Re: [PATCH v2 1/2] mm/rmap: fix and simplify reusing mergeable anon_vma as parent when fork

["lixinhai.lxh@gmail.com" <lixinhai.lxh@gmail.com>]

On 2020-01-07 at 18:19 Konstantin Khlebnikov wrote:
>This fixes some misconceptions in commit 4e4a9eb92133 ("mm/rmap.c: reuse
>mergeable anon_vma as parent when fork"). It merges anon-vma in unexpected
>way but fortunately still produces valid anon-vma tree, so nothing crashes.
>
>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
>fork indepndent anon-vmas: ANON1 ANON2 .. ANONn (each is child of ANON0).
>Before this patch only DST1 will fork new ANON1 and following DST2 .. DSTn
>will share parent's ANON0 (i.e. anon-vma tree is valid but isn't optimal).
>With this patch DST1 will create new ANON1 and DST2 .. DSTn will share it.
>
>Root 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.
>
>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'.
>
>Third hidden problem is linking between VMA and anon-vmas whose pages it
>could contain. Loop in anon_vma_clone() attaches only parent's anon-vmas,
>shared anon-vma isn't attached. But every mapped page stays reachable in
>rmap because we erroneously share anon-vma from parent's previous VMA.
>
>This patch moves sharing logic out of anon_vma_clone() into more specific
>anon_vma_fork() because this supposed to work only at fork() and simply
>reuses anon_vma from previous VMA if it is forked from the same anon-vma.
>
>Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
>Reported-by: Li Xinhai <lixinhai.lxh@gmail.com>
>Fixes: 4e4a9eb92133 ("mm/rmap.c: reuse mergeable anon_vma as parent when fork")
>Link: https://lore.kernel.org/linux-mm/CALYGNiNzz+dxHX0g5-gNypUQc3B=8_Scp53-NTOh=zWsdUuHAw@mail.gmail.com/T/#t

The call to anon_vma_clone is nice. It looks fine to me.
Reviewed-by: Li Xinhai <lixinhai.lxh@gmail.com>

should I also add Suggested-by: Li Xinhai <lixinhai.lxh@gmail.com> ?

>---
> include/linux/rmap.h |    3 ++-
> kernel/fork.c        |    2 +-
> mm/rmap.c            |   23 +++++++++--------------
> 3 files changed, 12 insertions(+), 16 deletions(-)
>
>diff --git a/include/linux/rmap.h b/include/linux/rmap.h
>index 988d176472df..560e4480dcd0 100644
>--- a/include/linux/rmap.h
>+++ b/include/linux/rmap.h
>@@ -143,7 +143,8 @@ void anon_vma_init(void);	/* create anon_vma_cachep */
> int  __anon_vma_prepare(struct vm_area_struct *);
> void unlink_anon_vmas(struct vm_area_struct *);
> int anon_vma_clone(struct vm_area_struct *, struct vm_area_struct *);
>-int anon_vma_fork(struct vm_area_struct *, struct vm_area_struct *);
>+int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma,
>+	  struct vm_area_struct *prev);
>
> static inline int anon_vma_prepare(struct vm_area_struct *vma)
> {
>diff --git a/kernel/fork.c b/kernel/fork.c
>index 2508a4f238a3..c33626993831 100644
>--- a/kernel/fork.c
>+++ b/kernel/fork.c
>@@ -556,7 +556,7 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
> tmp->anon_vma = NULL;
> if (anon_vma_prepare(tmp))
> goto fail_nomem_anon_vma_fork;
>-	} else if (anon_vma_fork(tmp, mpnt))
>+	} else if (anon_vma_fork(tmp, mpnt, prev))
> goto fail_nomem_anon_vma_fork;
> tmp->vm_flags &= ~(VM_LOCKED | VM_LOCKONFAULT);
> tmp->vm_next = tmp->vm_prev = NULL;
>diff --git a/mm/rmap.c b/mm/rmap.c
>index b3e381919835..3c1e04389291 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;
>@@ -332,7 +319,8 @@ int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src)
>  * the corresponding VMA in the parent process is attached to.
>  * Returns 0 on success, non-zero on failure.
>  */
>-int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma)
>+int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma,
>+	  struct vm_area_struct *prev)
> {
> struct anon_vma_chain *avc;
> struct anon_vma *anon_vma;
>@@ -342,6 +330,13 @@ int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma)
> if (!pvma->anon_vma)
> return 0;
>
>+	/* Share anon_vma with previous VMA if it has the same parent. */
>+	if (prev && prev->anon_vma &&
>+	    prev->anon_vma->parent == pvma->anon_vma) {
>+	vma->anon_vma = prev->anon_vma;
>+	return anon_vma_clone(vma, prev);
>+	}
>+
> /* Drop inherited anon_vma, we'll reuse existing or allocate new. */
> vma->anon_vma = NULL;
>
>