Guarantee that rmap_walk() is called on locked folios so that threads changing folio->mapping and folio->index for non-KSM anon folios can serialize on fine-grained folio lock rather than anon_vma lock. Other folio types are already always locked before rmap_walk(). With this, we are going from 'not necessarily' locking the non-KSM anon folio to 'definitely' locking it during rmap walks. This patch is in preparation for removing anon_vma write-lock from UFFDIO_MOVE. With this patch, three functions are now expected to be called with a locked folio. To be careful of not missing any case, here is the exhaustive list of all their callers. 1) rmap_walk() is called from: a) folio_referenced() b) damon_folio_mkold() c) damon_folio_young() d) page_idle_clear_pte_refs() e) try_to_unmap() f) try_to_migrate() g) folio_mkclean() h) remove_migration_ptes() In the above list, first 4 are changed in this patch to try-lock non-KSM anon folios, similar to other types of folios. The remaining functions in the list already hold folio lock when calling rmap_walk(). 2) folio_lock_anon_vma_read() is called from following functions: a) collect_procs_anon() b) page_idle_clear_pte_refs() c) damon_folio_mkold() d) damon_folio_young() e) folio_referenced() f) try_to_unmap() g) try_to_migrate() All the functions in above list, except collect_procs_anon(), are covered by the rmap_walk() list above. For collect_procs_anon(), with kill_procs_now() changed to take folio lock in this patch ensures that all callers of folio_lock_anon_vma_read() now hold the lock. 3) folio_get_anon_vma() is called from following functions, all of which already hold the folio lock: a) move_pages_huge_pmd() b) __folio_split() c) move_pages_ptes() d) migrate_folio_unmap() e) unmap_and_move_huge_page() Functionally, this patch doesn't break the logic because rmap walkers generally do some other check to see if what is expected to mapped did happen so it's fine, or otherwise treat things as best-effort. Among the 4 functions changed in this patch, folio_referenced() is the only core-mm function, and is also frequently accessed. To assess the impact of locking non-KSM anon folios in shrink_active_list()->folio_referenced() path, we performed an app cycle test on an arm64 android device. During the whole duration of the test there were over 140k invocations of shrink_active_list(), out of which over 29k had at least one non-KSM anon folio on which folio_referenced() was called. In none of these invocations folio_trylock() failed. Of course, we now take a lock where we wouldn't previously have. In the past it would have had a major impact in causing a CoW write fault to copy a page in do_wp_page(), as commit 09854ba94c6a ("mm: do_wp_page() simplification") caused a failure to obtain folio lock to result in a page copy even if one wasn't necessary. However, since commit 6c287605fd56 ("mm: remember exclusively mapped anonymous pages with PG_anon_exclusive"), and the introduction of the folio anon exclusive flag, this issue is significantly mitigated. The only case remaining that we might worry about from this perspective is that of read-only folios immediately after fork where the anon exclusive bit will not have been set yet. We note however in the case of read-only just-forked folios that wp_can_reuse_anon_folio() will notice the raised reference count established by shrink_active_list() via isolate_lru_folios() and refuse to reuse in any case, so this will in fact have no impact - the folio lock is ultimately immaterial here. All-in-all it appears that there is little opportunity for meaningful negative impact from this change. CC: David Hildenbrand CC: Lorenzo Stoakes CC: Harry Yoo CC: Peter Xu CC: Suren Baghdasaryan CC: Barry Song CC: SeongJae Park Signed-off-by: Lokesh Gidra --- mm/damon/ops-common.c | 16 ++++------------ mm/memory-failure.c | 3 +++ mm/page_idle.c | 8 ++------ mm/rmap.c | 42 ++++++++++++------------------------------ 4 files changed, 21 insertions(+), 48 deletions(-) diff --git a/mm/damon/ops-common.c b/mm/damon/ops-common.c index 998c5180a603..f61d6dde13dc 100644 --- a/mm/damon/ops-common.c +++ b/mm/damon/ops-common.c @@ -162,21 +162,17 @@ void damon_folio_mkold(struct folio *folio) .rmap_one = damon_folio_mkold_one, .anon_lock = folio_lock_anon_vma_read, }; - bool need_lock; if (!folio_mapped(folio) || !folio_raw_mapping(folio)) { folio_set_idle(folio); return; } - need_lock = !folio_test_anon(folio) || folio_test_ksm(folio); - if (need_lock && !folio_trylock(folio)) + if (!folio_trylock(folio)) return; rmap_walk(folio, &rwc); - - if (need_lock) - folio_unlock(folio); + folio_unlock(folio); } @@ -228,7 +224,6 @@ bool damon_folio_young(struct folio *folio) .rmap_one = damon_folio_young_one, .anon_lock = folio_lock_anon_vma_read, }; - bool need_lock; if (!folio_mapped(folio) || !folio_raw_mapping(folio)) { if (folio_test_idle(folio)) @@ -237,14 +232,11 @@ bool damon_folio_young(struct folio *folio) return true; } - need_lock = !folio_test_anon(folio) || folio_test_ksm(folio); - if (need_lock && !folio_trylock(folio)) + if (!folio_trylock(folio)) return false; rmap_walk(folio, &rwc); - - if (need_lock) - folio_unlock(folio); + folio_unlock(folio); return accessed; } diff --git a/mm/memory-failure.c b/mm/memory-failure.c index a24806bb8e82..f698df156bf8 100644 --- a/mm/memory-failure.c +++ b/mm/memory-failure.c @@ -2143,7 +2143,10 @@ static void kill_procs_now(struct page *p, unsigned long pfn, int flags, { LIST_HEAD(tokill); + folio_lock(folio); collect_procs(folio, p, &tokill, flags & MF_ACTION_REQUIRED); + folio_unlock(folio); + kill_procs(&tokill, true, pfn, flags); } diff --git a/mm/page_idle.c b/mm/page_idle.c index a82b340dc204..9bf573d22e87 100644 --- a/mm/page_idle.c +++ b/mm/page_idle.c @@ -101,19 +101,15 @@ static void page_idle_clear_pte_refs(struct folio *folio) .rmap_one = page_idle_clear_pte_refs_one, .anon_lock = folio_lock_anon_vma_read, }; - bool need_lock; if (!folio_mapped(folio) || !folio_raw_mapping(folio)) return; - need_lock = !folio_test_anon(folio) || folio_test_ksm(folio); - if (need_lock && !folio_trylock(folio)) + if (!folio_trylock(folio)) return; rmap_walk(folio, &rwc); - - if (need_lock) - folio_unlock(folio); + folio_unlock(folio); } static ssize_t page_idle_bitmap_read(struct file *file, struct kobject *kobj, diff --git a/mm/rmap.c b/mm/rmap.c index 0bc7cf8b7359..fd9f18670440 100644 --- a/mm/rmap.c +++ b/mm/rmap.c @@ -489,17 +489,15 @@ void __init anon_vma_init(void) * if there is a mapcount, we can dereference the anon_vma after observing * those. * - * NOTE: the caller should normally hold folio lock when calling this. If - * not, the caller needs to double check the anon_vma didn't change after - * taking the anon_vma lock for either read or write (UFFDIO_MOVE can modify it - * concurrently without folio lock protection). See folio_lock_anon_vma_read() - * which has already covered that, and comment above remap_pages(). + * NOTE: the caller should hold folio lock when calling this. */ struct anon_vma *folio_get_anon_vma(const struct folio *folio) { struct anon_vma *anon_vma = NULL; unsigned long anon_mapping; + VM_WARN_ON_FOLIO(!folio_test_locked(folio), folio); + rcu_read_lock(); anon_mapping = (unsigned long)READ_ONCE(folio->mapping); if ((anon_mapping & FOLIO_MAPPING_FLAGS) != FOLIO_MAPPING_ANON) @@ -546,7 +544,8 @@ struct anon_vma *folio_lock_anon_vma_read(const struct folio *folio, struct anon_vma *root_anon_vma; unsigned long anon_mapping; -retry: + VM_WARN_ON_FOLIO(!folio_test_locked(folio), folio); + rcu_read_lock(); anon_mapping = (unsigned long)READ_ONCE(folio->mapping); if ((anon_mapping & FOLIO_MAPPING_FLAGS) != FOLIO_MAPPING_ANON) @@ -557,17 +556,6 @@ struct anon_vma *folio_lock_anon_vma_read(const struct folio *folio, anon_vma = (struct anon_vma *) (anon_mapping - FOLIO_MAPPING_ANON); root_anon_vma = READ_ONCE(anon_vma->root); if (down_read_trylock(&root_anon_vma->rwsem)) { - /* - * folio_move_anon_rmap() might have changed the anon_vma as we - * might not hold the folio lock here. - */ - if (unlikely((unsigned long)READ_ONCE(folio->mapping) != - anon_mapping)) { - up_read(&root_anon_vma->rwsem); - rcu_read_unlock(); - goto retry; - } - /* * If the folio is still mapped, then this anon_vma is still * its anon_vma, and holding the mutex ensures that it will @@ -602,18 +590,6 @@ struct anon_vma *folio_lock_anon_vma_read(const struct folio *folio, rcu_read_unlock(); anon_vma_lock_read(anon_vma); - /* - * folio_move_anon_rmap() might have changed the anon_vma as we might - * not hold the folio lock here. - */ - if (unlikely((unsigned long)READ_ONCE(folio->mapping) != - anon_mapping)) { - anon_vma_unlock_read(anon_vma); - put_anon_vma(anon_vma); - anon_vma = NULL; - goto retry; - } - if (atomic_dec_and_test(&anon_vma->refcount)) { /* * Oops, we held the last refcount, release the lock @@ -988,7 +964,7 @@ int folio_referenced(struct folio *folio, int is_locked, if (!folio_raw_mapping(folio)) return 0; - if (!is_locked && (!folio_test_anon(folio) || folio_test_ksm(folio))) { + if (!is_locked) { we_locked = folio_trylock(folio); if (!we_locked) return 1; @@ -2820,6 +2796,12 @@ static void rmap_walk_anon(struct folio *folio, pgoff_t pgoff_start, pgoff_end; struct anon_vma_chain *avc; + /* + * The folio lock ensures that folio->mapping can't be changed under us + * to an anon_vma with different root. + */ + VM_WARN_ON_FOLIO(!folio_test_locked(folio), folio); + if (locked) { anon_vma = folio_anon_vma(folio); /* anon_vma disappear under us? */ -- 2.51.0.534.gc79095c0ca-goog Now that rmap_walk() is guaranteed to be called with the folio lock held, we can stop serializing on the src VMA anon_vma lock when moving an exclusive folio from a src VMA to a dst VMA in UFFDIO_MOVE ioctl. When moving a folio, we modify folio->mapping through folio_move_anon_rmap() and adjust folio->index accordingly. Doing that while we could have concurrent RMAP walks would be dangerous. Therefore, to avoid that, we had to acquire anon_vma of src VMA in write-mode. That meant that when multiple threads called UFFDIO_MOVE concurrently on distinct pages of the same src VMA, they would serialize on it, hurting scalability. In addition to avoiding the scalability bottleneck, this patch also simplifies the complicated lock dance that UFFDIO_MOVE has to go through between RCU, folio-lock, ptl, and anon_vma. folio_move_anon_rmap() already enforces that the folio is locked. So when we have the folio locked we can no longer race with concurrent rmap_walk() as used by folio_referenced() and others who call it on unlocked non-KSM anon folios, and therefore the anon_vma lock is no longer required. Note that this handling is now the same as for other folio_move_anon_rmap() users that also do not hold the anon_vma lock -- namely COW reuse handling (do_wp_page()->wp_can_reuse_anon_folio(), do_huge_pmd_wp_page(), and hugetlb_wp()). These users never required the anon_vma lock as they are only moving the anon VMA closer to the anon_vma leaf of the VMA, for example, from an anon_vma root to a leaf of that root. rmap walks were always able to tolerate that scenario. CC: David Hildenbrand CC: Lorenzo Stoakes CC: Peter Xu CC: Suren Baghdasaryan CC: Barry Song Signed-off-by: Lokesh Gidra --- mm/huge_memory.c | 22 +---------------- mm/userfaultfd.c | 62 +++++++++--------------------------------------- 2 files changed, 12 insertions(+), 72 deletions(-) diff --git a/mm/huge_memory.c b/mm/huge_memory.c index 1b81680b4225..a16e3778b544 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -2533,7 +2533,6 @@ int move_pages_huge_pmd(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd, pm pmd_t _dst_pmd, src_pmdval; struct page *src_page; struct folio *src_folio; - struct anon_vma *src_anon_vma; spinlock_t *src_ptl, *dst_ptl; pgtable_t src_pgtable; struct mmu_notifier_range range; @@ -2582,23 +2581,9 @@ int move_pages_huge_pmd(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd, pm src_addr + HPAGE_PMD_SIZE); mmu_notifier_invalidate_range_start(&range); - if (src_folio) { + if (src_folio) folio_lock(src_folio); - /* - * split_huge_page walks the anon_vma chain without the page - * lock. Serialize against it with the anon_vma lock, the page - * lock is not enough. - */ - src_anon_vma = folio_get_anon_vma(src_folio); - if (!src_anon_vma) { - err = -EAGAIN; - goto unlock_folio; - } - anon_vma_lock_write(src_anon_vma); - } else - src_anon_vma = NULL; - dst_ptl = pmd_lockptr(mm, dst_pmd); double_pt_lock(src_ptl, dst_ptl); if (unlikely(!pmd_same(*src_pmd, src_pmdval) || @@ -2643,11 +2628,6 @@ int move_pages_huge_pmd(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd, pm pgtable_trans_huge_deposit(mm, dst_pmd, src_pgtable); unlock_ptls: double_pt_unlock(src_ptl, dst_ptl); - if (src_anon_vma) { - anon_vma_unlock_write(src_anon_vma); - put_anon_vma(src_anon_vma); - } -unlock_folio: /* unblock rmap walks */ if (src_folio) folio_unlock(src_folio); diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c index af61b95c89e4..6be65089085e 100644 --- a/mm/userfaultfd.c +++ b/mm/userfaultfd.c @@ -1035,8 +1035,7 @@ static inline bool is_pte_pages_stable(pte_t *dst_pte, pte_t *src_pte, */ static struct folio *check_ptes_for_batched_move(struct vm_area_struct *src_vma, unsigned long src_addr, - pte_t *src_pte, pte_t *dst_pte, - struct anon_vma *src_anon_vma) + pte_t *src_pte, pte_t *dst_pte) { pte_t orig_dst_pte, orig_src_pte; struct folio *folio; @@ -1052,8 +1051,7 @@ static struct folio *check_ptes_for_batched_move(struct vm_area_struct *src_vma, folio = vm_normal_folio(src_vma, src_addr, orig_src_pte); if (!folio || !folio_trylock(folio)) return NULL; - if (!PageAnonExclusive(&folio->page) || folio_test_large(folio) || - folio_anon_vma(folio) != src_anon_vma) { + if (!PageAnonExclusive(&folio->page) || folio_test_large(folio)) { folio_unlock(folio); return NULL; } @@ -1061,9 +1059,8 @@ static struct folio *check_ptes_for_batched_move(struct vm_area_struct *src_vma, } /* - * Moves src folios to dst in a batch as long as they share the same - * anon_vma as the first folio, are not large, and can successfully - * take the lock via folio_trylock(). + * Moves src folios to dst in a batch as long as they are not large, and can + * successfully take the lock via folio_trylock(). */ static long move_present_ptes(struct mm_struct *mm, struct vm_area_struct *dst_vma, @@ -1073,8 +1070,7 @@ static long move_present_ptes(struct mm_struct *mm, pte_t orig_dst_pte, pte_t orig_src_pte, pmd_t *dst_pmd, pmd_t dst_pmdval, spinlock_t *dst_ptl, spinlock_t *src_ptl, - struct folio **first_src_folio, unsigned long len, - struct anon_vma *src_anon_vma) + struct folio **first_src_folio, unsigned long len) { int err = 0; struct folio *src_folio = *first_src_folio; @@ -1132,8 +1128,8 @@ static long move_present_ptes(struct mm_struct *mm, src_pte++; folio_unlock(src_folio); - src_folio = check_ptes_for_batched_move(src_vma, src_addr, src_pte, - dst_pte, src_anon_vma); + src_folio = check_ptes_for_batched_move(src_vma, src_addr, + src_pte, dst_pte); if (!src_folio) break; } @@ -1263,7 +1259,6 @@ static long move_pages_ptes(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd pmd_t dummy_pmdval; pmd_t dst_pmdval; struct folio *src_folio = NULL; - struct anon_vma *src_anon_vma = NULL; struct mmu_notifier_range range; long ret = 0; @@ -1347,9 +1342,9 @@ static long move_pages_ptes(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd } /* - * Pin and lock both source folio and anon_vma. Since we are in - * RCU read section, we can't block, so on contention have to - * unmap the ptes, obtain the lock and retry. + * Pin and lock source folio. Since we are in RCU read section, + * we can't block, so on contention have to unmap the ptes, + * obtain the lock and retry. */ if (!src_folio) { struct folio *folio; @@ -1423,33 +1418,11 @@ static long move_pages_ptes(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd goto retry; } - if (!src_anon_vma) { - /* - * folio_referenced walks the anon_vma chain - * without the folio lock. Serialize against it with - * the anon_vma lock, the folio lock is not enough. - */ - src_anon_vma = folio_get_anon_vma(src_folio); - if (!src_anon_vma) { - /* page was unmapped from under us */ - ret = -EAGAIN; - goto out; - } - if (!anon_vma_trylock_write(src_anon_vma)) { - pte_unmap(src_pte); - pte_unmap(dst_pte); - src_pte = dst_pte = NULL; - /* now we can block and wait */ - anon_vma_lock_write(src_anon_vma); - goto retry; - } - } - ret = move_present_ptes(mm, dst_vma, src_vma, dst_addr, src_addr, dst_pte, src_pte, orig_dst_pte, orig_src_pte, dst_pmd, dst_pmdval, dst_ptl, src_ptl, &src_folio, - len, src_anon_vma); + len); } else { struct folio *folio = NULL; @@ -1515,10 +1488,6 @@ static long move_pages_ptes(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd } out: - if (src_anon_vma) { - anon_vma_unlock_write(src_anon_vma); - put_anon_vma(src_anon_vma); - } if (src_folio) { folio_unlock(src_folio); folio_put(src_folio); @@ -1792,15 +1761,6 @@ static void uffd_move_unlock(struct vm_area_struct *dst_vma, * virtual regions without knowing if there are transparent hugepage * in the regions or not, but preventing the risk of having to split * the hugepmd during the remap. - * - * If there's any rmap walk that is taking the anon_vma locks without - * first obtaining the folio lock (the only current instance is - * folio_referenced), they will have to verify if the folio->mapping - * has changed after taking the anon_vma lock. If it changed they - * should release the lock and retry obtaining a new anon_vma, because - * it means the anon_vma was changed by move_pages() before the lock - * could be obtained. This is the only additional complexity added to - * the rmap code to provide this anonymous page remapping functionality. */ ssize_t move_pages(struct userfaultfd_ctx *ctx, unsigned long dst_start, unsigned long src_start, unsigned long len, __u64 mode) -- 2.51.0.534.gc79095c0ca-goog