Re: [PATCH 2/2] MM: Discard NR_UNSTABLE_NFS, use NR_WRITEBACK instead.

[Michal Hocko]

On Tue 07-04-20 09:44:25, Neil Brown wrote:
> 
> After an NFS page has been written it is considered "unstable" until a
> COMMIT request succeeds.  If the COMMIT fails, the page will be
> re-written.
> 
> These "unstable" pages are currently accounted as "reclaimable", either
> in WB_RECLAIMABLE, or in NR_UNSTABLE_NFS which is included in a
> 'reclaimable' count.  This might have made sense when sending the COMMIT
> required a separate action by the VFS/MM (e.g.  releasepage() used to
> send a COMMIT).  However now that all writes generated by ->writepages()
> will automatically be followed by a COMMIT (since commit 919e3bd9a875
> ("NFS: Ensure we commit after writeback is complete")) it makes more
> sense to treat them as writeback pages.
> 
> So this patch removes NR_UNSTABLE_NFS and accounts unstable pages in
> NR_WRITEBACK and WB_WRITEBACK.
> 
> A particular effect of this change is that when
> wb_check_background_flush() calls wb_over_bg_threshold(), the latter
> will report 'true' a lot less often as the 'unstable' pages are no
> longer considered 'dirty' (as there is nothing that writeback can do
> about them anyway).
> 
> Currently wb_check_background_flush() will trigger writeback to NFS even
> when there are relatively few dirty pages (if there are lots of unstable
> pages), this can result in small writes going to the server (10s of
> Kilobytes rather than a Megabyte) which hurts throughput.
> With this patch, there are fewer writes which are each larger on average.
> 
> Where the NR_UNSTABLE_NFS count was included in statistics
> virtual-files, the entry is retained, but the value is hard-coded as
> zero.  static trace points which record this counter no longer report
> it.

I do not have sufficient insight to nfs so I cannot judge that part but
the core MM changes make sense and I do not see any problems there. It
is PITA to keep the counter in user visible interfaces like meminfo and
vmstat but I believe this really makes sense here as this is a counter
that is usually considered. Maybe its non-existence will not be fatal
for existing scripts but risking that is not worth it now. Maybe we can
drop the fake value in future.
 
> Signed-off-by: NeilBrown <neilb@xxxxxxx>

Acked-by: Michal Hocko <mhocko@xxxxxxxx> # for MM parts

Thanks!
> ---
>  Documentation/filesystems/proc.rst |  4 ++--
>  drivers/base/node.c                |  2 +-
>  fs/fs-writeback.c                  |  1 -
>  fs/nfs/internal.h                  | 10 +++++++---
>  fs/nfs/write.c                     |  4 ++--
>  fs/proc/meminfo.c                  |  3 +--
>  include/linux/mmzone.h             |  1 -
>  include/trace/events/writeback.h   |  5 +----
>  mm/memcontrol.c                    |  1 -
>  mm/page-writeback.c                | 17 ++++-------------
>  mm/page_alloc.c                    |  6 ++----
>  mm/vmstat.c                        | 13 ++++++++++---
>  12 files changed, 30 insertions(+), 37 deletions(-)
> 
> diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
> index 38b606991065..092b7b44d158 100644
> --- a/Documentation/filesystems/proc.rst
> +++ b/Documentation/filesystems/proc.rst
> @@ -1042,8 +1042,8 @@ PageTables
>                amount of memory dedicated to the lowest level of page
>                tables.
>  NFS_Unstable
> -              NFS pages sent to the server, but not yet committed to stable
> -	      storage
> +              Always zero. Previous counted pages which had been written to
> +              the server, but has not been committed to stable storage.
>  Bounce
>                Memory used for block device "bounce buffers"
>  WritebackTmp
> diff --git a/drivers/base/node.c b/drivers/base/node.c
> index 10d7e818e118..15f5ed6a8830 100644
> --- a/drivers/base/node.c
> +++ b/drivers/base/node.c
> @@ -439,7 +439,7 @@ static ssize_t node_read_meminfo(struct device *dev,
>  		       nid, K(i.sharedram),
>  		       nid, sum_zone_node_page_state(nid, NR_KERNEL_STACK_KB),
>  		       nid, K(sum_zone_node_page_state(nid, NR_PAGETABLE)),
> -		       nid, K(node_page_state(pgdat, NR_UNSTABLE_NFS)),
> +		       nid, 0,
>  		       nid, K(sum_zone_node_page_state(nid, NR_BOUNCE)),
>  		       nid, K(node_page_state(pgdat, NR_WRITEBACK_TEMP)),
>  		       nid, K(sreclaimable +
> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> index 76ac9c7d32ec..c5bdf46e3b4b 100644
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -1070,7 +1070,6 @@ static void bdi_split_work_to_wbs(struct backing_dev_info *bdi,
>  static unsigned long get_nr_dirty_pages(void)
>  {
>  	return global_node_page_state(NR_FILE_DIRTY) +
> -		global_node_page_state(NR_UNSTABLE_NFS) +
>  		get_nr_dirty_inodes();
>  }
>  
> diff --git a/fs/nfs/internal.h b/fs/nfs/internal.h
> index f80c47d5ff27..749da02b547a 100644
> --- a/fs/nfs/internal.h
> +++ b/fs/nfs/internal.h
> @@ -652,7 +652,8 @@ void nfs_super_set_maxbytes(struct super_block *sb, __u64 maxfilesize)
>  }
>  
>  /*
> - * Record the page as unstable and mark its inode as dirty.
> + * Record the page as unstable (an extra writeback period) and mark its
> + * inode as dirty.
>   */
>  static inline
>  void nfs_mark_page_unstable(struct page *page, struct nfs_commit_info *cinfo)
> @@ -660,8 +661,11 @@ void nfs_mark_page_unstable(struct page *page, struct nfs_commit_info *cinfo)
>  	if (!cinfo->dreq) {
>  		struct inode *inode = page_file_mapping(page)->host;
>  
> -		inc_node_page_state(page, NR_UNSTABLE_NFS);
> -		inc_wb_stat(&inode_to_bdi(inode)->wb, WB_RECLAIMABLE);
> +		/* This page is really still in write-back - just that the
> +		 * writeback is happening on the server now.
> +		 */
> +		inc_node_page_state(page, NR_WRITEBACK);
> +		inc_wb_stat(&inode_to_bdi(inode)->wb, WB_WRITEBACK);
>  		__mark_inode_dirty(inode, I_DIRTY_DATASYNC);
>  	}
>  }
> diff --git a/fs/nfs/write.c b/fs/nfs/write.c
> index c478b772cc49..2e15a56620b3 100644
> --- a/fs/nfs/write.c
> +++ b/fs/nfs/write.c
> @@ -958,9 +958,9 @@ nfs_mark_request_commit(struct nfs_page *req, struct pnfs_layout_segment *lseg,
>  static void
>  nfs_clear_page_commit(struct page *page)
>  {
> -	dec_node_page_state(page, NR_UNSTABLE_NFS);
> +	dec_node_page_state(page, NR_WRITEBACK);
>  	dec_wb_stat(&inode_to_bdi(page_file_mapping(page)->host)->wb,
> -		    WB_RECLAIMABLE);
> +		    WB_WRITEBACK);
>  }
>  
>  /* Called holding the request lock on @req */
> diff --git a/fs/proc/meminfo.c b/fs/proc/meminfo.c
> index 8c1f1bb1a5ce..9bd94b5a9658 100644
> --- a/fs/proc/meminfo.c
> +++ b/fs/proc/meminfo.c
> @@ -106,8 +106,7 @@ static int meminfo_proc_show(struct seq_file *m, void *v)
>  	show_val_kb(m, "PageTables:     ",
>  		    global_zone_page_state(NR_PAGETABLE));
>  
> -	show_val_kb(m, "NFS_Unstable:   ",
> -		    global_node_page_state(NR_UNSTABLE_NFS));
> +	show_val_kb(m, "NFS_Unstable:   ", 0);
>  	show_val_kb(m, "Bounce:         ",
>  		    global_zone_page_state(NR_BOUNCE));
>  	show_val_kb(m, "WritebackTmp:   ",
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index e84d448988b6..3937f2be27d8 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -237,7 +237,6 @@ enum node_stat_item {
>  	NR_FILE_THPS,
>  	NR_FILE_PMDMAPPED,
>  	NR_ANON_THPS,
> -	NR_UNSTABLE_NFS,	/* NFS unstable pages */
>  	NR_VMSCAN_WRITE,
>  	NR_VMSCAN_IMMEDIATE,	/* Prioritise for reclaim when writeback ends */
>  	NR_DIRTIED,		/* page dirtyings since bootup */
> diff --git a/include/trace/events/writeback.h b/include/trace/events/writeback.h
> index d94def25e4dc..45b5fbdb1f62 100644
> --- a/include/trace/events/writeback.h
> +++ b/include/trace/events/writeback.h
> @@ -542,7 +542,6 @@ TRACE_EVENT(global_dirty_state,
>  	TP_STRUCT__entry(
>  		__field(unsigned long,	nr_dirty)
>  		__field(unsigned long,	nr_writeback)
> -		__field(unsigned long,	nr_unstable)
>  		__field(unsigned long,	background_thresh)
>  		__field(unsigned long,	dirty_thresh)
>  		__field(unsigned long,	dirty_limit)
> @@ -553,7 +552,6 @@ TRACE_EVENT(global_dirty_state,
>  	TP_fast_assign(
>  		__entry->nr_dirty	= global_node_page_state(NR_FILE_DIRTY);
>  		__entry->nr_writeback	= global_node_page_state(NR_WRITEBACK);
> -		__entry->nr_unstable	= global_node_page_state(NR_UNSTABLE_NFS);
>  		__entry->nr_dirtied	= global_node_page_state(NR_DIRTIED);
>  		__entry->nr_written	= global_node_page_state(NR_WRITTEN);
>  		__entry->background_thresh = background_thresh;
> @@ -561,12 +559,11 @@ TRACE_EVENT(global_dirty_state,
>  		__entry->dirty_limit	= global_wb_domain.dirty_limit;
>  	),
>  
> -	TP_printk("dirty=%lu writeback=%lu unstable=%lu "
> +	TP_printk("dirty=%lu writeback=%lu "
>  		  "bg_thresh=%lu thresh=%lu limit=%lu "
>  		  "dirtied=%lu written=%lu",
>  		  __entry->nr_dirty,
>  		  __entry->nr_writeback,
> -		  __entry->nr_unstable,
>  		  __entry->background_thresh,
>  		  __entry->dirty_thresh,
>  		  __entry->dirty_limit,
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index ca194864d802..41b450b0ca29 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -4326,7 +4326,6 @@ void mem_cgroup_wb_stats(struct bdi_writeback *wb, unsigned long *pfilepages,
>  
>  	*pdirty = memcg_exact_page_state(memcg, NR_FILE_DIRTY);
>  
> -	/* this should eventually include NR_UNSTABLE_NFS */
>  	*pwriteback = memcg_exact_page_state(memcg, NR_WRITEBACK);
>  	*pfilepages = memcg_exact_page_state(memcg, NR_INACTIVE_FILE) +
>  			memcg_exact_page_state(memcg, NR_ACTIVE_FILE);
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index 4c9875971de5..d16f6a59bce4 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -504,7 +504,6 @@ bool node_dirty_ok(struct pglist_data *pgdat)
>  	unsigned long nr_pages = 0;
>  
>  	nr_pages += node_page_state(pgdat, NR_FILE_DIRTY);
> -	nr_pages += node_page_state(pgdat, NR_UNSTABLE_NFS);
>  	nr_pages += node_page_state(pgdat, NR_WRITEBACK);
>  
>  	return nr_pages <= limit;
> @@ -758,7 +757,7 @@ static void mdtc_calc_avail(struct dirty_throttle_control *mdtc,
>   * bounded by the bdi->min_ratio and/or bdi->max_ratio parameters, if set.
>   *
>   * Return: @wb's dirty limit in pages. The term "dirty" in the context of
> - * dirty balancing includes all PG_dirty, PG_writeback and NFS unstable pages.
> + * dirty balancing includes all PG_dirty and PG_writeback pages.
>   */
>  static unsigned long __wb_calc_thresh(struct dirty_throttle_control *dtc)
>  {
> @@ -1566,7 +1565,7 @@ static void balance_dirty_pages(struct bdi_writeback *wb,
>  	struct dirty_throttle_control * const mdtc = mdtc_valid(&mdtc_stor) ?
>  						     &mdtc_stor : NULL;
>  	struct dirty_throttle_control *sdtc;
> -	unsigned long nr_reclaimable;	/* = file_dirty + unstable_nfs */
> +	unsigned long nr_reclaimable;	/* = file_dirty */
>  	long period;
>  	long pause;
>  	long max_pause;
> @@ -1589,14 +1588,7 @@ static void balance_dirty_pages(struct bdi_writeback *wb,
>  		unsigned long m_thresh = 0;
>  		unsigned long m_bg_thresh = 0;
>  
> -		/*
> -		 * Unstable writes are a feature of certain networked
> -		 * filesystems (i.e. NFS) in which data may have been
> -		 * written to the server's write cache, but has not yet
> -		 * been flushed to permanent storage.
> -		 */
> -		nr_reclaimable = global_node_page_state(NR_FILE_DIRTY) +
> -					global_node_page_state(NR_UNSTABLE_NFS);
> +		nr_reclaimable = global_node_page_state(NR_FILE_DIRTY);
>  		gdtc->avail = global_dirtyable_memory();
>  		gdtc->dirty = nr_reclaimable + global_node_page_state(NR_WRITEBACK);
>  
> @@ -1940,8 +1932,7 @@ bool wb_over_bg_thresh(struct bdi_writeback *wb)
>  	 * as we're trying to decide whether to put more under writeback.
>  	 */
>  	gdtc->avail = global_dirtyable_memory();
> -	gdtc->dirty = global_node_page_state(NR_FILE_DIRTY) +
> -		      global_node_page_state(NR_UNSTABLE_NFS);
> +	gdtc->dirty = global_node_page_state(NR_FILE_DIRTY);
>  	domain_dirty_limits(gdtc);
>  
>  	if (gdtc->dirty > gdtc->bg_thresh)
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index e5f76da8cd4e..24678d6e308d 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5237,7 +5237,7 @@ void show_free_areas(unsigned int filter, nodemask_t *nodemask)
>  
>  	printk("active_anon:%lu inactive_anon:%lu isolated_anon:%lu\n"
>  		" active_file:%lu inactive_file:%lu isolated_file:%lu\n"
> -		" unevictable:%lu dirty:%lu writeback:%lu unstable:%lu\n"
> +		" unevictable:%lu dirty:%lu writeback:%lu unstable:0\n"
>  		" slab_reclaimable:%lu slab_unreclaimable:%lu\n"
>  		" mapped:%lu shmem:%lu pagetables:%lu bounce:%lu\n"
>  		" free:%lu free_pcp:%lu free_cma:%lu\n",
> @@ -5250,7 +5250,6 @@ void show_free_areas(unsigned int filter, nodemask_t *nodemask)
>  		global_node_page_state(NR_UNEVICTABLE),
>  		global_node_page_state(NR_FILE_DIRTY),
>  		global_node_page_state(NR_WRITEBACK),
> -		global_node_page_state(NR_UNSTABLE_NFS),
>  		global_node_page_state(NR_SLAB_RECLAIMABLE),
>  		global_node_page_state(NR_SLAB_UNRECLAIMABLE),
>  		global_node_page_state(NR_FILE_MAPPED),
> @@ -5283,7 +5282,7 @@ void show_free_areas(unsigned int filter, nodemask_t *nodemask)
>  			" anon_thp: %lukB"
>  #endif
>  			" writeback_tmp:%lukB"
> -			" unstable:%lukB"
> +			" unstable:0kB"
>  			" all_unreclaimable? %s"
>  			"\n",
>  			pgdat->node_id,
> @@ -5305,7 +5304,6 @@ void show_free_areas(unsigned int filter, nodemask_t *nodemask)
>  			K(node_page_state(pgdat, NR_ANON_THPS) * HPAGE_PMD_NR),
>  #endif
>  			K(node_page_state(pgdat, NR_WRITEBACK_TEMP)),
> -			K(node_page_state(pgdat, NR_UNSTABLE_NFS)),
>  			pgdat->kswapd_failures >= MAX_RECLAIM_RETRIES ?
>  				"yes" : "no");
>  	}
> diff --git a/mm/vmstat.c b/mm/vmstat.c
> index c9c0d71f917f..228d9f6e1c5c 100644
> --- a/mm/vmstat.c
> +++ b/mm/vmstat.c
> @@ -1108,7 +1108,7 @@ int fragmentation_index(struct zone *zone, unsigned int order)
>  					TEXT_FOR_HIGHMEM(xx) xx "_movable",
>  
>  const char * const vmstat_text[] = {
> -	/* enum zone_stat_item countes */
> +	/* enum zone_stat_item counters */
>  	"nr_free_pages",
>  	"nr_zone_inactive_anon",
>  	"nr_zone_active_anon",
> @@ -1162,7 +1162,6 @@ const char * const vmstat_text[] = {
>  	"nr_file_hugepages",
>  	"nr_file_pmdmapped",
>  	"nr_anon_transparent_hugepages",
> -	"nr_unstable",
>  	"nr_vmscan_write",
>  	"nr_vmscan_immediate_reclaim",
>  	"nr_dirtied",
> @@ -1707,8 +1706,16 @@ static void *vmstat_start(struct seq_file *m, loff_t *pos)
>  static void *vmstat_next(struct seq_file *m, void *arg, loff_t *pos)
>  {
>  	(*pos)++;
> -	if (*pos >= NR_VMSTAT_ITEMS)
> +	if (*pos >= NR_VMSTAT_ITEMS) {
> +		/*
> +		 * Deprecated counters which are no longer represented
> +		 * in vmstat arrays. We just lie about them to be always
> +		 * 0 to not break userspace which might expect them in
> +		 * the output.
> +		 */
> +		seq_puts(m, "nr_unstable 0");
>  		return NULL;
> +	}
>  	return (unsigned long *)m->private + *pos;
>  }
>  
> -- 
> 2.26.0
> 



-- 
Michal Hocko
SUSE Labs