| Message ID | 20191011025841.16801-1-richardw.yang@linux.intel.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [v3,1/2] mm/rmap.c: don't reuse anon_vma if we just want a copy | expand |
On Fri, Oct 11, 2019 at 10:58:40AM +0800, Wei Yang wrote: >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 stop reuse anon_vma for non-fork cases. > >Fix commit 7a3ef208e662 ("mm: prevent endless growth of anon_vma >hierarchy") > >Signed-off-by: Wei Yang <richardw.yang@linux.intel.com> > >--- >v3: > * use dst->anon_vma and src->anon_vma to get reuse state > pointed by Konstantin Khlebnikov >--- > mm/rmap.c | 12 +++++++++--- > 1 file changed, 9 insertions(+), 3 deletions(-) > >diff --git a/mm/rmap.c b/mm/rmap.c >index d9a23bb773bf..fc0aba7fb9b9 100644 >--- a/mm/rmap.c >+++ b/mm/rmap.c >@@ -250,7 +250,13 @@ static inline void unlock_anon_vma_root(struct 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_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 (reuse = !dst->anon_vma && src->anon_vma). >+ * >+ * If reuse 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 >@@ -262,6 +268,7 @@ 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; >+ bool reuse = !dst->anon_vma && src->anon_vma; > > list_for_each_entry_reverse(pavc, &src->anon_vma_chain, same_vma) { > struct anon_vma *anon_vma; >@@ -286,8 +293,7 @@ int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src) > * 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 (reuse && anon_vma != src->anon_vma && anon_vma->degree < 2) > dst->anon_vma = anon_vma; What a shame. dst->anon_vma would be changed in the loop, so we only need to assign it when dst->anon_vma == NULL. > } > if (dst->anon_vma) >-- >2.17.1
diff --git a/mm/rmap.c b/mm/rmap.c index d9a23bb773bf..fc0aba7fb9b9 100644 --- a/mm/rmap.c +++ b/mm/rmap.c @@ -250,7 +250,13 @@ static inline void unlock_anon_vma_root(struct 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_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 (reuse = !dst->anon_vma && src->anon_vma). + * + * If reuse 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 @@ -262,6 +268,7 @@ 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; + bool reuse = !dst->anon_vma && src->anon_vma; list_for_each_entry_reverse(pavc, &src->anon_vma_chain, same_vma) { struct anon_vma *anon_vma; @@ -286,8 +293,7 @@ int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src) * 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 (reuse && anon_vma != src->anon_vma && anon_vma->degree < 2) dst->anon_vma = anon_vma; } if (dst->anon_vma)
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 stop reuse anon_vma for non-fork cases. Fix commit 7a3ef208e662 ("mm: prevent endless growth of anon_vma hierarchy") Signed-off-by: Wei Yang <richardw.yang@linux.intel.com> --- v3: * use dst->anon_vma and src->anon_vma to get reuse state pointed by Konstantin Khlebnikov --- mm/rmap.c | 12 +++++++++--- 1 file changed, 9 insertions(+), 3 deletions(-)