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Date: Thu, 21 Mar 2013 17:32:27 +0100
From: Michal Hocko <mhocko@...e.cz>
To: Mel Gorman <mgorman@...e.de>
Cc: Linux-MM <linux-mm@...ck.org>, Jiri Slaby <jslaby@...e.cz>,
Valdis Kletnieks <Valdis.Kletnieks@...edu>,
Rik van Riel <riel@...hat.com>,
Zlatko Calusic <zcalusic@...sync.net>,
Johannes Weiner <hannes@...xchg.org>,
dormando <dormando@...ia.net>,
Satoru Moriya <satoru.moriya@....com>,
LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 07/10 -v2r1] mm: vmscan: Block kswapd if it is
encountering pages under writeback
Here is what you have in your mm-vmscan-limit-reclaim-v2r1 branch:
> commit 0dae7d4be56e6a7fe3f128284679f5efc0cc2383
> Author: Mel Gorman <mgorman@...e.de>
> Date: Tue Mar 12 10:33:31 2013 +0000
>
> mm: vmscan: Block kswapd if it is encountering pages under writeback
>
> Historically, kswapd used to congestion_wait() at higher priorities if it
> was not making forward progress. This made no sense as the failure to make
> progress could be completely independent of IO. It was later replaced by
> wait_iff_congested() and removed entirely by commit 258401a6 (mm: don't
> wait on congested zones in balance_pgdat()) as it was duplicating logic
> in shrink_inactive_list().
>
> This is problematic. If kswapd encounters many pages under writeback and
> it continues to scan until it reaches the high watermark then it will
> quickly skip over the pages under writeback and reclaim clean young
> pages or push applications out to swap.
>
> The use of wait_iff_congested() is not suited to kswapd as it will only
> stall if the underlying BDI is really congested or a direct reclaimer was
> unable to write to the underlying BDI. kswapd bypasses the BDI congestion
> as it sets PF_SWAPWRITE but even if this was taken into account then it
> would cause direct reclaimers to stall on writeback which is not desirable.
>
> This patch sets a ZONE_WRITEBACK flag if direct reclaim or kswapd is
> encountering too many pages under writeback. If this flag is set and
> kswapd encounters a PageReclaim page under writeback then it'll assume
> that the LRU lists are being recycled too quickly before IO can complete
> and block waiting for some IO to complete.
>
> Signed-off-by: Mel Gorman <mgorman@...e.de>
Looks reasonable to me.
Reviewed-by: Michal Hocko <mhocko@...e.cz>
>
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index afedd1d..dd0d266 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -499,6 +499,9 @@ typedef enum {
> * many dirty file pages at the tail
> * of the LRU.
> */
> + ZONE_WRITEBACK, /* reclaim scanning has recently found
> + * many pages under writeback
> + */
> } zone_flags_t;
>
> static inline void zone_set_flag(struct zone *zone, zone_flags_t flag)
> @@ -526,6 +529,11 @@ static inline int zone_is_reclaim_dirty(const struct zone *zone)
> return test_bit(ZONE_TAIL_LRU_DIRTY, &zone->flags);
> }
>
> +static inline int zone_is_reclaim_writeback(const struct zone *zone)
> +{
> + return test_bit(ZONE_WRITEBACK, &zone->flags);
> +}
> +
> static inline int zone_is_reclaim_locked(const struct zone *zone)
> {
> return test_bit(ZONE_RECLAIM_LOCKED, &zone->flags);
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index a8b94fa..e87de90 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -723,25 +723,51 @@ static unsigned long shrink_page_list(struct list_head *page_list,
> may_enter_fs = (sc->gfp_mask & __GFP_FS) ||
> (PageSwapCache(page) && (sc->gfp_mask & __GFP_IO));
>
> + /*
> + * If a page at the tail of the LRU is under writeback, there
> + * are three cases to consider.
> + *
> + * 1) If reclaim is encountering an excessive number of pages
> + * under writeback and this page is both under writeback and
> + * PageReclaim then it indicates that pages are being queued
> + * for IO but are being recycled through the LRU before the
> + * IO can complete. In this case, wait on the IO to complete
> + * and then clear the ZONE_WRITEBACK flag to recheck if the
> + * condition exists.
> + *
> + * 2) Global reclaim encounters a page, memcg encounters a
> + * page that is not marked for immediate reclaim or
> + * the caller does not have __GFP_IO. In this case mark
> + * the page for immediate reclaim and continue scanning.
> + *
> + * __GFP_IO is checked because a loop driver thread might
> + * enter reclaim, and deadlock if it waits on a page for
> + * which it is needed to do the write (loop masks off
> + * __GFP_IO|__GFP_FS for this reason); but more thought
> + * would probably show more reasons.
> + *
> + * Don't require __GFP_FS, since we're not going into the
> + * FS, just waiting on its writeback completion. Worryingly,
> + * ext4 gfs2 and xfs allocate pages with
> + * grab_cache_page_write_begin(,,AOP_FLAG_NOFS), so testing
> + * may_enter_fs here is liable to OOM on them.
> + *
> + * 3) memcg encounters a page that is not already marked
> + * PageReclaim. memcg does not have any dirty pages
> + * throttling so we could easily OOM just because too many
> + * pages are in writeback and there is nothing else to
> + * reclaim. Wait for the writeback to complete.
> + */
> if (PageWriteback(page)) {
> - /*
> - * memcg doesn't have any dirty pages throttling so we
> - * could easily OOM just because too many pages are in
> - * writeback and there is nothing else to reclaim.
> - *
> - * Check __GFP_IO, certainly because a loop driver
> - * thread might enter reclaim, and deadlock if it waits
> - * on a page for which it is needed to do the write
> - * (loop masks off __GFP_IO|__GFP_FS for this reason);
> - * but more thought would probably show more reasons.
> - *
> - * Don't require __GFP_FS, since we're not going into
> - * the FS, just waiting on its writeback completion.
> - * Worryingly, ext4 gfs2 and xfs allocate pages with
> - * grab_cache_page_write_begin(,,AOP_FLAG_NOFS), so
> - * testing may_enter_fs here is liable to OOM on them.
> - */
> - if (global_reclaim(sc) ||
> + /* Case 1 above */
> + if (current_is_kswapd() &&
> + PageReclaim(page) &&
> + zone_is_reclaim_writeback(zone)) {
> + wait_on_page_writeback(page);
> + zone_clear_flag(zone, ZONE_WRITEBACK);
> +
> + /* Case 2 above */
> + } else if (global_reclaim(sc) ||
> !PageReclaim(page) || !(sc->gfp_mask & __GFP_IO)) {
> /*
> * This is slightly racy - end_page_writeback()
> @@ -756,9 +782,13 @@ static unsigned long shrink_page_list(struct list_head *page_list,
> */
> SetPageReclaim(page);
> nr_writeback++;
> +
> goto keep_locked;
> +
> + /* Case 3 above */
> + } else {
> + wait_on_page_writeback(page);
> }
> - wait_on_page_writeback(page);
> }
>
> if (!force_reclaim)
> @@ -1373,8 +1403,10 @@ shrink_inactive_list(unsigned long nr_to_scan, struct lruvec *lruvec,
> * isolated page is PageWriteback
> */
> if (nr_writeback && nr_writeback >=
> - (nr_taken >> (DEF_PRIORITY - sc->priority)))
> + (nr_taken >> (DEF_PRIORITY - sc->priority))) {
> wait_iff_congested(zone, BLK_RW_ASYNC, HZ/10);
> + zone_set_flag(zone, ZONE_WRITEBACK);
> + }
>
> /*
> * Similarly, if many dirty pages are encountered that are not
> @@ -2639,8 +2671,8 @@ static bool prepare_kswapd_sleep(pg_data_t *pgdat, int order, long remaining,
> * kswapd shrinks the zone by the number of pages required to reach
> * the high watermark.
> *
> - * Returns true if kswapd scanned at least the requested number of
> - * pages to reclaim.
> + * Returns true if kswapd scanned at least the requested number of pages to
> + * reclaim or if the lack of process was due to pages under writeback.
> */
> static bool kswapd_shrink_zone(struct zone *zone,
> struct scan_control *sc,
> @@ -2663,6 +2695,8 @@ static bool kswapd_shrink_zone(struct zone *zone,
> if (nr_slab == 0 && !zone_reclaimable(zone))
> zone->all_unreclaimable = 1;
>
> + zone_clear_flag(zone, ZONE_WRITEBACK);
> +
> return sc->nr_scanned >= sc->nr_to_reclaim;
> }
--
Michal Hocko
SUSE Labs
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