| Message ID | 20191011072256.16275-1-richardw.yang@linux.intel.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [v4,1/2] mm/rmap.c: don't reuse anon_vma if we just want a copy | expand |
On 11/10/2019 10.22, 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> Yes, reusing heuristic was designed for fork. But this isn't strictly necessary - any vmas could share anon_vma. For example all vmas in system could be linked with single anon_vma. Acked-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru> > > --- > v4: > * check dst->anon_vma in each iteration > v3: > * use dst->anon_vma and src->anon_vma to get reuse state > pointed by Konstantin Khlebnikov > --- > mm/rmap.c | 24 +++++++++++++++--------- > 1 file changed, 15 insertions(+), 9 deletions(-) > > diff --git a/mm/rmap.c b/mm/rmap.c > index d9a23bb773bf..c34414567474 100644 > --- a/mm/rmap.c > +++ b/mm/rmap.c > @@ -250,13 +250,19 @@ 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 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 > - * child isn't reused even if there was no alive vma, thus rmap walker has a > - * good chance of avoiding scanning the whole hierarchy when it searches where > - * page is mapped. > + * 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 checking (!dst->anon_vma && src->anon_vma). > + * > + * If (!dst->anon_vma && src->anon_vma) 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 child isn't reused even if there was no alive vma, thus rmap > + * walker has a good chance of avoiding scanning the whole hierarchy when it > + * searches where page is mapped. > */ > int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src) > { > @@ -286,8 +292,8 @@ 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 (!dst->anon_vma && src->anon_vma && > + anon_vma != src->anon_vma && anon_vma->degree < 2) > dst->anon_vma = anon_vma; > } > if (dst->anon_vma) >
On Fri, Oct 11, 2019 at 11:04:16AM +0300, Konstantin Khlebnikov wrote: >On 11/10/2019 10.22, 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> > >Yes, reusing heuristic was designed for fork. >But this isn't strictly necessary - any vmas could share anon_vma. >For example all vmas in system could be linked with single anon_vma. Yes, agree with you. Thanks for your comment :-) > >Acked-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru> > >> >> --- >> v4: >> * check dst->anon_vma in each iteration >> v3: >> * use dst->anon_vma and src->anon_vma to get reuse state >> pointed by Konstantin Khlebnikov >> --- >> mm/rmap.c | 24 +++++++++++++++--------- >> 1 file changed, 15 insertions(+), 9 deletions(-) >> >> diff --git a/mm/rmap.c b/mm/rmap.c >> index d9a23bb773bf..c34414567474 100644 >> --- a/mm/rmap.c >> +++ b/mm/rmap.c >> @@ -250,13 +250,19 @@ 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 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 >> - * child isn't reused even if there was no alive vma, thus rmap walker has a >> - * good chance of avoiding scanning the whole hierarchy when it searches where >> - * page is mapped. >> + * 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 checking (!dst->anon_vma && src->anon_vma). >> + * >> + * If (!dst->anon_vma && src->anon_vma) 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 child isn't reused even if there was no alive vma, thus rmap >> + * walker has a good chance of avoiding scanning the whole hierarchy when it >> + * searches where page is mapped. >> */ >> int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src) >> { >> @@ -286,8 +292,8 @@ 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 (!dst->anon_vma && src->anon_vma && >> + anon_vma != src->anon_vma && anon_vma->degree < 2) >> dst->anon_vma = anon_vma; >> } >> if (dst->anon_vma) >>
diff --git a/mm/rmap.c b/mm/rmap.c index d9a23bb773bf..c34414567474 100644 --- a/mm/rmap.c +++ b/mm/rmap.c @@ -250,13 +250,19 @@ 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 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 - * child isn't reused even if there was no alive vma, thus rmap walker has a - * good chance of avoiding scanning the whole hierarchy when it searches where - * page is mapped. + * 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 checking (!dst->anon_vma && src->anon_vma). + * + * If (!dst->anon_vma && src->anon_vma) 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 child isn't reused even if there was no alive vma, thus rmap + * walker has a good chance of avoiding scanning the whole hierarchy when it + * searches where page is mapped. */ int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src) { @@ -286,8 +292,8 @@ 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 (!dst->anon_vma && src->anon_vma && + 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> --- v4: * check dst->anon_vma in each iteration v3: * use dst->anon_vma and src->anon_vma to get reuse state pointed by Konstantin Khlebnikov --- mm/rmap.c | 24 +++++++++++++++--------- 1 file changed, 15 insertions(+), 9 deletions(-)