Re: [PATCH] mm,thp: stop leaking unreleased file pages

[Hugh Dickins]

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@xxxxxxxxxx>
> > Fixes: 99cb0dbd47a1 ("mm,thp: add read-only THP support for (non-shmem) FS")
> > Cc: stable@xxxxxxxxxxxxxxx # v5.4+
> 
> Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>