From: Andrea Arcangeli Date: Wed, 27 May 2020 23:06:24 +0000 (-0400) Subject: [RAMEN9610-21992]mm: thp: make the THP mapcount atomic against __split_huge_pmd_locked() X-Git-Tag: MMI-RSA31.Q1-48-36-11~4 X-Git-Url: https://git.stricted.de/?a=commitdiff_plain;h=47dab6f3ad15eaa875632f14ea0869fd331df1ed;p=GitHub%2FMotorolaMobilityLLC%2Fkernel-slsi.git [RAMEN9610-21992]mm: thp: make the THP mapcount atomic against __split_huge_pmd_locked() commit c444eb564fb16645c172d550359cb3d75fe8a040 upstream. Write protect anon page faults require an accurate mapcount to decide if to break the COW or not. This is implemented in the THP path with reuse_swap_page() -> page_trans_huge_map_swapcount()/page_trans_huge_mapcount(). If the COW triggers while the other processes sharing the page are under a huge pmd split, to do an accurate reading, we must ensure the mapcount isn't computed while it's being transferred from the head page to the tail pages. reuse_swap_cache() already runs serialized by the page lock, so it's enough to add the page lock around __split_huge_pmd_locked too, in order to add the missing serialization. Note: the commit in "Fixes" is just to facilitate the backporting, because the code before such commit didn't try to do an accurate THP mapcount calculation and it instead used the page_count() to decide if to COW or not. Both the page_count and the pin_count are THP-wide refcounts, so they're inaccurate if used in reuse_swap_page(). Reverting such commit (besides the unrelated fix to the local anon_vma assignment) would have also opened the window for memory corruption side effects to certain workloads as documented in such commit header. Change-Id: I702663cf38fd55f76fa2772e4b1cc8b1d0a45a0a Signed-off-by: Andrea Arcangeli Suggested-by: Jann Horn Reported-by: Jann Horn Acked-by: Kirill A. Shutemov Fixes: 6d0a07edd17c ("mm: thp: calculate the mapcount correctly for THP pages during WP faults") Cc: stable@vger.kernel.org Signed-off-by: Linus Torvalds Signed-off-by: Greg Kroah-Hartman --- diff --git a/mm/huge_memory.c b/mm/huge_memory.c index 4ef967ad24ec..4c71da233b60 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -2190,6 +2190,8 @@ void __split_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd, spinlock_t *ptl; struct mm_struct *mm = vma->vm_mm; unsigned long haddr = address & HPAGE_PMD_MASK; + bool was_locked = false; + pmd_t _pmd; mmu_notifier_invalidate_range_start(mm, haddr, haddr + HPAGE_PMD_SIZE); ptl = pmd_lock(mm, pmd); @@ -2199,11 +2201,32 @@ void __split_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd, * pmd against. Otherwise we can end up replacing wrong page. */ VM_BUG_ON(freeze && !page); - if (page && page != pmd_page(*pmd)) - goto out; + if (page) { + VM_WARN_ON_ONCE(!PageLocked(page)); + was_locked = true; + if (page != pmd_page(*pmd)) + goto out; + } +repeat: if (pmd_trans_huge(*pmd)) { - page = pmd_page(*pmd); + if (!page) { + page = pmd_page(*pmd); + if (unlikely(!trylock_page(page))) { + get_page(page); + _pmd = *pmd; + spin_unlock(ptl); + lock_page(page); + spin_lock(ptl); + if (unlikely(!pmd_same(*pmd, _pmd))) { + unlock_page(page); + put_page(page); + page = NULL; + goto repeat; + } + put_page(page); + } + } if (PageMlocked(page)) clear_page_mlock(page); } else if (!(pmd_devmap(*pmd) || is_pmd_migration_entry(*pmd))) @@ -2211,6 +2234,8 @@ void __split_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd, __split_huge_pmd_locked(vma, pmd, haddr, freeze); out: spin_unlock(ptl); + if (!was_locked && page) + unlock_page(page); mmu_notifier_invalidate_range_end(mm, haddr, haddr + HPAGE_PMD_SIZE); }