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

[Naresh Kamboju]

On Thu, 28 May 2020 at 02:58, Hugh Dickins <hughd@xxxxxxxxxx> wrote:
>
> 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).

My two cents,
Do you have a test case / test steps to validate this patch ?

- Naresh