Messages in this thread | | | Subject | Re: [PATCH] mm/rmap: fix reusing mergeable anon_vma as parent when fork | From | Konstantin Khlebnikov <> | Date | Tue, 7 Jan 2020 11:24:47 +0300 |
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On 07/01/2020 05.44, lixinhai.lxh@gmail.com wrote: > On 2020-01-07 at 04:35 Konstantin Khlebnikov wrote: >> On Mon, Jan 6, 2020 at 1:42 PM Konstantin Khlebnikov >> <khlebnikov@yandex-team.ru> wrote: >>> >>> This fixes couple misconceptions in commit 4e4a9eb92133 ("mm/rmap.c: reuse >>> mergeable anon_vma as parent when fork"). >>> >>> First problem caused by initialization order in dup_mmap(): vma->vm_prev >>> is set after calling anon_vma_fork(). Thus in anon_vma_fork() it points to >>> previous VMA in parent mm. This is fixed by rearrangement in dup_mmap(). >>> >>> If in parent VMAs: SRC1 SRC2 .. SRCn share anon-vma ANON0, then after fork >>> before all patches in child process related VMAs: DST1 DST2 .. DSTn will >>> use different anon-vmas: ANON1 ANON2 .. ANONn. Before this patch only DST1 >>> will fork new ANON1 and following DST2 .. DSTn will share parent's ANON0. >>> With this patch DST1 will create new ANON1 and DST2 .. DSTn will share it. >>> >>> Also this patch moves sharing logic out of anon_vma_clone() into more >>> specific anon_vma_fork() because this supposed to work only at fork(). >>> Function anon_vma_clone() is more generic is also used at splitting VMAs. >>> >>> Second problem is hidden behind first one: assumption "Parent has vm_prev, >>> which implies we have vm_prev" is wrong if first VMA in parent mm has set >>> flag VM_DONTCOPY. Luckily prev->anon_vma doesn't dereference NULL pointer >>> because in current code 'prev' actually is same as 'pprev'. To avoid that >>> this patch just checks pointer and compares vm_start to verify relation >>> between previous VMAs in parent and child. >>> >>> Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru> >>> Fixes: 4e4a9eb92133 ("mm/rmap.c: reuse mergeable anon_vma as parent when fork") >> >> Oops, I've forgot to mention that Li Xinhai <lixinhai.lxh@gmail.com> >> found and reported this suspicious code. Sorry. >> >> Reported-by: Li Xinhai <lixinhai.lxh@gmail.com> >> Link: https://lore.kernel.org/linux-mm/CALYGNiNzz+dxHX0g5-gNypUQc3B=8_Scp53-NTOh=zWsdUuHAw@mail.gmail.com/T/#t >> > > Can we change the interface > int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma), > to > int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma, struct vm_area_struct *pcvma), > and 'pcvma' means previous child vma. > so highlight the use of that vma, and the current code sequence for linking 'tmp' vma > in dup_mmap() is not changed(in case some code would have dependency on that > linking sequence)
There should be no dependency on linking sequence. But we could generalize sharing: cloned vma could share prev anon-vma (or any other actually) if anon_vma->parent == src->anon_vma. This is more clear than sharing only between related vmas.
> > Another issue is for linking the avc for the reused anon_vma. anon_vma_clone() > use the iteration > list_for_each_entry_reverse(pavc, &src->anon_vma_chain, same_vma), > to link avc for child vma, and it is unable to reach the resued anon_vma because > that is from the previous vma not from parent vma. So, in anon_vma_fork(), > we need to setup the avc link for vma->anon.
Oh, yes. That's another example where current code miraculously stays correct.
> >>> --- >>> kernel/fork.c | 4 ++-- >>> mm/rmap.c | 25 ++++++++++++------------- >>> 2 files changed, 14 insertions(+), 15 deletions(-) >>> >>> diff --git a/kernel/fork.c b/kernel/fork.c >>> index 2508a4f238a3..04ee5e243f65 100644 >>> --- a/kernel/fork.c >>> +++ b/kernel/fork.c >>> @@ -548,6 +548,8 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm, >>> if (retval) >>> goto fail_nomem_policy; >>> tmp->vm_mm = mm; >>> + tmp->vm_prev = prev; /* anon_vma_fork use this */ >>> + tmp->vm_next = NULL; >>> retval = dup_userfaultfd(tmp, &uf); >>> if (retval) >>> goto fail_nomem_anon_vma_fork; >>> @@ -559,7 +561,6 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm, >>> } else if (anon_vma_fork(tmp, mpnt)) >>> goto fail_nomem_anon_vma_fork; >>> tmp->vm_flags &= ~(VM_LOCKED | VM_LOCKONFAULT); >>> - tmp->vm_next = tmp->vm_prev = NULL; >>> file = tmp->vm_file; >>> if (file) { >>> struct inode *inode = file_inode(file); >>> @@ -592,7 +593,6 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm, >>> */ >>> *pprev = tmp; >>> pprev = &tmp->vm_next; >>> - tmp->vm_prev = prev; >>> prev = tmp; >>> >>> __vma_link_rb(mm, tmp, rb_link, rb_parent); >>> diff --git a/mm/rmap.c b/mm/rmap.c >>> index b3e381919835..77b3aa38d5c2 100644 >>> --- a/mm/rmap.c >>> +++ b/mm/rmap.c >>> @@ -269,19 +269,6 @@ int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src) >>> { >>> struct anon_vma_chain *avc, *pavc; >>> struct anon_vma *root = NULL; >>> - struct vm_area_struct *prev = dst->vm_prev, *pprev = src->vm_prev; >>> - >>> - /* >>> - * If parent share anon_vma with its vm_prev, keep this sharing in in >>> - * child. >>> - * >>> - * 1. Parent has vm_prev, which implies we have vm_prev. >>> - * 2. Parent and its vm_prev have the same anon_vma. >>> - */ >>> - if (!dst->anon_vma && src->anon_vma && >>> - pprev && pprev->anon_vma == src->anon_vma) >>> - dst->anon_vma = prev->anon_vma; >>> - >>> >>> list_for_each_entry_reverse(pavc, &src->anon_vma_chain, same_vma) { >>> struct anon_vma *anon_vma; >>> @@ -334,6 +321,7 @@ int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src) >>> */ >>> int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma) >>> { >>> + struct vm_area_struct *prev = vma->vm_prev, *pprev = pvma->vm_prev; >>> struct anon_vma_chain *avc; >>> struct anon_vma *anon_vma; >>> int error; >>> @@ -345,6 +333,17 @@ int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma) >>> /* Drop inherited anon_vma, we'll reuse existing or allocate new. */ >>> vma->anon_vma = NULL; >>> >>> + /* >>> + * If parent shares anon_vma with its vm_prev, keep this sharing. >>> + * >>> + * Previous VMA could be missing or not match previuos in parent >>> + * if VM_DONTCOPY is set: compare vm_start to avoid this case. >>> + */ >>> + if (pvma->anon_vma && pprev && prev && >>> + pprev->anon_vma == pvma->anon_vma && >>> + pprev->vm_start == prev->vm_start) >>> + vma->anon_vma = prev->anon_vma; >>> + >>> /* >>> * First, attach the new VMA to the parent VMA's anon_vmas, >>> * so rmap can find non-COWed pages in child processes. >>> >> >
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