On 2020-01-06 at 18:43 Konstantin Khlebnikov wrote: >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. This logic was not fully clarified in https://lore.kernel.org/linux-mm/20191011072256.16275-2-richardw.yang@xxxxxxxxxxxxxxx/ I've assumed that sharing parent vma's anon_vma with child vma was the purpose of that patch, and it intentionally want the first child has its own new anon_vma (don't sharing as done by other child vma). > >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@xxxxxxxxx> >> Cc: Wei Yang <richardw.yang@xxxxxxxxxxxxxxx> >> Cc: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx> >> Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx> >> --- >> 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; >>
