lib/vsprintf: avoid redundant work with 0 size

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@redhat.com
Link: https://lkml.kernel.org/r/20220202203036.744010-2-longman@redhat.com
Signed-off-by: Waiman Long <longman@redhat.com>
Acked-by: David Rientjes <rientjes@google.com>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Acked-by: Roman Gushchin <guro@fb.com>
Acked-by: Rafael Aquini <aquini@redhat.com>
Acked-by: Mike Rapoport <rppt@linux.ibm.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
Cc: Petr Mladek <pmladek@suse.com>
Cc: Steven Rostedt (Google) <rostedt@goodmis.org>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc: Ira Weiny <ira.weiny@intel.com>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>