Re: [PATCH 2/2] mm: zswap: remove unnecessary tree cleanups in zswap_swapoff()

[Yosry Ahmed <yosryahmed@google.com>]

On Thu, Jan 25, 2024 at 12:30 AM Chengming Zhou
<zhouchengming@bytedance.com> wrote:
>
> On 2024/1/25 15:53, Yosry Ahmed wrote:
> >> Hello,
> >>
> >> I also thought about this problem for some time, maybe something like below
> >> can be changed to fix it? It's likely I missed something, just some thoughts.
> >>
> >> IMHO, the problem is caused by the different way in which we use zswap entry
> >> in the writeback, that should be much like zswap_load().
> >>
> >> The zswap_load() comes in with the folio locked in swap cache, so it has
> >> stable zswap tree to search and lock... But in writeback case, we don't,
> >> shrink_memcg_cb() comes in with only a zswap entry with lru list lock held,
> >> then release lru lock to get tree lock, which maybe freed already.
> >>
> >> So we should change here, we read swpentry from entry with lru list lock held,
> >> then release lru lock, to try to lock corresponding folio in swap cache,
> >> if we success, the following things is much the same like zswap_load().
> >> We can get tree lock, to recheck the invalidate race, if no race happened,
> >> we can make sure the entry is still right and get refcount of it, then
> >> release the tree lock.
> >
> > Hmm I think you may be onto something here. Moving the swap cache
> > allocation ahead before referencing the tree should give us the same
> > guarantees as zswap_load() indeed. We can also consolidate the
> > invalidate race checks (right now we have one in shrink_memcg_cb() and
> > another one inside zswap_writeback_entry()).
>
> Right, if we successfully lock folio in the swap cache, we can get the
> tree lock and check the invalidate race, only once.
>
> >
> > We will have to be careful about the error handling path to make sure
> > we delete the folio from the swap cache only after we know the tree
> > won't be referenced anymore. Anyway, I think this can work.
>
> Yes, we can't reference tree if we early return or after unlocking folio,
> since the reference of zswap entry can't protect the tree.
>
> >
> > On a separate note, I think there is a bug in zswap_writeback_entry()
> > when we delete a folio from the swap cache. I think we are missing a
> > folio_unlock() there.
>
> Ah, yes, and folio_put().

Yes. I am preparing a fix.

>
> >
> >>
> >> The main differences between this writeback with zswap_load() is the handling
> >> of lru entry and the tree lifetime. The whole zswap_load() function has the
> >> stable reference of zswap tree, but it's not for shrink_memcg_cb() bottom half
> >> after __swap_writepage() since we unlock the folio after that. So we can't
> >> reference the tree after that.
> >>
> >> This problem is easy to fix, we can zswap_invalidate_entry(tree, entry) early
> >> in tree lock, since thereafter writeback can't fail. BTW, I think we should
> >> also zswap_invalidate_entry() early in zswap_load() and only support the
> >> zswap_exclusive_loads_enabled mode, but that's another topic.
> >
> > zswap_invalidate_entry() actually doesn't seem to be using the tree at all.
> >
> >>
> >> The second difference is the handling of lru entry, which is easy that we
> >> just zswap_lru_del() in tree lock.
> >
> > Why do we need zswap_lru_del() at all? We should have already isolated
> > the entry at that point IIUC.
>
> I was thinking how to handle the "zswap_lru_putback()" if not writeback,
> in which case we can't use the entry actually since we haven't got reference
> of it. So we can don't isolate at the entry, and only zswap_lru_del() when
> we are going to writeback actually.

Why not just call zswap_lru_putback() before we unlock the folio?