Re: [PATCH 4/5] mm: vmscan: Immediately reclaim end-of-LRU dirty pages when writeback completes

Mel Gorman
2011-07-13T17:16:01Z

On Wed, Jul 13, 2011 at 06:40:40PM +0200, Johannes Weiner wrote:
> On Wed, Jul 13, 2011 at 03:31:26PM +0100, Mel Gorman wrote:
> > When direct reclaim encounters a dirty page, it gets recycled around
> > the LRU for another cycle. This patch marks the page PageReclaim using
> > deactivate_page() so that the page gets reclaimed almost immediately
> > after the page gets cleaned. This is to avoid reclaiming clean pages
> > that are younger than a dirty page encountered at the end of the LRU
> > that might have been something like a use-once page.
> >
> > Signed-off-by: Mel Gorman <mgorman@suse.de>
> > ---
> > include/linux/mmzone.h | 2 +-
> > mm/vmscan.c | 10 ++++++++--
> > mm/vmstat.c | 2 +-
> > 3 files changed, 10 insertions(+), 4 deletions(-)
> >
> > diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> > index c4508a2..bea7858 100644
> > --- a/include/linux/mmzone.h
> > +++ b/include/linux/mmzone.h
> > @@ -100,7 +100,7 @@ enum zone_stat_item {
> > NR_UNSTABLE_NFS, /* NFS unstable pages */
> > NR_BOUNCE,
> > NR_VMSCAN_WRITE,
> > - NR_VMSCAN_WRITE_SKIP,
> > + NR_VMSCAN_INVALIDATE,
> > NR_VMSCAN_THROTTLED,
> > NR_WRITEBACK_TEMP, /* Writeback using temporary buffers */
> > NR_ISOLATED_ANON, /* Temporary isolated pages from anon lru */
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 9826086..8e00aee 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -834,8 +834,13 @@ static unsigned long shrink_page_list(struct list_head *page_list,
> > */
> > if (page_is_file_cache(page) &&
> > (!current_is_kswapd() || priority >= DEF_PRIORITY - 2)) {
> > - inc_zone_page_state(page, NR_VMSCAN_WRITE_SKIP);
> > - goto keep_locked;
> > + inc_zone_page_state(page, NR_VMSCAN_INVALIDATE);
> > +
> > + /* Immediately reclaim when written back */
> > + unlock_page(page);
> > + deactivate_page(page);
> > +
> > + goto keep_dirty;
> > }
> >
> > if (references == PAGEREF_RECLAIM_CLEAN)
> > @@ -956,6 +961,7 @@ keep:
> > reset_reclaim_mode(sc);
> > keep_lumpy:
> > list_add(&page->lru, &ret_pages);
> > +keep_dirty:
> > VM_BUG_ON(PageLRU(page) || PageUnevictable(page));
> > }
>
> I really like the idea behind this patch, but I think all those pages
> are lost as PageLRU is cleared on isolation and lru_deactivate_fn
> bails on them in turn.
>
> If I'm not mistaken, the reference from the isolation is also leaked.

I think you're right. This patch was rushed and not thought through
properly. The surprise it appeared to work at all. Will rework
it. Thanks.

--
Mel Gorman
SUSE Labs