Matthew Wilcox <willy@xxxxxxxxxxxxx> writes: > On Fri, Apr 14, 2023 at 11:00:43AM -0700, Suren Baghdasaryan wrote: >> When page fault is handled under VMA lock protection, all swap page >> faults are retried with mmap_lock because folio_lock_or_retry >> implementation has to drop and reacquire mmap_lock if folio could >> not be immediately locked. >> Instead of retrying all swapped page faults, retry only when folio >> locking fails. > > Reviewed-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx> > > Let's just review what can now be handled under the VMA lock instead of > the mmap_lock, in case somebody knows better than me that it's not safe. > > - We can call migration_entry_wait(). This will wait for PG_locked to > become clear (in migration_entry_wait_on_locked()). As previously > discussed offline, I think this is safe to do while holding the VMA > locked. Do we even need to be holding the VMA locked while in migration_entry_wait()? My understanding is we're just waiting for PG_locked to be cleared so we can return with a reasonable chance the migration entry is gone. If for example it has been unmapped or protections downgraded we will simply refault. > - We can call remove_device_exclusive_entry(). That calls > folio_lock_or_retry(), which will fail if it can't get the VMA lock. Looks ok to me. > - We can call pgmap->ops->migrate_to_ram(). Perhaps somebody familiar > with Nouveau and amdkfd could comment on how safe this is? Currently this won't work because drives assume mmap_lock is held during pgmap->ops->migrate_to_ram(). Primarily this is because migrate_vma_setup()/migrate_vma_pages() is used to handle the fault and that asserts mmap_lock is taken in walk_page_range() and also migrate_vma_insert_page(). So I don't think we can call that case without mmap_lock. At a glance it seems it should be relatively easy to move to using lock_vma_under_rcu(). Drivers will need updating as well though because migrate_vma_setup() is called outside of fault handling paths so drivers will currently take mmap_lock rather than vma lock when looking up the vma. See for example nouveau_svmm_bind(). > - I believe we can't call handle_pte_marker() because we exclude UFFD > VMAs earlier. > - We can call swap_readpage() if we allocate a new folio. I haven't > traced through all this code to tell if it's OK. > > So ... I believe this is all OK, but we're definitely now willing to > wait for I/O from the swap device while holding the VMA lock when we > weren't before. And maybe we should make a bigger deal of it in the > changelog. > > And maybe we shouldn't just be failing the folio_lock_or_retry(), > maybe we should be waiting for the folio lock with the VMA locked.
