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From: Yosry Ahmed <yosryahmed@google.com>
Date: Thu, 25 Jan 2024 00:42:27 -0800
Message-ID: <CAJD7tkbrQw7FWx-EDKKCtH_E03xEd5Y+8BqRjE8d29JSOCGybg@mail.gmail.com>
Subject: Re: [PATCH 2/2] mm: zswap: remove unnecessary tree cleanups in zswap_swapoff()
To: Chengming Zhou <zhouchengming@bytedance.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>, Andrew Morton <akpm@linux-foundation.org>, 
	Nhat Pham <nphamcs@gmail.com>, Chris Li <chrisl@kernel.org>, Huang Ying <ying.huang@intel.com>, 
	linux-mm@kvack.org, linux-kernel@vger.kernel.org
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On Thu, Jan 25, 2024 at 12:30=E2=80=AFAM 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 th=
oughts.
> >>
> >> 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 h=
as
> >> 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 lo=
ck held,
> >> then release lru lock, to try to lock corresponding folio in swap cach=
e,
> >> 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 happe=
ned,
> >> 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 h=
andling
> >> of lru entry and the tree lifetime. The whole zswap_load() function ha=
s the
> >> stable reference of zswap tree, but it's not for shrink_memcg_cb() bot=
tom half
> >> after __swap_writepage() since we unlock the folio after that. So we c=
an'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 s=
hould
> >> also zswap_invalidate_entry() early in zswap_load() and only support t=
he
> >> 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 refere=
nce
> of it. So we can don't isolate at the entry, and only zswap_lru_del() whe=
n
> we are going to writeback actually.

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