The patch titled Subject: mm/rmap.c: don't reuse anon_vma if we just want a copy has been added to the -mm tree. Its filename is mm-rmapc-dont-reuse-anon_vma-if-we-just-want-a-copy.patch This patch should soon appear at http://ozlabs.org/~akpm/mmots/broken-out/mm-rmapc-dont-reuse-anon_vma-if-we-just-want-a-copy.patch and later at http://ozlabs.org/~akpm/mmotm/broken-out/mm-rmapc-dont-reuse-anon_vma-if-we-just-want-a-copy.patch Before you just go and hit "reply", please: a) Consider who else should be cc'ed b) Prefer to cc a suitable mailing list as well c) Ideally: find the original patch on the mailing list and do a reply-to-all to that, adding suitable additional cc's *** Remember to use Documentation/process/submit-checklist.rst when testing your code *** The -mm tree is included into linux-next and is updated there every 3-4 working days ------------------------------------------------------ From: Wei Yang <richardw.yang@xxxxxxxxxxxxxxx> Subject: mm/rmap.c: don't reuse anon_vma if we just want a copy 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@xxxxxxxxxxxxxxx Fixes: 7a3ef208e662 ("mm: prevent endless growth of anon_vma hierarchy") Signed-off-by: Wei Yang <richardw.yang@xxxxxxxxxxxxxxx> Acked-by: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx> Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx> Cc: "Jérôme Glisse" <jglisse@xxxxxxxxxx> Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx> Cc: Rik van Riel <riel@xxxxxxxxxxx> Cc: Qian Cai <cai@xxxxxx> Cc: Shakeel Butt <shakeelb@xxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- mm/rmap.c | 24 +++++++++++++++--------- 1 file changed, 15 insertions(+), 9 deletions(-) --- a/mm/rmap.c~mm-rmapc-dont-reuse-anon_vma-if-we-just-want-a-copy +++ a/mm/rmap.c @@ -250,13 +250,19 @@ static inline void unlock_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) { @@ -286,8 +292,8 @@ int anon_vma_clone(struct vm_area_struct * 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) _ Patches currently in -mm which might be from richardw.yang@xxxxxxxxxxxxxxx are mm-mmapc-remove-a-never-trigger-warning-in-__vma_adjust.patch mm-mmapc-prev-could-be-retrieved-from-vma-vm_prev.patch mm-mmapc-__vma_unlink_prev-is-not-necessary-now.patch mm-mmapc-extract-__vma_unlink_list-as-counter-part-for-__vma_link_list.patch mm-mmapc-rb_parent-is-not-necessary-in-__vma_link_list.patch mm-rmapc-dont-reuse-anon_vma-if-we-just-want-a-copy.patch mm-rmapc-reuse-mergeable-anon_vma-as-parent-when-fork.patch hugetlb-remove-unused-hstate-in-hugetlb_fault_mutex_hash.patch userfaultfd-use-vma_pagesize-for-all-huge-page-size-calculation.patch userfaultfd-remove-unnecessary-warn_on-in-__mcopy_atomic_hugetlb.patch userfaultfd-wrap-the-common-dst_vma-check-into-an-inlined-function.patch mm-fix-typo-in-the-comment-when-calling-function-__setpageuptodate.patch