gup users will only care about the actual physical +hugetlbfs). Most GUP users will only care about the actual physical address of the page and its temporary pinning to release after the I/O is complete, so they won't ever notice the fact the page is huge. But if any driver is going to mangle over the page structure of the tail page (like for checking page->mapping or other bits that are relevant for the head page and not the tail page), it should be updated to jump -to check head page instead. Taking reference on any head/tail page would -prevent page from being split by anyone. +to check head page instead. Taking a reference on any head/tail page would +prevent the page from being split by anyone. .. note:: these aren't new constraints to the GUP API, and they match the - same constrains that applies to hugetlbfs too, so any driver capable + same constraints that apply to hugetlbfs too, so any driver capable of handling GUP on hugetlbfs will also work fine on transparent hugepage backed mappings. In case you can't handle compound pages if they're returned by -follow_page, the FOLL_SPLIT bit can be specified as parameter to +follow_page, the FOLL_SPLIT bit can be specified as a parameter to follow_page, so that it will split the hugepages before returning -them. Migration for example passes FOLL_SPLIT as parameter to -follow_page because it's not hugepage aware and in fact it can't work -at all on hugetlbfs (but it instead works fine on transparent -hugepages thanks to FOLL_SPLIT). migration simply can't deal with -hugepages being returned (as it's not only checking the pfn of the -page and pinning it during the copy but it pretends to migrate the -memory in regular page sizes and with regular pte/pmd mappings). +them. Graceful fallback ================= @@ -72,11 +67,11 @@ pmd_offset. It's trivial to make the code transparent hugepage aware by just grepping for "pmd_offset" and adding split_huge_pmd where missing after pmd_offset returns the pmd. Thanks to the graceful fallback design, with a one liner change, you can avoid to write -hundred if not thousand of lines of complex code to make your code +hundreds if not thousands of lines of complex code to make your code hugepage aware. If you're not walking pagetables but you run into a physical hugepage -but you can't handle it natively in your code, you can split it by +that you can't handle natively in your code, you can split it by calling split_huge_page(page). This is what the Linux VM does before it tries to swapout the hugepage for example. split_huge_page() can fail if the page is pinned and you must handle this correctly. @@ -103,18 +98,18 @@ split_huge_page() or split_huge_pmd() has a cost. To make pagetable walks huge pmd aware, all you need to do is to call pmd_trans_huge() on the pmd returned by pmd_offset. You must hold the -mmap_sem in read (or write) mode to be sure an huge pmd cannot be +mmap_sem in read (or write) mode to be sure a huge pmd cannot be created from under you by khugepaged (khugepaged collapse_huge_page takes the mmap_sem in write mode in addition to the anon_vma lock). If pmd_trans_huge returns false, you just fallback in the old code paths. If instead pmd_trans_huge returns true, you have to take the page table lock (pmd_lock()) and re-run pmd_trans_huge. Taking the -page table lock will prevent the huge pmd to be converted into a +page table lock will prevent the huge pmd being converted into a regular pmd from under you (split_huge_pmd can run in parallel to the pagetable walk). If the second pmd_trans_huge returns false, you should just drop the page table lock and fallback to the old code as -before. Otherwise you can proceed to process the huge pmd and the -hugepage natively. Once finished you can drop the page table lock. +before. Otherwise, you can proceed to process the huge pmd and the +hugepage natively. Once finished, you can drop the page table lock. Refcounts and transparent huge pages ==================================== @@ -122,61 +117,61 @@ Refcounts and transparent huge pages Refcounting on THP is mostly consistent with refcounting on other compound pages: - - get_page()/put_page() and GUP operate in head page's ->_refcount. + - get_page()/put_page() and GUP operate on head page's ->_refcount. - ->_refcount in tail pages is always zero: get_page_unless_zero() never - succeed on tail pages. + succeeds on tail pages. - map/unmap of the pages with PTE entry increment/decrement ->_mapcount on relevant sub-page of the compound page. - - map/unmap of the whole compound page accounted in compound_mapcount + - map/unmap of the whole compound page is accounted for in compound_mapcount (stored in first tail page). For file huge pages, we also increment ->_mapcount of all sub-pages in order to have race-free detection of last unmap of subpages. PageDoubleMap() indicates that the page is *possibly* mapped with PTEs. -For anonymous pages PageDoubleMap() also indicates ->_mapcount in all +For anonymous pages, PageDoubleMap() also indicates ->_mapcount in all subpages is offset up by one. This additional reference is required to get race-free detection of unmap of subpages when we have them mapped with both PMDs and PTEs. -This is optimization required to lower overhead of per-subpage mapcount -tracking. The alternative is alter ->_mapcount in all subpages on each +This optimization is required to lower the overhead of per-subpage mapcount +tracking. The alternative is to alter ->_mapcount in all subpages on each map/unmap of the whole compound page. -For anonymous pages, we set PG_double_map when a PMD of the page got split -for the first time, but still have PMD mapping. The additional references -go away with last compound_mapcount. +For anonymous pages, we set PG_double_map when a PMD of the page is split +for the first time, but still have a PMD mapping. The additional references +go away with the last compound_mapcount. -File pages get PG_double_map set on first map of the page with PTE and -goes away when the page gets evicted from page cache. +File pages get PG_double_map set on the first map of the page with PTE and +goes away when the page gets evicted from the page cache. split_huge_page internally has to distribute the refcounts in the head page to the tail pages before clearing all PG_head/tail bits from the page structures. It can be done easily for refcounts taken by page table -entries. But we don't have enough information on how to distribute any +entries, but we don't have enough information on how to distribute any additional pins (i.e. from get_user_pages). split_huge_page() fails any -requests to split pinned huge page: it expects page count to be equal to -sum of mapcount of all sub-pages plus one (split_huge_page caller must -have reference for head page). +requests to split pinned huge pages: it expects page count to be equal to +the sum of mapcount of all sub-pages plus one (split_huge_page caller must +have a reference to the head page). split_huge_page uses migration entries to stabilize page->_refcount and -page->_mapcount of anonymous pages. File pages just got unmapped. +page->_mapcount of anonymous pages. File pages just get unmapped. -We safe against physical memory scanners too: the only legitimate way -scanner can get reference to a page is get_page_unless_zero(). +We are safe against physical memory scanners too: the only legitimate way +a scanner can get a reference to a page is get_page_unless_zero(). All tail pages have zero ->_refcount until atomic_add(). This prevents the scanner from getting a reference to the tail page up to that point. After the -atomic_add() we don't care about the ->_refcount value. We already known how +atomic_add() we don't care about the ->_refcount value. We already know how many references should be uncharged from the head page. For head page get_page_unless_zero() will succeed and we don't mind. It's -clear where reference should go after split: it will stay on head page. +clear where references should go after split: it will stay on the head page. -Note that split_huge_pmd() doesn't have any limitation on refcounting: +Note that split_huge_pmd() doesn't have any limitations on refcounting: pmd can be split at any point and never fails. Partial unmap and deferred_split_huge_page() @@ -188,10 +183,10 @@ in page_remove_rmap() and queue the THP for splitting if memory pressure comes. Splitting will free up unused subpages. Splitting the page right away is not an option due to locking context in -the place where we can detect partial unmap. It's also might be +the place where we can detect partial unmap. It also might be counterproductive since in many cases partial unmap happens during exit(2) if a THP crosses a VMA boundary. -Function deferred_split_huge_page() is used to queue page for splitting. +The function deferred_split_huge_page() is used to queue a page for splitting. The splitting itself will happen when we get memory pressure via shrinker interface. |