On 01.12.20 00:06, Peter Xu wrote: > Faulting around for reads are in most cases helpful for the performance so that > continuous memory accesses may avoid another trip of page fault. However it > may not always work as expected. > > For example, userfaultfd registered regions may not be the best candidate for > pre-faults around the reads. > > For missing mode uffds, fault around does not help because if the page cache > existed, then the page should be there already. If the page cache is not > there, nothing else we can do, either. If the fault-around code is destined to > be helpless for userfault-missing vmas, then ideally we can skip it. > > For wr-protected mode uffds, errornously fault in those pages around could lead > to threads accessing the pages without uffd server's awareness. For example, > when punching holes on uffd-wp registered shmem regions, we'll first try to > unmap all the pages before evicting the page cache but without locking the > page (please refer to shmem_fallocate(), where unmap_mapping_range() is called > before shmem_truncate_range()). When fault-around happens near a hole being > punched, we might errornously fault in the "holes" right before it will be > punched. Then there's a small window before the page cache was finally > dropped, and after the page will be writable again (NOTE: the uffd-wp protect > information is totally lost due to the pre-unmap in shmem_fallocate(), so the > page can be writable within the small window). That's severe data loss. > > Let's grant the userspace full control of the uffd-registered ranges, rather > than trying to do the tricks. > > Cc: Hugh Dickins <hughd@xxxxxxxxxx> > Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx> > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> > Cc: Mike Rapoport <rppt@xxxxxxxxxxxxxxxxxx> > Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx> > Reviewed-by: Mike Rapoport <rppt@xxxxxxxxxxxxxxxxxx> > Signed-off-by: Peter Xu <peterx@xxxxxxxxxx> > --- > > v2: > - use userfaultfd_armed() directly [Mike] > > Note that since no file-backed uffd-wp support is there yet upstream, so the > uffd-wp check is actually not really functioning. However since we have all > the necessary uffd-wp concepts already upstream, maybe it's better to do it > once and for all. > > This patch comes from debugging a data loss issue when working on the uffd-wp > support on shmem/hugetlbfs. I posted this out for early review and comments, > but also because it should already start to benefit missing mode userfaultfd to > avoid trying to fault around on reads. > --- > mm/memory.c | 17 +++++++++++++++++ > 1 file changed, 17 insertions(+) > > diff --git a/mm/memory.c b/mm/memory.c > index eeae590e526a..59b2be22565e 100644 > --- a/mm/memory.c > +++ b/mm/memory.c > @@ -3933,6 +3933,23 @@ static vm_fault_t do_fault_around(struct vm_fault *vmf) > int off; > vm_fault_t ret = 0; > > + /* > + * Be extremely careful with uffd-armed regions. > + * > + * For missing mode uffds, fault around does not help because if the > + * page cache existed, then the page should be there already. If the > + * page cache is not there, nothing else we can do either. > + * > + * For wr-protected mode uffds, errornously fault in those pages around > + * could lead to threads accessing the pages without uffd server's > + * awareness, finally it could cause ghostly data corruption. > + * > + * The idea is that, every single page of uffd regions should be > + * governed by the userspace on which page to fault in. > + */ > + if (unlikely(userfaultfd_armed(vmf->vma))) > + return 0; > + > nr_pages = READ_ONCE(fault_around_bytes) >> PAGE_SHIFT; > mask = ~(nr_pages * PAGE_SIZE - 1) & PAGE_MASK; > > Thanks for the clarifying comment. Acked-by: David Hildenbrand <david@xxxxxxxxxx> -- Thanks, David / dhildenb
