| Message ID | 20231009064230.2952396-2-surenb@google.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | userfaultfd move option | expand |
On Sun, Oct 08, 2023 at 11:42:26PM -0700, Suren Baghdasaryan wrote: > From: Andrea Arcangeli <aarcange@redhat.com> > > For now, folio_move_anon_rmap() was only used to move a folio to a > different anon_vma after fork(), whereby the root anon_vma stayed > unchanged. For that, it was sufficient to hold the folio lock when > calling folio_move_anon_rmap(). > > However, we want to make use of folio_move_anon_rmap() to move folios > between VMAs that have a different root anon_vma. As folio_referenced() > performs an RMAP walk without holding the folio lock but only holding the > anon_vma in read mode, holding the folio lock is insufficient. > > When moving to an anon_vma with a different root anon_vma, we'll have to > hold both, the folio lock and the anon_vma lock in write mode. > Consequently, whenever we succeeded in folio_lock_anon_vma_read() to > read-lock the anon_vma, we have to re-check if the mapping was changed > in the meantime. If that was the case, we have to retry. > > Note that folio_move_anon_rmap() must only be called if the anon page is > exclusive to a process, and must not be called on KSM folios. > > This is a preparation for UFFDIO_MOVE, which will hold the folio lock, > the anon_vma lock in write mode, and the mmap_lock in read mode. > > Signed-off-by: Andrea Arcangeli <aarcange@redhat.com> > Signed-off-by: Suren Baghdasaryan <surenb@google.com> > --- > mm/rmap.c | 24 ++++++++++++++++++++++++ > 1 file changed, 24 insertions(+) > > diff --git a/mm/rmap.c b/mm/rmap.c > index c1f11c9dbe61..f9ddc50269d2 100644 > --- a/mm/rmap.c > +++ b/mm/rmap.c > @@ -542,7 +542,9 @@ struct anon_vma *folio_lock_anon_vma_read(struct folio *folio, > struct anon_vma *root_anon_vma; > unsigned long anon_mapping; > > +retry: > rcu_read_lock(); > +retry_under_rcu: > anon_mapping = (unsigned long)READ_ONCE(folio->mapping); > if ((anon_mapping & PAGE_MAPPING_FLAGS) != PAGE_MAPPING_ANON) > goto out; > @@ -552,6 +554,16 @@ struct anon_vma *folio_lock_anon_vma_read(struct folio *folio, > anon_vma = (struct anon_vma *) (anon_mapping - PAGE_MAPPING_ANON); > root_anon_vma = READ_ONCE(anon_vma->root); > if (down_read_trylock(&root_anon_vma->rwsem)) { > + /* > + * folio_move_anon_rmap() might have changed the anon_vma as we > + * might not hold the folio lock here. > + */ > + if (unlikely((unsigned long)READ_ONCE(folio->mapping) != > + anon_mapping)) { > + up_read(&root_anon_vma->rwsem); > + goto retry_under_rcu; Is adding this specific label worthwhile? How about rcu unlock and goto retry (then it'll also be clear that we won't hold rcu read lock for unpredictable time)? > + } > + > /* > * If the folio is still mapped, then this anon_vma is still > * its anon_vma, and holding the mutex ensures that it will
On 13.10.23 00:01, Peter Xu wrote: > On Sun, Oct 08, 2023 at 11:42:26PM -0700, Suren Baghdasaryan wrote: >> From: Andrea Arcangeli <aarcange@redhat.com> >> >> For now, folio_move_anon_rmap() was only used to move a folio to a >> different anon_vma after fork(), whereby the root anon_vma stayed >> unchanged. For that, it was sufficient to hold the folio lock when >> calling folio_move_anon_rmap(). >> >> However, we want to make use of folio_move_anon_rmap() to move folios >> between VMAs that have a different root anon_vma. As folio_referenced() >> performs an RMAP walk without holding the folio lock but only holding the >> anon_vma in read mode, holding the folio lock is insufficient. >> >> When moving to an anon_vma with a different root anon_vma, we'll have to >> hold both, the folio lock and the anon_vma lock in write mode. >> Consequently, whenever we succeeded in folio_lock_anon_vma_read() to >> read-lock the anon_vma, we have to re-check if the mapping was changed >> in the meantime. If that was the case, we have to retry. >> >> Note that folio_move_anon_rmap() must only be called if the anon page is >> exclusive to a process, and must not be called on KSM folios. >> >> This is a preparation for UFFDIO_MOVE, which will hold the folio lock, >> the anon_vma lock in write mode, and the mmap_lock in read mode. >> >> Signed-off-by: Andrea Arcangeli <aarcange@redhat.com> >> Signed-off-by: Suren Baghdasaryan <surenb@google.com> >> --- >> mm/rmap.c | 24 ++++++++++++++++++++++++ >> 1 file changed, 24 insertions(+) >> >> diff --git a/mm/rmap.c b/mm/rmap.c >> index c1f11c9dbe61..f9ddc50269d2 100644 >> --- a/mm/rmap.c >> +++ b/mm/rmap.c >> @@ -542,7 +542,9 @@ struct anon_vma *folio_lock_anon_vma_read(struct folio *folio, >> struct anon_vma *root_anon_vma; >> unsigned long anon_mapping; >> >> +retry: >> rcu_read_lock(); >> +retry_under_rcu: >> anon_mapping = (unsigned long)READ_ONCE(folio->mapping); >> if ((anon_mapping & PAGE_MAPPING_FLAGS) != PAGE_MAPPING_ANON) >> goto out; >> @@ -552,6 +554,16 @@ struct anon_vma *folio_lock_anon_vma_read(struct folio *folio, >> anon_vma = (struct anon_vma *) (anon_mapping - PAGE_MAPPING_ANON); >> root_anon_vma = READ_ONCE(anon_vma->root); >> if (down_read_trylock(&root_anon_vma->rwsem)) { >> + /* >> + * folio_move_anon_rmap() might have changed the anon_vma as we >> + * might not hold the folio lock here. >> + */ >> + if (unlikely((unsigned long)READ_ONCE(folio->mapping) != >> + anon_mapping)) { >> + up_read(&root_anon_vma->rwsem); >> + goto retry_under_rcu; > > Is adding this specific label worthwhile? How about rcu unlock and goto > retry (then it'll also be clear that we won't hold rcu read lock for > unpredictable time)? +1, sounds good to me
On Fri, Oct 13, 2023 at 1:04 AM David Hildenbrand <david@redhat.com> wrote: > > On 13.10.23 00:01, Peter Xu wrote: > > On Sun, Oct 08, 2023 at 11:42:26PM -0700, Suren Baghdasaryan wrote: > >> From: Andrea Arcangeli <aarcange@redhat.com> > >> > >> For now, folio_move_anon_rmap() was only used to move a folio to a > >> different anon_vma after fork(), whereby the root anon_vma stayed > >> unchanged. For that, it was sufficient to hold the folio lock when > >> calling folio_move_anon_rmap(). > >> > >> However, we want to make use of folio_move_anon_rmap() to move folios > >> between VMAs that have a different root anon_vma. As folio_referenced() > >> performs an RMAP walk without holding the folio lock but only holding the > >> anon_vma in read mode, holding the folio lock is insufficient. > >> > >> When moving to an anon_vma with a different root anon_vma, we'll have to > >> hold both, the folio lock and the anon_vma lock in write mode. > >> Consequently, whenever we succeeded in folio_lock_anon_vma_read() to > >> read-lock the anon_vma, we have to re-check if the mapping was changed > >> in the meantime. If that was the case, we have to retry. > >> > >> Note that folio_move_anon_rmap() must only be called if the anon page is > >> exclusive to a process, and must not be called on KSM folios. > >> > >> This is a preparation for UFFDIO_MOVE, which will hold the folio lock, > >> the anon_vma lock in write mode, and the mmap_lock in read mode. > >> > >> Signed-off-by: Andrea Arcangeli <aarcange@redhat.com> > >> Signed-off-by: Suren Baghdasaryan <surenb@google.com> > >> --- > >> mm/rmap.c | 24 ++++++++++++++++++++++++ > >> 1 file changed, 24 insertions(+) > >> > >> diff --git a/mm/rmap.c b/mm/rmap.c > >> index c1f11c9dbe61..f9ddc50269d2 100644 > >> --- a/mm/rmap.c > >> +++ b/mm/rmap.c > >> @@ -542,7 +542,9 @@ struct anon_vma *folio_lock_anon_vma_read(struct folio *folio, > >> struct anon_vma *root_anon_vma; > >> unsigned long anon_mapping; > >> > >> +retry: > >> rcu_read_lock(); > >> +retry_under_rcu: > >> anon_mapping = (unsigned long)READ_ONCE(folio->mapping); > >> if ((anon_mapping & PAGE_MAPPING_FLAGS) != PAGE_MAPPING_ANON) > >> goto out; > >> @@ -552,6 +554,16 @@ struct anon_vma *folio_lock_anon_vma_read(struct folio *folio, > >> anon_vma = (struct anon_vma *) (anon_mapping - PAGE_MAPPING_ANON); > >> root_anon_vma = READ_ONCE(anon_vma->root); > >> if (down_read_trylock(&root_anon_vma->rwsem)) { > >> + /* > >> + * folio_move_anon_rmap() might have changed the anon_vma as we > >> + * might not hold the folio lock here. > >> + */ > >> + if (unlikely((unsigned long)READ_ONCE(folio->mapping) != > >> + anon_mapping)) { > >> + up_read(&root_anon_vma->rwsem); > >> + goto retry_under_rcu; > > > > Is adding this specific label worthwhile? How about rcu unlock and goto > > retry (then it'll also be clear that we won't hold rcu read lock for > > unpredictable time)? > > +1, sounds good to me Sorry for the delay, I was travelling for a week. I was hesitant about RCU unlocking and then immediately re-locking but your point about holding it for unpredictable time makes sense. Will change. Thanks! > > -- > Cheers, > > David / dhildenb >
diff --git a/mm/rmap.c b/mm/rmap.c index c1f11c9dbe61..f9ddc50269d2 100644 --- a/mm/rmap.c +++ b/mm/rmap.c @@ -542,7 +542,9 @@ struct anon_vma *folio_lock_anon_vma_read(struct folio *folio, struct anon_vma *root_anon_vma; unsigned long anon_mapping; +retry: rcu_read_lock(); +retry_under_rcu: anon_mapping = (unsigned long)READ_ONCE(folio->mapping); if ((anon_mapping & PAGE_MAPPING_FLAGS) != PAGE_MAPPING_ANON) goto out; @@ -552,6 +554,16 @@ struct anon_vma *folio_lock_anon_vma_read(struct folio *folio, anon_vma = (struct anon_vma *) (anon_mapping - PAGE_MAPPING_ANON); root_anon_vma = READ_ONCE(anon_vma->root); if (down_read_trylock(&root_anon_vma->rwsem)) { + /* + * folio_move_anon_rmap() might have changed the anon_vma as we + * might not hold the folio lock here. + */ + if (unlikely((unsigned long)READ_ONCE(folio->mapping) != + anon_mapping)) { + up_read(&root_anon_vma->rwsem); + goto retry_under_rcu; + } + /* * If the folio is still mapped, then this anon_vma is still * its anon_vma, and holding the mutex ensures that it will @@ -586,6 +598,18 @@ struct anon_vma *folio_lock_anon_vma_read(struct folio *folio, rcu_read_unlock(); anon_vma_lock_read(anon_vma); + /* + * folio_move_anon_rmap() might have changed the anon_vma as we might + * not hold the folio lock here. + */ + if (unlikely((unsigned long)READ_ONCE(folio->mapping) != + anon_mapping)) { + anon_vma_unlock_read(anon_vma); + put_anon_vma(anon_vma); + anon_vma = NULL; + goto retry; + } + if (atomic_dec_and_test(&anon_vma->refcount)) { /* * Oops, we held the last refcount, release the lock