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Date: Tue, 27 Feb 2024 11:17:52 +0100
From: Greg KH <gregkh@linuxfoundation.org>
To: Yosry Ahmed <yosryahmed@google.com>
Cc: stable@vger.kernel.org, Chengming Zhou <zhouchengming@bytedance.com>,
	Johannes Weiner <hannes@cmpxchg.org>, Nhat Pham <nphamcs@gmail.com>,
	Domenico Cerasuolo <cerasuolodomenico@gmail.com>,
	Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH] mm: zswap: fix missing folio cleanup in writeback race
 path
Message-ID: <2024022730-passenger-moonlike-62f3@gregkh>
References: <2024022611-tropics-deferred-2483@gregkh>
 <20240226221017.1332778-1-yosryahmed@google.com>
 <2024022755-amplify-vocation-854e@gregkh>
 <CAJD7tkYYWPWA-3fGAjMOqVy+8Mcwq++6yw0R6odeOsByugre+Q@mail.gmail.com>
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In-Reply-To: <CAJD7tkYYWPWA-3fGAjMOqVy+8Mcwq++6yw0R6odeOsByugre+Q@mail.gmail.com>

On Tue, Feb 27, 2024 at 12:58:27AM -0800, Yosry Ahmed wrote:
> On Tue, Feb 27, 2024 at 12:55 AM Greg KH <greg@kroah.com> wrote:
> >
> > On Mon, Feb 26, 2024 at 10:10:17PM +0000, Yosry Ahmed wrote:
> > > In zswap_writeback_entry(), after we get a folio from
> > > __read_swap_cache_async(), we grab the tree lock again to check that the
> > > swap entry was not invalidated and recycled.  If it was, we delete the
> > > folio we just added to the swap cache and exit.
> > >
> > > However, __read_swap_cache_async() returns the folio locked when it is
> > > newly allocated, which is always true for this path, and the folio is
> > > ref'd.  Make sure to unlock and put the folio before returning.
> > >
> > > This was discovered by code inspection, probably because this path handles
> > > a race condition that should not happen often, and the bug would not crash
> > > the system, it will only strand the folio indefinitely.
> > >
> > > Link: https://lkml.kernel.org/r/20240125085127.1327013-1-yosryahmed@google.com
> > > Fixes: 04fc7816089c ("mm: fix zswap writeback race condition")
> > > Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
> > > Reviewed-by: Chengming Zhou <zhouchengming@bytedance.com>
> > > Acked-by: Johannes Weiner <hannes@cmpxchg.org>
> > > Reviewed-by: Nhat Pham <nphamcs@gmail.com>
> > > Cc: Domenico Cerasuolo <cerasuolodomenico@gmail.com>
> > > Cc: <stable@vger.kernel.org>
> > > Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> > > (cherry picked from commit e3b63e966cac0bf78aaa1efede1827a252815a1d)
> > > Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
> >
> > What tree is this for?
> 
> This is for 6.6.y, sorry I messed up the subject prefix. Do I need to resend?

Ah, here is the 6.6.y version, missed it, sorry for the noise.  I'll go
queue this and the 6.1.y version up now.

thanks,

greg k-h