Re: [PATCH 09/10] vmscan: Do not writeback filesystem pages indirect reclaim

From: Wu Fengguang
Date: Mon Sep 13 2010 - 09:32:07 EST

Mel,

Sorry for being late, I'm doing pretty much prework these days ;)

On Mon, Sep 06, 2010 at 06:47:32PM +0800, Mel Gorman wrote:
> When memory is under enough pressure, a process may enter direct
> reclaim to free pages in the same manner kswapd does. If a dirty page is
> encountered during the scan, this page is written to backing storage using
> mapping->writepage. This can result in very deep call stacks, particularly
> if the target storage or filesystem are complex. It has already been observed
> on XFS that the stack overflows but the problem is not XFS-specific.
>
> This patch prevents direct reclaim writing back filesystem pages by checking
> if current is kswapd or the page is anonymous before writing back. If the
> dirty pages cannot be written back, they are placed back on the LRU lists
> for either background writing by the BDI threads or kswapd. If in direct
> lumpy reclaim and dirty pages are encountered, the process will stall for
> the background flusher before trying to reclaim the pages again.
>
> As the call-chain for writing anonymous pages is not expected to be deep
> and they are not cleaned by flusher threads, anonymous pages are still
> written back in direct reclaim.
>
> Signed-off-by: Mel Gorman <mel@xxxxxxxxx>
> Acked-by: Rik van Riel <riel@xxxxxxxxxx>
> Reviewed-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> ---
> mm/vmscan.c | 49 ++++++++++++++++++++++++++++++++++++++++++++++---
> 1 files changed, 46 insertions(+), 3 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index ff52b46..408c101 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -145,6 +145,9 @@ static DECLARE_RWSEM(shrinker_rwsem);
> #define scanning_global_lru(sc) (1)
> #endif
>
> +/* Direct lumpy reclaim waits up to five seconds for background cleaning */
> +#define MAX_SWAP_CLEAN_WAIT 50
> +
> static struct zone_reclaim_stat *get_reclaim_stat(struct zone *zone,
> struct scan_control *sc)
> {
> @@ -682,11 +685,13 @@ static noinline_for_stack void free_page_list(struct list_head *free_pages)
> * shrink_page_list() returns the number of reclaimed pages
> */
> static unsigned long shrink_page_list(struct list_head *page_list,
> - struct scan_control *sc)
> + struct scan_control *sc,
> + unsigned long *nr_still_dirty)
> {
> LIST_HEAD(ret_pages);
> LIST_HEAD(free_pages);
> int pgactivate = 0;
> + unsigned long nr_dirty = 0;
> unsigned long nr_reclaimed = 0;
>
> cond_resched();
> @@ -785,6 +790,15 @@ static unsigned long shrink_page_list(struct list_head *page_list,
> }
>
> if (PageDirty(page)) {
> + /*
> + * Only kswapd can writeback filesystem pages to
> + * avoid risk of stack overflow
> + */
> + if (page_is_file_cache(page) && !current_is_kswapd()) {
> + nr_dirty++;
> + goto keep_locked;
> + }
> +
> if (references == PAGEREF_RECLAIM_CLEAN)
> goto keep_locked;
> if (!may_enter_fs)
> @@ -908,6 +922,8 @@ keep_lumpy:
> free_page_list(&free_pages);
>
> list_splice(&ret_pages, page_list);
> +
> + *nr_still_dirty = nr_dirty;
> count_vm_events(PGACTIVATE, pgactivate);
> return nr_reclaimed;
> }
> @@ -1312,6 +1328,10 @@ static inline bool should_reclaim_stall(unsigned long nr_taken,
> if (sc->lumpy_reclaim_mode == LUMPY_MODE_NONE)
> return false;
>
> + /* If we cannot writeback, there is no point stalling */
> + if (!sc->may_writepage)
> + return false;
> +
> /* If we have relaimed everything on the isolated list, no stall */
> if (nr_freed == nr_taken)
> return false;
> @@ -1339,11 +1359,13 @@ shrink_inactive_list(unsigned long nr_to_scan, struct zone *zone,
> struct scan_control *sc, int priority, int file)
> {
> LIST_HEAD(page_list);
> + LIST_HEAD(putback_list);
> unsigned long nr_scanned;
> unsigned long nr_reclaimed = 0;
> unsigned long nr_taken;
> unsigned long nr_anon;
> unsigned long nr_file;
> + unsigned long nr_dirty;
>
> while (unlikely(too_many_isolated(zone, file, sc))) {
> congestion_wait(BLK_RW_ASYNC, HZ/10);
> @@ -1392,14 +1414,35 @@ shrink_inactive_list(unsigned long nr_to_scan, struct zone *zone,
>
> spin_unlock_irq(&zone->lru_lock);
>
> - nr_reclaimed = shrink_page_list(&page_list, sc);
> + nr_reclaimed = shrink_page_list(&page_list, sc, &nr_dirty);
>
> /* Check if we should syncronously wait for writeback */
> if (should_reclaim_stall(nr_taken, nr_reclaimed, priority, sc)) {

It is possible to OOM if the LRU list is small and/or the storage is slow, so
that the flusher cannot clean enough pages before the LRU is fully scanned.

So we may need do waits on dirty/writeback pages on *order-0*
direct reclaims, when priority goes rather low (such as < 3).

> + int dirty_retry = MAX_SWAP_CLEAN_WAIT;
> set_lumpy_reclaim_mode(priority, sc, true);
> - nr_reclaimed += shrink_page_list(&page_list, sc);
> +
> + while (nr_reclaimed < nr_taken && nr_dirty && dirty_retry--) {
> + struct page *page, *tmp;
> +

> + /* Take off the clean pages marked for activation */
> + list_for_each_entry_safe(page, tmp, &page_list, lru) {
> + if (PageDirty(page) || PageWriteback(page))
> + continue;
> +
> + list_del(&page->lru);
> + list_add(&page->lru, &putback_list);
> + }

nitpick: I guess the above loop is optional code to avoid overheads
of shrink_page_list() repeatedly going through some unfreeable pages?
Considering this is the slow code path, I'd prefer to keep the code
simple than to do such optimizations.

> + wakeup_flusher_threads(laptop_mode ? 0 : nr_dirty);

how about
if (!laptop_mode)
wakeup_flusher_threads(nr_dirty);

> + wait_iff_congested(zone, BLK_RW_ASYNC, HZ/10);
> +
> + nr_reclaimed = shrink_page_list(&page_list, sc,
> + &nr_dirty);
> + }
> }
>
> + list_splice(&putback_list, &page_list);
> +
> local_irq_disable();
> if (current_is_kswapd())
> __count_vm_events(KSWAPD_STEAL, nr_reclaimed);
> --
> 1.7.1
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