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From: Barry Song <21cnbao@gmail.com>
Date: Sat, 23 Mar 2024 10:41:32 +1300
Message-ID: <CAGsJ_4yc-XB3+FkcZTy1aZ0n6ZKEkfWVYk_TjqqrdcROa5VYtA@mail.gmail.com>
Subject: Re: [RFC PATCH] mm: add folio in swapcache if swapin from zswap
To: Yosry Ahmed <yosryahmed@google.com>
Cc: chengming.zhou@linux.dev, hannes@cmpxchg.org, nphamcs@gmail.com, 
	akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, 
	Zhongkun He <hezhongkun.hzk@bytedance.com>
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On Sat, Mar 23, 2024 at 8:38=E2=80=AFAM Yosry Ahmed <yosryahmed@google.com>=
 wrote:
>
> On Fri, Mar 22, 2024 at 9:40=E2=80=AFAM <chengming.zhou@linux.dev> wrote:
> >
> > From: Chengming Zhou <chengming.zhou@linux.dev>
> >
> > There is a report of data corruption caused by double swapin, which is
> > only possible in the skip swapcache path on SWP_SYNCHRONOUS_IO backends=
.
> >
> > The root cause is that zswap is not like other "normal" swap backends,
> > it won't keep the copy of data after the first time of swapin. So if

I don't quite understand this, so once we load a page from zswap, zswap
will free it even though do_swap_page might not set it to PTE?

shouldn't zswap free the memory after notify_free just like zram?

> > the folio in the first time of swapin can't be installed in the pagetab=
le
> > successfully and we just free it directly. Then in the second time of
> > swapin, we can't find anything in zswap and read wrong data from swapfi=
le,
> > so this data corruption problem happened.
> >
> > We can fix it by always adding the folio into swapcache if we know the
> > pinned swap entry can be found in zswap, so it won't get freed even tho=
ugh
> > it can't be installed successfully in the first time of swapin.
>
> A concurrent faulting thread could have already checked the swapcache
> before we add the folio to it, right? In this case, that thread will
> go ahead and call swap_read_folio() anyway.
>
> Also, I suspect the zswap lookup might hurt performance. Would it be
> better to add the folio back to zswap upon failure? This should be
> detectable by checking if the folio is dirty as I mentioned in the bug
> report thread.

I don't like the idea either as sync-io is the fast path for zram etc.
or, can we use
the way of zram to free compressed data?

>