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Date:   Wed, 27 May 2020 14:28:10 -0700 (PDT)
From:   Hugh Dickins <hughd@google.com>
To:     Johannes Weiner <hannes@cmpxchg.org>
cc:     Hugh Dickins <hughd@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Song Liu <songliubraving@fb.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH] mm,thp: stop leaking unreleased file pages
In-Reply-To: <20200526160140.GC850116@cmpxchg.org>
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References: <alpine.LSU.2.11.2005231837500.1766@eggly.anvils> <20200526160140.GC850116@cmpxchg.org>
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On Tue, 26 May 2020, Johannes Weiner wrote:
> On Sat, May 23, 2020 at 06:50:15PM -0700, Hugh Dickins wrote:
> > When collapse_file() calls try_to_release_page(), it has already
> > isolated the page: so if releasing buffers happens to fail (as it
> > sometimes does), remember to putback_lru_page(): otherwise that page is
> > left unreclaimable and unfreeable, and the file extent uncollapsible.
> 
> Oof, I could imagine that was painful to debug (unless you already
> suspected file THP due to a targeted test or similar). Kudos.

Thanks, but I have to refuse both your admiration and sympathy:
mercifully, it was just something I noticed by source inspection
when working there.

I did then put in a debug count to see if it ever got hit in practice,
and checked after big multi-testing runs: it was sometimes hit, but
certainly not often, and I don't know what it was racing with when
it happened - would depend on filesystem anyway (ext4 in our case).

Saying "source inspection" reminds me: there is another funny in there,
but I don't think it matters very much in practice, and might need
rather a lot of testing to justify any particular patch: where
page_cache_sync_readahead() asks for PAGE_SIZE pages!

"end - index" seems a more reasonable number to me: but then we
might find that reading ahead into the next huge extent had actually
been a useful optimization (and those readahead functions impose
their own caps, so PAGE_SIZE shouldn't work out too outrageously).

Hugh

> 
> > Signed-off-by: Hugh Dickins <hughd@google.com>
> > Fixes: 99cb0dbd47a1 ("mm,thp: add read-only THP support for (non-shmem) FS")
> > Cc: stable@vger.kernel.org # v5.4+
> 
> Acked-by: Johannes Weiner <hannes@cmpxchg.org>