The patch titled
Subject: lib/vsprintf: avoid redundant work with 0 size
has been added to the -mm tree. Its filename is
lib-vsprintf-avoid-redundant-work-with-0-size.patch
This patch should soon appear at
https://ozlabs.org/~akpm/mmots/broken-out/lib-vsprintf-avoid-redundant-work-with-0-size.patch
and later at
https://ozlabs.org/~akpm/mmotm/broken-out/lib-vsprintf-avoid-redundant-work-with-0-size.patch
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------------------------------------------------------
From: Waiman Long <longman@xxxxxxxxxx>
Subject: lib/vsprintf: avoid redundant work with 0 size
Patch series "mm/page_owner: Extend page_owner to show memcg information", v4.
While debugging the constant increase in percpu memory consumption on a
system that spawned large number of containers, it was found that a lot of
offline mem_cgroup structures remained in place without being freed.
Further investigation indicated that those mem_cgroup structures were
pinned by some pages.
In order to find out what those pages are, the existing page_owner
debugging tool is extended to show memory cgroup information and whether
those memcgs are offline or not. With the enhanced page_owner tool, the
following is a typical page that pinned the mem_cgroup structure in my
test case:
Page allocated via order 0, mask 0x1100cca(GFP_HIGHUSER_MOVABLE), pid 162970 (podman), ts 1097761405537 ns, free_ts 1097760838089 ns
PFN 1925700 type Movable Block 3761 type Movable Flags 0x17ffffc00c001c(uptodate|dirty|lru|reclaim|swapbacked|node=0|zone=2|lastcpupid=0x1fffff)
prep_new_page+0xac/0xe0
get_page_from_freelist+0x1327/0x14d0
__alloc_pages+0x191/0x340
alloc_pages_vma+0x84/0x250
shmem_alloc_page+0x3f/0x90
shmem_alloc_and_acct_page+0x76/0x1c0
shmem_getpage_gfp+0x281/0x940
shmem_write_begin+0x36/0xe0
generic_perform_write+0xed/0x1d0
__generic_file_write_iter+0xdc/0x1b0
generic_file_write_iter+0x5d/0xb0
new_sync_write+0x11f/0x1b0
vfs_write+0x1ba/0x2a0
ksys_write+0x59/0xd0
do_syscall_64+0x37/0x80
entry_SYSCALL_64_after_hwframe+0x44/0xae
Charged to offline memcg libpod-conmon-15e4f9c758422306b73b2dd99f9d50a5ea53cbb16b4a13a2c2308a4253cc0ec8.
So the page was not freed because it was part of a shmem segment. That is
useful information that can help users to diagnose similar problems.
With cgroup v1, /proc/cgroups can be read to find out the total number of
memory cgroups (online + offline). With cgroup v2, the cgroup.stat of the
root cgroup can be read to find the number of dying cgroups (most likely
pinned by dying memcgs).
The page_owner feature is not supposed to be enabled for production system
due to its memory overhead. However, if it is suspected that dying memcgs
are increasing over time, a test environment with page_owner enabled can
then be set up with appropriate workload for further analysis on what may
be causing the increasing number of dying memcgs.
This patch (of 4):
For *scnprintf(), vsnprintf() is always called even if the input size is
0. That is a waste of time, so just return 0 in this case.
Note that vsnprintf() will never return -1 to indicate an error. So
skipping the call to vsnprintf() when size is 0 will have no functional
impact at all.
Link: https://lkml.kernel.org/r/20220202203036.744010-1-longman@xxxxxxxxxx
Link: https://lkml.kernel.org/r/20220202203036.744010-2-longman@xxxxxxxxxx
Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
Acked-by: David Rientjes <rientjes@xxxxxxxxxx>
Reviewed-by: Sergey Senozhatsky <senozhatsky@xxxxxxxxxxxx>
Acked-by: Roman Gushchin <guro@xxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxxxx>
Cc: Vladimir Davydov <vdavydov.dev@xxxxxxxxx>
Cc: Petr Mladek <pmladek@xxxxxxxx>
Cc: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
Cc: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
Cc: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx>
Cc: Ira Weiny <ira.weiny@xxxxxxxxx>
Cc: Mike Rapoport <rppt@xxxxxxxxxx>
Cc: David Rientjes <rientjes@xxxxxxxxxx>
Cc: Rafael Aquini <aquini@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
lib/vsprintf.c | 8 +++++---
1 file changed, 5 insertions(+), 3 deletions(-)
--- a/lib/vsprintf.c~lib-vsprintf-avoid-redundant-work-with-0-size
+++ a/lib/vsprintf.c
@@ -2895,13 +2895,15 @@ int vscnprintf(char *buf, size_t size, c
{
int i;
+ if (unlikely(!size))
+ return 0;
+
i = vsnprintf(buf, size, fmt, args);
if (likely(i < size))
return i;
- if (size != 0)
- return size - 1;
- return 0;
+
+ return size - 1;
}
EXPORT_SYMBOL(vscnprintf);
_
Patches currently in -mm which might be from longman@xxxxxxxxxx are
lib-vsprintf-avoid-redundant-work-with-0-size.patch
mm-page_owner-use-scnprintf-to-avoid-excessive-buffer-overrun-check.patch
mm-page_owner-print-memcg-information.patch
mm-page_owner-record-task-command-name.patch
mm-sparsemem-fix-mem_section-will-never-be-null-gcc-12-warning.patch
ipc-mqueue-use-get_tree_nodev-in-mqueue_get_tree.patch
