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Message-ID: <CAJnrk1YcA9MBC+KQdLE7B-CspoO5=xjkAf78swP6Q6UPijJaug@mail.gmail.com>
Date: Wed, 25 Jun 2025 14:38:01 -0700
From: Joanne Koong <joannelkoong@...il.com>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
"Matthew Wilcox (Oracle)" <willy@...radead.org>, Tejun Heo <tj@...nel.org>,
Maxim Patlasov <mpatlasov@...allels.com>, Jan Kara <jack@...e.cz>,
"Zach O'Keefe" <zokeefe@...gle.com>, Jonathan Corbet <corbet@....net>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>, "Rafael J. Wysocki" <rafael@...nel.org>,
Danilo Krummrich <dakr@...nel.org>, Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>,
"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>, Brendan Jackman <jackmanb@...gle.com>,
Johannes Weiner <hannes@...xchg.org>, Zi Yan <ziy@...dia.com>,
Jingbo Xu <jefflexu@...ux.alibaba.com>, Jeff Layton <jlayton@...nel.org>,
Miklos Szeredi <mszeredi@...hat.com>, linux-kernel@...r.kernel.org,
linux-fsdevel@...r.kernel.org, linux-doc@...r.kernel.org, linux-mm@...ck.org,
Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH] mm, vmstat: remove the NR_WRITEBACK_TEMP node_stat_item counter
On Wed, Jun 25, 2025 at 8:51 AM Vlastimil Babka <vbabka@...e.cz> wrote:
>
> The only user of the counter (FUSE) was removed in commit 0c58a97f919c
> ("fuse: remove tmp folio for writebacks and internal rb tree") so follow
> the established pattern of removing the counter and hardcoding 0 in
> meminfo output, as done recently with NR_BOUNCE. Update documentation
> for procfs, including for the value for Bounce that was missed when
> removing its counter.
>
> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
> ---
> The removal of the counter is straightforward. The reason for the large
> Cc list is that there is a comment in mm/page-writeback.c function
> wb_position_ratio() that mentions NR_WRITEBACK_TEMP, and just deleting
> the sentence feels to me it could be the wrong thing to do - maybe the
> strictlimit feature itself is now obsolete? It sure does mention FUSE
> as the main reason to exist, but commit 5a53748568f79 that introduced it
> also mentions slow USB sticks as a possibile scenario. Has that
> happened? I'm not familiar enough with this so I'd rather highlight this
> and ask for input here than make "git grep NR_WRITEBACK_TEMP" return
> nothing.
My understanding is that even without the fuse use case, strictlimit
is still used for other devices via the /sys/class/bdi interface (eg
/sys/class/bdi/<bdi>/strict_limit) so I don't think the feature itself
is obsolete.
It's not clear to me whether fuse still needs strictlimit now that it
doesn't have tmp writeback pages, but it'd be great to get an answer
to this, as strictlimit currently leads to too much dirty throttling
when large folios are enabled in fuse.
> ---
> Documentation/filesystems/proc.rst | 8 +++++---
> drivers/base/node.c | 2 +-
> fs/proc/meminfo.c | 3 +--
> include/linux/mmzone.h | 1 -
> mm/show_mem.c | 2 --
> mm/vmstat.c | 1 -
> 6 files changed, 7 insertions(+), 10 deletions(-)
>
> diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
> index 5236cb52e357dcd00496b26be8578e1dec0a345e..2971551b7235345c9a7ec3c84a87a16adcda5901 100644
> --- a/Documentation/filesystems/proc.rst
> +++ b/Documentation/filesystems/proc.rst
> @@ -1196,12 +1196,14 @@ SecPageTables
> Memory consumed by secondary page tables, this currently includes
> KVM mmu and IOMMU allocations on x86 and arm64.
> NFS_Unstable
> - Always zero. Previous counted pages which had been written to
> + Always zero. Previously 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"
> + Always zero. Previously memory used for block device
> + "bounce buffers".
> WritebackTmp
> - Memory used by FUSE for temporary writeback buffers
> + Always zero. Previously memory used by FUSE for temporary
> + writeback buffers.
> CommitLimit
> Based on the overcommit ratio ('vm.overcommit_ratio'),
> this is the total amount of memory currently available to
> diff --git a/drivers/base/node.c b/drivers/base/node.c
> index 6d66382dae6533a0c8481f72ad67c35021e331d3..e434cb260e6182468e0d617b559134c6fbe128f4 100644
> --- a/drivers/base/node.c
> +++ b/drivers/base/node.c
> @@ -500,7 +500,7 @@ static ssize_t node_read_meminfo(struct device *dev,
> nid, K(node_page_state(pgdat, NR_SECONDARY_PAGETABLE)),
> nid, 0UL,
> nid, 0UL,
> - nid, K(node_page_state(pgdat, NR_WRITEBACK_TEMP)),
> + nid, 0UL,
> nid, K(sreclaimable +
> node_page_state(pgdat, NR_KERNEL_MISC_RECLAIMABLE)),
> nid, K(sreclaimable + sunreclaimable),
> diff --git a/fs/proc/meminfo.c b/fs/proc/meminfo.c
> index bc2bc60c36ccc1dab8913913056f5ff20b448490..a458f1e112fdbc63019239a79ce39c5576b5f963 100644
> --- a/fs/proc/meminfo.c
> +++ b/fs/proc/meminfo.c
> @@ -121,8 +121,7 @@ static int meminfo_proc_show(struct seq_file *m, void *v)
>
> show_val_kb(m, "NFS_Unstable: ", 0);
> show_val_kb(m, "Bounce: ", 0);
> - show_val_kb(m, "WritebackTmp: ",
> - global_node_page_state(NR_WRITEBACK_TEMP));
> + show_val_kb(m, "WritebackTmp: ", 0);
> show_val_kb(m, "CommitLimit: ", vm_commit_limit());
> show_val_kb(m, "Committed_AS: ", committed);
> seq_printf(m, "VmallocTotal: %8lu kB\n",
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 1d1bb2b7f40d25b430932c9ef9096d97bf1c29de..0c5da9141983b795018c0aa2457b065507416564 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -206,7 +206,6 @@ enum node_stat_item {
> NR_FILE_PAGES,
> NR_FILE_DIRTY,
> NR_WRITEBACK,
> - NR_WRITEBACK_TEMP, /* Writeback using temporary buffers */
> NR_SHMEM, /* shmem pages (included tmpfs/GEM pages) */
> NR_SHMEM_THPS,
> NR_SHMEM_PMDMAPPED,
> diff --git a/mm/show_mem.c b/mm/show_mem.c
> index 0cf8bf5d832d6b339b4c9a6c7b8b3ab41683bcfe..41999e94a56d623726ea92f3f38785e8b218afe5 100644
> --- a/mm/show_mem.c
> +++ b/mm/show_mem.c
> @@ -246,7 +246,6 @@ static void show_free_areas(unsigned int filter, nodemask_t *nodemask, int max_z
> " shmem_pmdmapped:%lukB"
> " anon_thp:%lukB"
> #endif
> - " writeback_tmp:%lukB"
> " kernel_stack:%lukB"
> #ifdef CONFIG_SHADOW_CALL_STACK
> " shadow_call_stack:%lukB"
> @@ -273,7 +272,6 @@ static void show_free_areas(unsigned int filter, nodemask_t *nodemask, int max_z
> K(node_page_state(pgdat, NR_SHMEM_PMDMAPPED)),
> K(node_page_state(pgdat, NR_ANON_THPS)),
> #endif
> - K(node_page_state(pgdat, NR_WRITEBACK_TEMP)),
> node_page_state(pgdat, NR_KERNEL_STACK_KB),
> #ifdef CONFIG_SHADOW_CALL_STACK
> node_page_state(pgdat, NR_KERNEL_SCS_KB),
> diff --git a/mm/vmstat.c b/mm/vmstat.c
> index c3114b8826e4c3b6969fd4af4b0cd32173c42d7b..e0fcd9057f344170b2dc5c82beafea4ec18359bb 100644
> --- a/mm/vmstat.c
> +++ b/mm/vmstat.c
> @@ -1251,7 +1251,6 @@ const char * const vmstat_text[] = {
> [I(NR_FILE_PAGES)] = "nr_file_pages",
> [I(NR_FILE_DIRTY)] = "nr_dirty",
> [I(NR_WRITEBACK)] = "nr_writeback",
> - [I(NR_WRITEBACK_TEMP)] = "nr_writeback_temp",
> [I(NR_SHMEM)] = "nr_shmem",
> [I(NR_SHMEM_THPS)] = "nr_shmem_hugepages",
> [I(NR_SHMEM_PMDMAPPED)] = "nr_shmem_pmdmapped",
>
> ---
> base-commit: 4216fd45fc9156da0ee33fcb25cc0a5265049e32
> change-id: 20250625-nr_writeback_removal-4eca139cf09a
>
> Best regards,
> --
> Vlastimil Babka <vbabka@...e.cz>
>
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