Yosry Ahmed <yosryahmed@xxxxxxxxxx> writes: > On Tue, Jan 23, 2024 at 1:01 AM Huang, Ying <ying.huang@xxxxxxxxx> wrote: >> >> Yosry Ahmed <yosryahmed@xxxxxxxxxx> writes: >> >> > In swap_range_free(), we update inuse_pages then do some cleanups (arch >> > invalidation, zswap invalidation, swap cache cleanups, etc). During >> > swapoff, try_to_unuse() uses inuse_pages to make sure all swap entries >> > are freed. Make sure we only update inuse_pages after we are done with >> > the cleanups. >> > >> > In practice, this shouldn't matter, because swap_range_free() is called >> > with the swap info lock held, and the swapoff code will spin for that >> > lock after try_to_unuse() anyway. >> > >> > The goal is to make it obvious and more future proof that once >> > try_to_unuse() returns, all cleanups are done. >> >> Defines "all cleanups". Apparently, some other operations are still >> to be done after try_to_unuse() in swap_off(). > > I am referring to the cleanups in swap_range_free() that I mentioned above. > > How about s/all the cleanups/all the cleanups in swap_range_free()? Sounds good for me. >> >> > This also facilitates a >> > following zswap cleanup patch which uses this fact to simplify >> > zswap_swapoff(). >> > >> > Signed-off-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx> >> > --- >> > mm/swapfile.c | 4 ++-- >> > 1 file changed, 2 insertions(+), 2 deletions(-) >> > >> > diff --git a/mm/swapfile.c b/mm/swapfile.c >> > index 556ff7347d5f0..2fedb148b9404 100644 >> > --- a/mm/swapfile.c >> > +++ b/mm/swapfile.c >> > @@ -737,8 +737,6 @@ static void swap_range_free(struct swap_info_struct *si, unsigned long offset, >> > if (was_full && (si->flags & SWP_WRITEOK)) >> > add_to_avail_list(si); >> > } >> > - atomic_long_add(nr_entries, &nr_swap_pages); >> > - WRITE_ONCE(si->inuse_pages, si->inuse_pages - nr_entries); >> > if (si->flags & SWP_BLKDEV) >> > swap_slot_free_notify = >> > si->bdev->bd_disk->fops->swap_slot_free_notify; >> > @@ -752,6 +750,8 @@ static void swap_range_free(struct swap_info_struct *si, unsigned long offset, >> > offset++; >> > } >> > clear_shadow_from_swap_cache(si->type, begin, end); >> > + atomic_long_add(nr_entries, &nr_swap_pages); >> > + WRITE_ONCE(si->inuse_pages, si->inuse_pages - nr_entries); >> >> This isn't enough. You need to use smp_wmb() here and smp_rmb() in >> somewhere reading si->inuse_pages. > > Hmm, good point. Although as I mentioned in the commit message, this > shouldn't matter today as swap_range_free() executes with the lock > held, and we spin on the lock after try_to_unuse() returns. Yes. IIUC, this patch isn't needed too because we have spinlock already. > It may still be more future-proof to add the memory barriers. Yes. Without memory barriers, moving code doesn't guarantee memory order. > In swap_range_free, we want to make sure that the write to > si->inuse_pages in swap_range_free() happens *after* the cleanups > (specifically zswap_invalidate() in this case). > In swap_off, we want to make sure that the cleanups following > try_to_unuse() (e.g. zswap_swapoff) happen *after* reading > si->inuse_pages == 0 in try_to_unuse(). > > So I think we want smp_wmb() in swap_range_free() and smp_mb() in > try_to_unuse(). Does the below look correct to you? > > diff --git a/mm/swapfile.c b/mm/swapfile.c > index 2fedb148b9404..a2fa2f65a8ddd 100644 > --- a/mm/swapfile.c > +++ b/mm/swapfile.c > @@ -750,6 +750,12 @@ static void swap_range_free(struct > swap_info_struct *si, unsigned long offset, > offset++; > } > clear_shadow_from_swap_cache(si->type, begin, end); > + > + /* > + * Make sure that try_to_unuse() observes si->inuse_pages reaching 0 > + * only after the above cleanups are done. > + */ > + smp_wmb(); > atomic_long_add(nr_entries, &nr_swap_pages); > WRITE_ONCE(si->inuse_pages, si->inuse_pages - nr_entries); > } > @@ -2130,6 +2136,11 @@ static int try_to_unuse(unsigned int type) > return -EINTR; > } > > + /* > + * Make sure that further cleanups after try_to_unuse() returns happen > + * after swap_range_free() reduces si->inuse_pages to 0. > + */ > + smp_mb(); > return 0; > } We need to take care of "si->inuse_pages" checking at the beginning of try_to_unuse() too. Otherwise, it looks good to me. > Alternatively, we may just hold the spinlock in try_to_unuse() when we > check si->inuse_pages at the end. This will also ensure that any calls > to swap_range_free() have completed. Let me know what you prefer. Personally, I prefer memory barriers here. -- Best Regards, Huang, Ying
