Re: [PATCH 1/5] mm: vmscan: Do not writeback filesystem pages in direct reclaim

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Wed, 13 Jul 2011 15:31:23 +0100
Mel Gorman <mgorman@suse.de> wrote:

> From: Mel Gorman <mel@csn.ul.ie>
> 
> When kswapd is failing to keep zones above the min watermark, a process
> will enter direct reclaim 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 causes two problems. First, it can result in very deep call
> stacks, particularly if the target storage or filesystem are complex.
> Some filesystems ignore write requests from direct reclaim as a result.
> The second is that a single-page flush is inefficient in terms of IO.
> While there is an expectation that the elevator will merge requests,
> this does not always happen. Quoting Christoph Hellwig;
> 
> 	The elevator has a relatively small window it can operate on,
> 	and can never fix up a bad large scale writeback pattern.
> 
> This patch prevents direct reclaim writing back filesystem pages by
> checking if current is kswapd. Anonymous pages are still written to
> swap as there is not the equivalent of a flusher thread for anonymos
> pages. If the dirty pages cannot be written back, they are placed
> back on the LRU lists.
> 
> Signed-off-by: Mel Gorman <mgorman@suse.de>

Hm.


> ---
>  include/linux/mmzone.h |    1 +
>  mm/vmscan.c            |    9 +++++++++
>  mm/vmstat.c            |    1 +
>  3 files changed, 11 insertions(+), 0 deletions(-)
> 
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 9f7c3eb..b70a0c0 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -100,6 +100,7 @@ enum zone_stat_item {
>  	NR_UNSTABLE_NFS,	/* NFS unstable pages */
>  	NR_BOUNCE,
>  	NR_VMSCAN_WRITE,
> +	NR_VMSCAN_WRITE_SKIP,
>  	NR_WRITEBACK_TEMP,	/* Writeback using temporary buffers */
>  	NR_ISOLATED_ANON,	/* Temporary isolated pages from anon lru */
>  	NR_ISOLATED_FILE,	/* Temporary isolated pages from file lru */
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 4f49535..2d3e5b6 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -825,6 +825,15 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>  		if (PageDirty(page)) {
>  			nr_dirty++;
>  
> +			/*
> +			 * Only kswapd can writeback filesystem pages to
> +			 * avoid risk of stack overflow
> +			 */
> +			if (page_is_file_cache(page) && !current_is_kswapd()) {
> +				inc_zone_page_state(page, NR_VMSCAN_WRITE_SKIP);
> +				goto keep_locked;
> +			}
> +


This will cause tons of memcg OOM kill because we have no help of kswapd (now).

Could you make this

	if (scanning_global_lru(sc) && page_is_file_cache(page) && !current_is_kswapd())
...


Then...sorry, please keep file system hook for a while. I'll do memcg dirty_ratio work
by myself if Greg will not post new version until the next month. After that, we can
remove scanning_global_lru(sc), I think.

Thanks,
-Kame

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