| Message ID | b53be6a4-7715-51f9-aad-f1347dcb7c4@google.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | mm: free retracted page table by RCU | expand |
Hi, On 2023/7/12 12:42, Hugh Dickins wrote: > Bring collapse_and_free_pmd() back into collapse_pte_mapped_thp(). > It does need mmap_read_lock(), but it does not need mmap_write_lock(), > nor vma_start_write() nor i_mmap lock nor anon_vma lock. All racing > paths are relying on pte_offset_map_lock() and pmd_lock(), so use those. > > Follow the pattern in retract_page_tables(); and using pte_free_defer() > removes most of the need for tlb_remove_table_sync_one() here; but call > pmdp_get_lockless_sync() to use it in the PAE case. > > First check the VMA, in case page tables are being torn down: from JannH. > Confirm the preliminary find_pmd_or_thp_or_none() once page lock has been > acquired and the page looks suitable: from then on its state is stable. > > However, collapse_pte_mapped_thp() was doing something others don't: > freeing a page table still containing "valid" entries. i_mmap lock did > stop a racing truncate from double-freeing those pages, but we prefer > collapse_pte_mapped_thp() to clear the entries as usual. Their TLB > flush can wait until the pmdp_collapse_flush() which follows, but the > mmu_notifier_invalidate_range_start() has to be done earlier. > > Do the "step 1" checking loop without mmu_notifier: it wouldn't be good > for khugepaged to keep on repeatedly invalidating a range which is then > found unsuitable e.g. contains COWs. "step 2", which does the clearing, > must then be more careful (after dropping ptl to do mmu_notifier), with > abort prepared to correct the accounting like "step 3". But with those > entries now cleared, "step 4" (after dropping ptl to do pmd_lock) is kept > safe by the huge page lock, which stops new PTEs from being faulted in. > > Signed-off-by: Hugh Dickins <hughd@google.com> > --- > mm/khugepaged.c | 172 ++++++++++++++++++++++---------------------------- > 1 file changed, 77 insertions(+), 95 deletions(-) > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > index 3bb05147961b..46986eb4eebb 100644 > --- a/mm/khugepaged.c > +++ b/mm/khugepaged.c > @@ -1483,7 +1483,7 @@ static bool khugepaged_add_pte_mapped_thp(struct mm_struct *mm, > return ret; > } > > -/* hpage must be locked, and mmap_lock must be held in write */ > +/* hpage must be locked, and mmap_lock must be held */ > static int set_huge_pmd(struct vm_area_struct *vma, unsigned long addr, > pmd_t *pmdp, struct page *hpage) > { > @@ -1495,7 +1495,7 @@ static int set_huge_pmd(struct vm_area_struct *vma, unsigned long addr, > }; > > VM_BUG_ON(!PageTransHuge(hpage)); > - mmap_assert_write_locked(vma->vm_mm); > + mmap_assert_locked(vma->vm_mm); > > if (do_set_pmd(&vmf, hpage)) > return SCAN_FAIL; > @@ -1504,48 +1504,6 @@ static int set_huge_pmd(struct vm_area_struct *vma, unsigned long addr, > return SCAN_SUCCEED; > } > > -/* > - * A note about locking: > - * Trying to take the page table spinlocks would be useless here because those > - * are only used to synchronize: > - * > - * - modifying terminal entries (ones that point to a data page, not to another > - * page table) > - * - installing *new* non-terminal entries > - * > - * Instead, we need roughly the same kind of protection as free_pgtables() or > - * mm_take_all_locks() (but only for a single VMA): > - * The mmap lock together with this VMA's rmap locks covers all paths towards > - * the page table entries we're messing with here, except for hardware page > - * table walks and lockless_pages_from_mm(). > - */ > -static void collapse_and_free_pmd(struct mm_struct *mm, struct vm_area_struct *vma, > - unsigned long addr, pmd_t *pmdp) > -{ > - pmd_t pmd; > - struct mmu_notifier_range range; > - > - mmap_assert_write_locked(mm); > - if (vma->vm_file) > - lockdep_assert_held_write(&vma->vm_file->f_mapping->i_mmap_rwsem); > - /* > - * All anon_vmas attached to the VMA have the same root and are > - * therefore locked by the same lock. > - */ > - if (vma->anon_vma) > - lockdep_assert_held_write(&vma->anon_vma->root->rwsem); > - > - mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, mm, addr, > - addr + HPAGE_PMD_SIZE); > - mmu_notifier_invalidate_range_start(&range); > - pmd = pmdp_collapse_flush(vma, addr, pmdp); > - tlb_remove_table_sync_one(); > - mmu_notifier_invalidate_range_end(&range); > - mm_dec_nr_ptes(mm); > - page_table_check_pte_clear_range(mm, addr, pmd); > - pte_free(mm, pmd_pgtable(pmd)); > -} > - > /** > * collapse_pte_mapped_thp - Try to collapse a pte-mapped THP for mm at > * address haddr. > @@ -1561,26 +1519,29 @@ static void collapse_and_free_pmd(struct mm_struct *mm, struct vm_area_struct *v > int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr, > bool install_pmd) > { > + struct mmu_notifier_range range; > + bool notified = false; > unsigned long haddr = addr & HPAGE_PMD_MASK; > struct vm_area_struct *vma = vma_lookup(mm, haddr); > struct page *hpage; > pte_t *start_pte, *pte; > - pmd_t *pmd; > - spinlock_t *ptl; > - int count = 0, result = SCAN_FAIL; > + pmd_t *pmd, pgt_pmd; > + spinlock_t *pml, *ptl; > + int nr_ptes = 0, result = SCAN_FAIL; > int i; > > - mmap_assert_write_locked(mm); > + mmap_assert_locked(mm); > + > + /* First check VMA found, in case page tables are being torn down */ > + if (!vma || !vma->vm_file || > + !range_in_vma(vma, haddr, haddr + HPAGE_PMD_SIZE)) > + return SCAN_VMA_CHECK; > > /* Fast check before locking page if already PMD-mapped */ > result = find_pmd_or_thp_or_none(mm, haddr, &pmd); > if (result == SCAN_PMD_MAPPED) > return result; > > - if (!vma || !vma->vm_file || > - !range_in_vma(vma, haddr, haddr + HPAGE_PMD_SIZE)) > - return SCAN_VMA_CHECK; > - > /* > * If we are here, we've succeeded in replacing all the native pages > * in the page cache with a single hugepage. If a mm were to fault-in > @@ -1610,6 +1571,7 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr, > goto drop_hpage; > } > > + result = find_pmd_or_thp_or_none(mm, haddr, &pmd); > switch (result) { > case SCAN_SUCCEED: > break; > @@ -1623,27 +1585,10 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr, > goto drop_hpage; > } > > - /* Lock the vma before taking i_mmap and page table locks */ > - vma_start_write(vma); > - > - /* > - * We need to lock the mapping so that from here on, only GUP-fast and > - * hardware page walks can access the parts of the page tables that > - * we're operating on. > - * See collapse_and_free_pmd(). > - */ > - i_mmap_lock_write(vma->vm_file->f_mapping); > - > - /* > - * This spinlock should be unnecessary: Nobody else should be accessing > - * the page tables under spinlock protection here, only > - * lockless_pages_from_mm() and the hardware page walker can access page > - * tables while all the high-level locks are held in write mode. > - */ > result = SCAN_FAIL; > start_pte = pte_offset_map_lock(mm, pmd, haddr, &ptl); > - if (!start_pte) > - goto drop_immap; > + if (!start_pte) /* mmap_lock + page lock should prevent this */ > + goto drop_hpage; > > /* step 1: check all mapped PTEs are to the right huge page */ > for (i = 0, addr = haddr, pte = start_pte; > @@ -1670,10 +1615,18 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr, > */ > if (hpage + i != page) > goto abort; > - count++; > } > > - /* step 2: adjust rmap */ > + pte_unmap_unlock(start_pte, ptl); > + mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, mm, > + haddr, haddr + HPAGE_PMD_SIZE); > + mmu_notifier_invalidate_range_start(&range); > + notified = true; > + start_pte = pte_offset_map_lock(mm, pmd, haddr, &ptl); > + if (!start_pte) /* mmap_lock + page lock should prevent this */ > + goto abort; > + > + /* step 2: clear page table and adjust rmap */ > for (i = 0, addr = haddr, pte = start_pte; > i < HPAGE_PMD_NR; i++, addr += PAGE_SIZE, pte++) { > struct page *page; > @@ -1681,47 +1634,76 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr, > > if (pte_none(ptent)) > continue; > - page = vm_normal_page(vma, addr, ptent); > - if (WARN_ON_ONCE(page && is_zone_device_page(page))) > + /* > + * We dropped ptl after the first scan, to do the mmu_notifier: > + * page lock stops more PTEs of the hpage being faulted in, but > + * does not stop write faults COWing anon copies from existing > + * PTEs; and does not stop those being swapped out or migrated. > + */ > + if (!pte_present(ptent)) { > + result = SCAN_PTE_NON_PRESENT; > goto abort; > + } > + page = vm_normal_page(vma, addr, ptent); > + if (hpage + i != page) > + goto abort; > + > + /* > + * Must clear entry, or a racing truncate may re-remove it. > + * TLB flush can be left until pmdp_collapse_flush() does it. > + * PTE dirty? Shmem page is already dirty; file is read-only. > + */ > + pte_clear(mm, addr, pte); This is not non-present PTE entry, so we should call ptep_clear() to let page_table_check track the PTE clearing operation, right? Otherwise it may lead to false positives? Thanks, Qi > page_remove_rmap(page, vma, false); > + nr_ptes++; > } > > pte_unmap_unlock(start_pte, ptl); > > /* step 3: set proper refcount and mm_counters. */ > - if (count) { > - page_ref_sub(hpage, count); > - add_mm_counter(vma->vm_mm, mm_counter_file(hpage), -count); > + if (nr_ptes) { > + page_ref_sub(hpage, nr_ptes); > + add_mm_counter(mm, mm_counter_file(hpage), -nr_ptes); > } > > - /* step 4: remove pte entries */ > - /* we make no change to anon, but protect concurrent anon page lookup */ > - if (vma->anon_vma) > - anon_vma_lock_write(vma->anon_vma); > + /* step 4: remove page table */ > > - collapse_and_free_pmd(mm, vma, haddr, pmd); > + /* Huge page lock is still held, so page table must remain empty */ > + pml = pmd_lock(mm, pmd); > + if (ptl != pml) > + spin_lock_nested(ptl, SINGLE_DEPTH_NESTING); > + pgt_pmd = pmdp_collapse_flush(vma, haddr, pmd); > + pmdp_get_lockless_sync(); > + if (ptl != pml) > + spin_unlock(ptl); > + spin_unlock(pml); > > - if (vma->anon_vma) > - anon_vma_unlock_write(vma->anon_vma); > - i_mmap_unlock_write(vma->vm_file->f_mapping); > + mmu_notifier_invalidate_range_end(&range); > + > + mm_dec_nr_ptes(mm); > + page_table_check_pte_clear_range(mm, haddr, pgt_pmd); > + pte_free_defer(mm, pmd_pgtable(pgt_pmd)); > > maybe_install_pmd: > /* step 5: install pmd entry */ > result = install_pmd > ? set_huge_pmd(vma, haddr, pmd, hpage) > : SCAN_SUCCEED; > - > + goto drop_hpage; > +abort: > + if (nr_ptes) { > + flush_tlb_mm(mm); > + page_ref_sub(hpage, nr_ptes); > + add_mm_counter(mm, mm_counter_file(hpage), -nr_ptes); > + } > + if (start_pte) > + pte_unmap_unlock(start_pte, ptl); > + if (notified) > + mmu_notifier_invalidate_range_end(&range); > drop_hpage: > unlock_page(hpage); > put_page(hpage); > return result; > - > -abort: > - pte_unmap_unlock(start_pte, ptl); > -drop_immap: > - i_mmap_unlock_write(vma->vm_file->f_mapping); > - goto drop_hpage; > } > > static void khugepaged_collapse_pte_mapped_thps(struct khugepaged_mm_slot *mm_slot) > @@ -2855,9 +2837,9 @@ int madvise_collapse(struct vm_area_struct *vma, struct vm_area_struct **prev, > case SCAN_PTE_MAPPED_HUGEPAGE: > BUG_ON(mmap_locked); > BUG_ON(*prev); > - mmap_write_lock(mm); > + mmap_read_lock(mm); > result = collapse_pte_mapped_thp(mm, addr, true); > - mmap_write_unlock(mm); > + mmap_locked = true; > goto handle_result; > /* Whitelisted set of results where continuing OK */ > case SCAN_PMD_NULL:
On Thu, 3 Aug 2023, Qi Zheng wrote: > On 2023/7/12 12:42, Hugh Dickins wrote: > > Bring collapse_and_free_pmd() back into collapse_pte_mapped_thp(). > > It does need mmap_read_lock(), but it does not need mmap_write_lock(), > > nor vma_start_write() nor i_mmap lock nor anon_vma lock. All racing > > paths are relying on pte_offset_map_lock() and pmd_lock(), so use those. ... > > @@ -1681,47 +1634,76 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, > > unsigned long addr, > > > > if (pte_none(ptent)) > > continue; > > - page = vm_normal_page(vma, addr, ptent); > > - if (WARN_ON_ONCE(page && is_zone_device_page(page))) > > + /* > > + * We dropped ptl after the first scan, to do the > > mmu_notifier: > > + * page lock stops more PTEs of the hpage being faulted in, > > but > > + * does not stop write faults COWing anon copies from existing > > + * PTEs; and does not stop those being swapped out or > > migrated. > > + */ > > + if (!pte_present(ptent)) { > > + result = SCAN_PTE_NON_PRESENT; > > goto abort; > > + } > > + page = vm_normal_page(vma, addr, ptent); > > + if (hpage + i != page) > > + goto abort; > > + > > + /* > > + * Must clear entry, or a racing truncate may re-remove it. > > + * TLB flush can be left until pmdp_collapse_flush() does it. > > + * PTE dirty? Shmem page is already dirty; file is read-only. > > + */ > > + pte_clear(mm, addr, pte); > > This is not non-present PTE entry, so we should call ptep_clear() to let > page_table_check track the PTE clearing operation, right? Otherwise it > may lead to false positives? You are right: thanks a lot for catching that: fix patch follows. Hugh
On 2023/8/6 11:55, Hugh Dickins wrote: > On Thu, 3 Aug 2023, Qi Zheng wrote: >> On 2023/7/12 12:42, Hugh Dickins wrote: >>> Bring collapse_and_free_pmd() back into collapse_pte_mapped_thp(). >>> It does need mmap_read_lock(), but it does not need mmap_write_lock(), >>> nor vma_start_write() nor i_mmap lock nor anon_vma lock. All racing >>> paths are relying on pte_offset_map_lock() and pmd_lock(), so use those. > ... >>> @@ -1681,47 +1634,76 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, >>> unsigned long addr, >>> >>> if (pte_none(ptent)) >>> continue; >>> - page = vm_normal_page(vma, addr, ptent); >>> - if (WARN_ON_ONCE(page && is_zone_device_page(page))) >>> + /* >>> + * We dropped ptl after the first scan, to do the >>> mmu_notifier: >>> + * page lock stops more PTEs of the hpage being faulted in, >>> but >>> + * does not stop write faults COWing anon copies from existing >>> + * PTEs; and does not stop those being swapped out or >>> migrated. >>> + */ >>> + if (!pte_present(ptent)) { >>> + result = SCAN_PTE_NON_PRESENT; >>> goto abort; >>> + } >>> + page = vm_normal_page(vma, addr, ptent); >>> + if (hpage + i != page) >>> + goto abort; >>> + >>> + /* >>> + * Must clear entry, or a racing truncate may re-remove it. >>> + * TLB flush can be left until pmdp_collapse_flush() does it. >>> + * PTE dirty? Shmem page is already dirty; file is read-only. >>> + */ >>> + pte_clear(mm, addr, pte); >> >> This is not non-present PTE entry, so we should call ptep_clear() to let >> page_table_check track the PTE clearing operation, right? Otherwise it >> may lead to false positives? > > You are right: thanks a lot for catching that: fix patch follows. With fix patch: Reviewed-by: Qi Zheng <zhengqi.arch@bytedance.com> Thanks. > > Hugh
diff --git a/mm/khugepaged.c b/mm/khugepaged.c index 3bb05147961b..46986eb4eebb 100644 --- a/mm/khugepaged.c +++ b/mm/khugepaged.c @@ -1483,7 +1483,7 @@ static bool khugepaged_add_pte_mapped_thp(struct mm_struct *mm, return ret; } -/* hpage must be locked, and mmap_lock must be held in write */ +/* hpage must be locked, and mmap_lock must be held */ static int set_huge_pmd(struct vm_area_struct *vma, unsigned long addr, pmd_t *pmdp, struct page *hpage) { @@ -1495,7 +1495,7 @@ static int set_huge_pmd(struct vm_area_struct *vma, unsigned long addr, }; VM_BUG_ON(!PageTransHuge(hpage)); - mmap_assert_write_locked(vma->vm_mm); + mmap_assert_locked(vma->vm_mm); if (do_set_pmd(&vmf, hpage)) return SCAN_FAIL; @@ -1504,48 +1504,6 @@ static int set_huge_pmd(struct vm_area_struct *vma, unsigned long addr, return SCAN_SUCCEED; } -/* - * A note about locking: - * Trying to take the page table spinlocks would be useless here because those - * are only used to synchronize: - * - * - modifying terminal entries (ones that point to a data page, not to another - * page table) - * - installing *new* non-terminal entries - * - * Instead, we need roughly the same kind of protection as free_pgtables() or - * mm_take_all_locks() (but only for a single VMA): - * The mmap lock together with this VMA's rmap locks covers all paths towards - * the page table entries we're messing with here, except for hardware page - * table walks and lockless_pages_from_mm(). - */ -static void collapse_and_free_pmd(struct mm_struct *mm, struct vm_area_struct *vma, - unsigned long addr, pmd_t *pmdp) -{ - pmd_t pmd; - struct mmu_notifier_range range; - - mmap_assert_write_locked(mm); - if (vma->vm_file) - lockdep_assert_held_write(&vma->vm_file->f_mapping->i_mmap_rwsem); - /* - * All anon_vmas attached to the VMA have the same root and are - * therefore locked by the same lock. - */ - if (vma->anon_vma) - lockdep_assert_held_write(&vma->anon_vma->root->rwsem); - - mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, mm, addr, - addr + HPAGE_PMD_SIZE); - mmu_notifier_invalidate_range_start(&range); - pmd = pmdp_collapse_flush(vma, addr, pmdp); - tlb_remove_table_sync_one(); - mmu_notifier_invalidate_range_end(&range); - mm_dec_nr_ptes(mm); - page_table_check_pte_clear_range(mm, addr, pmd); - pte_free(mm, pmd_pgtable(pmd)); -} - /** * collapse_pte_mapped_thp - Try to collapse a pte-mapped THP for mm at * address haddr. @@ -1561,26 +1519,29 @@ static void collapse_and_free_pmd(struct mm_struct *mm, struct vm_area_struct *v int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr, bool install_pmd) { + struct mmu_notifier_range range; + bool notified = false; unsigned long haddr = addr & HPAGE_PMD_MASK; struct vm_area_struct *vma = vma_lookup(mm, haddr); struct page *hpage; pte_t *start_pte, *pte; - pmd_t *pmd; - spinlock_t *ptl; - int count = 0, result = SCAN_FAIL; + pmd_t *pmd, pgt_pmd; + spinlock_t *pml, *ptl; + int nr_ptes = 0, result = SCAN_FAIL; int i; - mmap_assert_write_locked(mm); + mmap_assert_locked(mm); + + /* First check VMA found, in case page tables are being torn down */ + if (!vma || !vma->vm_file || + !range_in_vma(vma, haddr, haddr + HPAGE_PMD_SIZE)) + return SCAN_VMA_CHECK; /* Fast check before locking page if already PMD-mapped */ result = find_pmd_or_thp_or_none(mm, haddr, &pmd); if (result == SCAN_PMD_MAPPED) return result; - if (!vma || !vma->vm_file || - !range_in_vma(vma, haddr, haddr + HPAGE_PMD_SIZE)) - return SCAN_VMA_CHECK; - /* * If we are here, we've succeeded in replacing all the native pages * in the page cache with a single hugepage. If a mm were to fault-in @@ -1610,6 +1571,7 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr, goto drop_hpage; } + result = find_pmd_or_thp_or_none(mm, haddr, &pmd); switch (result) { case SCAN_SUCCEED: break; @@ -1623,27 +1585,10 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr, goto drop_hpage; } - /* Lock the vma before taking i_mmap and page table locks */ - vma_start_write(vma); - - /* - * We need to lock the mapping so that from here on, only GUP-fast and - * hardware page walks can access the parts of the page tables that - * we're operating on. - * See collapse_and_free_pmd(). - */ - i_mmap_lock_write(vma->vm_file->f_mapping); - - /* - * This spinlock should be unnecessary: Nobody else should be accessing - * the page tables under spinlock protection here, only - * lockless_pages_from_mm() and the hardware page walker can access page - * tables while all the high-level locks are held in write mode. - */ result = SCAN_FAIL; start_pte = pte_offset_map_lock(mm, pmd, haddr, &ptl); - if (!start_pte) - goto drop_immap; + if (!start_pte) /* mmap_lock + page lock should prevent this */ + goto drop_hpage; /* step 1: check all mapped PTEs are to the right huge page */ for (i = 0, addr = haddr, pte = start_pte; @@ -1670,10 +1615,18 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr, */ if (hpage + i != page) goto abort; - count++; } - /* step 2: adjust rmap */ + pte_unmap_unlock(start_pte, ptl); + mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, mm, + haddr, haddr + HPAGE_PMD_SIZE); + mmu_notifier_invalidate_range_start(&range); + notified = true; + start_pte = pte_offset_map_lock(mm, pmd, haddr, &ptl); + if (!start_pte) /* mmap_lock + page lock should prevent this */ + goto abort; + + /* step 2: clear page table and adjust rmap */ for (i = 0, addr = haddr, pte = start_pte; i < HPAGE_PMD_NR; i++, addr += PAGE_SIZE, pte++) { struct page *page; @@ -1681,47 +1634,76 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr, if (pte_none(ptent)) continue; - page = vm_normal_page(vma, addr, ptent); - if (WARN_ON_ONCE(page && is_zone_device_page(page))) + /* + * We dropped ptl after the first scan, to do the mmu_notifier: + * page lock stops more PTEs of the hpage being faulted in, but + * does not stop write faults COWing anon copies from existing + * PTEs; and does not stop those being swapped out or migrated. + */ + if (!pte_present(ptent)) { + result = SCAN_PTE_NON_PRESENT; goto abort; + } + page = vm_normal_page(vma, addr, ptent); + if (hpage + i != page) + goto abort; + + /* + * Must clear entry, or a racing truncate may re-remove it. + * TLB flush can be left until pmdp_collapse_flush() does it. + * PTE dirty? Shmem page is already dirty; file is read-only. + */ + pte_clear(mm, addr, pte); page_remove_rmap(page, vma, false); + nr_ptes++; } pte_unmap_unlock(start_pte, ptl); /* step 3: set proper refcount and mm_counters. */ - if (count) { - page_ref_sub(hpage, count); - add_mm_counter(vma->vm_mm, mm_counter_file(hpage), -count); + if (nr_ptes) { + page_ref_sub(hpage, nr_ptes); + add_mm_counter(mm, mm_counter_file(hpage), -nr_ptes); } - /* step 4: remove pte entries */ - /* we make no change to anon, but protect concurrent anon page lookup */ - if (vma->anon_vma) - anon_vma_lock_write(vma->anon_vma); + /* step 4: remove page table */ - collapse_and_free_pmd(mm, vma, haddr, pmd); + /* Huge page lock is still held, so page table must remain empty */ + pml = pmd_lock(mm, pmd); + if (ptl != pml) + spin_lock_nested(ptl, SINGLE_DEPTH_NESTING); + pgt_pmd = pmdp_collapse_flush(vma, haddr, pmd); + pmdp_get_lockless_sync(); + if (ptl != pml) + spin_unlock(ptl); + spin_unlock(pml); - if (vma->anon_vma) - anon_vma_unlock_write(vma->anon_vma); - i_mmap_unlock_write(vma->vm_file->f_mapping); + mmu_notifier_invalidate_range_end(&range); + + mm_dec_nr_ptes(mm); + page_table_check_pte_clear_range(mm, haddr, pgt_pmd); + pte_free_defer(mm, pmd_pgtable(pgt_pmd)); maybe_install_pmd: /* step 5: install pmd entry */ result = install_pmd ? set_huge_pmd(vma, haddr, pmd, hpage) : SCAN_SUCCEED; - + goto drop_hpage; +abort: + if (nr_ptes) { + flush_tlb_mm(mm); + page_ref_sub(hpage, nr_ptes); + add_mm_counter(mm, mm_counter_file(hpage), -nr_ptes); + } + if (start_pte) + pte_unmap_unlock(start_pte, ptl); + if (notified) + mmu_notifier_invalidate_range_end(&range); drop_hpage: unlock_page(hpage); put_page(hpage); return result; - -abort: - pte_unmap_unlock(start_pte, ptl); -drop_immap: - i_mmap_unlock_write(vma->vm_file->f_mapping); - goto drop_hpage; } static void khugepaged_collapse_pte_mapped_thps(struct khugepaged_mm_slot *mm_slot) @@ -2855,9 +2837,9 @@ int madvise_collapse(struct vm_area_struct *vma, struct vm_area_struct **prev, case SCAN_PTE_MAPPED_HUGEPAGE: BUG_ON(mmap_locked); BUG_ON(*prev); - mmap_write_lock(mm); + mmap_read_lock(mm); result = collapse_pte_mapped_thp(mm, addr, true); - mmap_write_unlock(mm); + mmap_locked = true; goto handle_result; /* Whitelisted set of results where continuing OK */ case SCAN_PMD_NULL:
Bring collapse_and_free_pmd() back into collapse_pte_mapped_thp(). It does need mmap_read_lock(), but it does not need mmap_write_lock(), nor vma_start_write() nor i_mmap lock nor anon_vma lock. All racing paths are relying on pte_offset_map_lock() and pmd_lock(), so use those. Follow the pattern in retract_page_tables(); and using pte_free_defer() removes most of the need for tlb_remove_table_sync_one() here; but call pmdp_get_lockless_sync() to use it in the PAE case. First check the VMA, in case page tables are being torn down: from JannH. Confirm the preliminary find_pmd_or_thp_or_none() once page lock has been acquired and the page looks suitable: from then on its state is stable. However, collapse_pte_mapped_thp() was doing something others don't: freeing a page table still containing "valid" entries. i_mmap lock did stop a racing truncate from double-freeing those pages, but we prefer collapse_pte_mapped_thp() to clear the entries as usual. Their TLB flush can wait until the pmdp_collapse_flush() which follows, but the mmu_notifier_invalidate_range_start() has to be done earlier. Do the "step 1" checking loop without mmu_notifier: it wouldn't be good for khugepaged to keep on repeatedly invalidating a range which is then found unsuitable e.g. contains COWs. "step 2", which does the clearing, must then be more careful (after dropping ptl to do mmu_notifier), with abort prepared to correct the accounting like "step 3". But with those entries now cleared, "step 4" (after dropping ptl to do pmd_lock) is kept safe by the huge page lock, which stops new PTEs from being faulted in. Signed-off-by: Hugh Dickins <hughd@google.com> --- mm/khugepaged.c | 172 ++++++++++++++++++++++---------------------------- 1 file changed, 77 insertions(+), 95 deletions(-)