Re: [PATCH] kswapd shall not sleep during page shortage

Andrew Morton
2004-11-10T22:08:40Z

Marcelo Tosatti <marcelo.tosatti@cyclades.com> wrote:
>
> On Tue, Nov 09, 2004 at 04:28:01PM -0800, Andrew Morton wrote:
> > Marcelo Tosatti <marcelo.tosatti@cyclades.com> wrote:
> > >
> > > Back to arguing in favour of my patch - it seemed to me that kswapd could
> > > go to sleep leaving allocators which can't reclaim pages themselves in a
> > > bad situation.
> >
> > Yes, but those processes would be sleeping in blk_congestion_wait() during,
> > say, a GFP_NOIO/GFP_NOFS allocation attempt. And in that case, they may be
> > holding locks whcih prevent kswapd from being able to do any work either.
> >
> > > It would have to be waken up by another instance of alloc_pages to then
> > > execute and start doing its job, while if it was executing already (madly
> > > scanning as you say), the chance it would find freeable pages quite
> > > earlier.
> > >
> > > Note that not only disk IO can cause pages to become freeable. A user
> > > can give up its reference on pagecache page for example (leaving
> > > the page on LRU to be found and freed by kswapd).
> >
> > yup. Or munlock(), or direct-io completion.
>
> Andrew,
>
> Shouldnt the kernel ideally clear zone->all_unreclaimable in those
> situations? (munlock, direct-io completion, last reference on pagecache
> page, etc).

The design intent here is that a zone shouldn't enter the all-unreclaimable
state until we've absolutely scanned the crap out of it. So we assume that
once a zone is all-unreclaimable then it will stay that way for a
relatively long time. We do little, short scans just to poll the status of
the zone. If one of those short scans ends up freeing a page then the zone
is removed from the all_unreclaimable state.

So if someone does one of the above things then we hope that a subsequent
short-scan will free a page and will wake the zone up. This has the obvious
drawback that it might take us a number of scanning passes before we
discover a reclaimable page. 1<<DEF_PRIORITY passes, worst-case.

For munlock we'd need to actually examine the zone of each affected page,
which is a bunch of new code - a full pte walk. We don't want munlocks of
ZONE_HIGHMEM to trigger these huge scans of a lower zone.

We could possibly put special-case code in the direct-io completion
handler, but it's all a bit weird.