From 47b390d23bf81894395c8773acf6f73c66465dc4 Mon Sep 17 00:00:00 2001 From: Wei Yang Date: Sat, 30 Nov 2019 17:50:56 -0800 Subject: mm/rmap.c: don't reuse anon_vma if we just want a copy MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Before commit 7a3ef208e662 ("mm: prevent endless growth of anon_vma hierarchy"), anon_vma_clone() doesn't change dst->anon_vma. While after this commit, anon_vma_clone() will try to reuse an exist one on forking. But this commit go a little bit further for the case not forking. anon_vma_clone() is called from __vma_split(), __split_vma(), copy_vma() and anon_vma_fork(). For the first three places, the purpose here is get a copy of src and we don't expect to touch dst->anon_vma even it is NULL. While after that commit, it is possible to reuse an anon_vma when dst->anon_vma is NULL. This is not we intend to have. This patch stops reuse of anon_vma for non-fork cases. Link: http://lkml.kernel.org/r/20191011072256.16275-1-richardw.yang@linux.intel.com Fixes: 7a3ef208e662 ("mm: prevent endless growth of anon_vma hierarchy") Signed-off-by: Wei Yang Acked-by: Konstantin Khlebnikov Cc: Kirill A. Shutemov Cc: "Jérôme Glisse" Cc: Mike Kravetz Cc: Rik van Riel Cc: Qian Cai Cc: Shakeel Butt Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/rmap.c | 24 +++++++++++++++--------- 1 file changed, 15 insertions(+), 9 deletions(-) (limited to 'mm/rmap.c') diff --git a/mm/rmap.c b/mm/rmap.c index 0c7b2a9400d4..ca254f5e4337 100644 --- a/mm/rmap.c +++ b/mm/rmap.c @@ -251,13 +251,19 @@ static inline void unlock_anon_vma_root(struct anon_vma *root) * Attach the anon_vmas from src to dst. * Returns 0 on success, -ENOMEM on failure. * - * If dst->anon_vma is NULL this function tries to find and reuse existing - * anon_vma which has no vmas and only one child anon_vma. This prevents - * degradation of anon_vma hierarchy to endless linear chain in case of - * constantly forking task. On the other hand, an anon_vma with more than one - * child isn't reused even if there was no alive vma, thus rmap walker has a - * good chance of avoiding scanning the whole hierarchy when it searches where - * page is mapped. + * anon_vma_clone() is called by __vma_split(), __split_vma(), copy_vma() and + * anon_vma_fork(). The first three want an exact copy of src, while the last + * one, anon_vma_fork(), may try to reuse an existing anon_vma to prevent + * endless growth of anon_vma. Since dst->anon_vma is set to NULL before call, + * we can identify this case by checking (!dst->anon_vma && src->anon_vma). + * + * If (!dst->anon_vma && src->anon_vma) is true, this function tries to find + * and reuse existing anon_vma which has no vmas and only one child anon_vma. + * This prevents degradation of anon_vma hierarchy to endless linear chain in + * case of constantly forking task. On the other hand, an anon_vma with more + * than one child isn't reused even if there was no alive vma, thus rmap + * walker has a good chance of avoiding scanning the whole hierarchy when it + * searches where page is mapped. */ int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src) { @@ -287,8 +293,8 @@ int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src) * will always reuse it. Root anon_vma is never reused: * it has self-parent reference and at least one child. */ - if (!dst->anon_vma && anon_vma != src->anon_vma && - anon_vma->degree < 2) + if (!dst->anon_vma && src->anon_vma && + anon_vma != src->anon_vma && anon_vma->degree < 2) dst->anon_vma = anon_vma; } if (dst->anon_vma) -- cgit