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

From: Trond Myklebust
Date: Mon Apr 06 2020 - 22:34:56 EST

On Tue, 2020-04-07 at 09:44 +1000, NeilBrown 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'm OK with this. It puts the control at the right level IMO. I'm
assuming it will go through the MM maintainers, so:

Acked-by: Trond Myklebust <trond.myklebust@xxxxxxxxxxxxxxx>

>
> Signed-off-by: NeilBrown <neilb@xxxxxxx>
> ---
> 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_UNSTA
> BLE_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;
> }
>
--
Trond Myklebust
Linux NFS client maintainer, Hammerspace
trond.myklebust@xxxxxxxxxxxxxxx