Re: [PATCH] mm/rmap.c: remove useless checking to child vma->vm_prev in anon_vma_clone

[Konstantin Khlebnikov <khlebnikov@yandex-team.ru>]

On 06/01/2020 09.37, Li Xinhai wrote:
> For fork case, the dst->vm_prev is always same as src->vm_prev when
> anon_vma_clone() is called. Removing the assignment from
> dst->vm_prev->anon_vma to dst->anon_vma, and explictly assign from
> anon_vma which is shared by its parent vmas.

This doesn't sound right.

I see dst->vm_prev is set after anon_vma_fork(), so here it still points to parent prev.
So, this thing works isn't as is supposed to be.

I expect this logic: If parent SRC1 SRC2 .. SRCn share ANON0
then in child related DST1 DST2 .. DSTn should fork and share ANON1:
Forking DST1 creates new ANON1 and then DST2 and following share it.

Also this assumption is wrong:
 > Parent has vm_prev, which implies we have vm_prev.
If in parent prev VMA has VM_DONTCOPY then in child prev VMA will
not match pprev or even could be NULL if it was first in mm.

See patch:
https://lore.kernel.org/lkml/157830736034.8148.7070851958306750616.stgit@buzz/T/#u

I've tested it using this:

--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -847,6 +847,12 @@ static int show_smap(struct seq_file *m, void *v)
                 seq_printf(m, "ProtectionKey:  %8u\n", vma_pkey(vma));
         show_smap_vma_flags(m, vma);

+       if (vma->anon_vma)
+               seq_printf(m, "AnonVMA: %p %p %d\n",
+                          vma->anon_vma,
+                          vma->anon_vma->parent,
+                          vma->anon_vma->degree);
+
         m_cache_vma(m, vma);

         return 0;

---

#include <sys/mman.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <stdio.h>

int main(int argc, char **argv) {
	void *ptr;
	char buf[100];

	ptr = mmap(NULL, 0x3000, PROT_READ | PROT_WRITE, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
	memset(ptr, 0, 0x3000);
	mprotect(ptr + 0x1000, 0x1000, PROT_READ);

	sprintf(buf, "cat /proc/%d/smaps", getpid());
	system(buf);

	if (fork()) {
		wait(NULL);
	} else {
		printf("\n\n\n");
		fflush(stdout);
		sprintf(buf, "cat /proc/%d/smaps", getpid());
		system(buf);
	}
}

---

> 
> Signed-off-by: Li Xinhai <lixinhai.lxh@gmail.com>
> Cc: Wei Yang <richardw.yang@linux.intel.com>
> Cc: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
> Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> ---
>   mm/rmap.c | 7 +++----
>   1 file changed, 3 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/rmap.c b/mm/rmap.c
> index b3e3819..3c912a6c 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -269,10 +269,10 @@ 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;
> +	struct vm_area_struct *pprev = src->vm_prev;
>   
>   	/*
> -	 * If parent share anon_vma with its vm_prev, keep this sharing in in
> +	 * If parent share anon_vma with its vm_prev, keep this sharing in
>   	 * child.
>   	 *
>   	 * 1. Parent has vm_prev, which implies we have vm_prev.
> @@ -280,8 +280,7 @@ int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src)
>   	 */
>   	if (!dst->anon_vma && src->anon_vma &&
>   	    pprev && pprev->anon_vma == src->anon_vma)
> -		dst->anon_vma = prev->anon_vma;
> -
> +		dst->anon_vma = pprev->anon_vma;
>   
>   	list_for_each_entry_reverse(pavc, &src->anon_vma_chain, same_vma) {
>   		struct anon_vma *anon_vma;
>