[PATCH v3 0/2] mm/show_mem: Bug fix for print mem alloc info

[Yueyang Pan <pyyjason@gmail.com>]

This patch set fixes two issues we saw in production rollout. 

The first issue is that we saw all zero output of memory allocation 
profiling information from show_mem() if CONFIG_MEM_ALLOC_PROFILING 
is set and sysctl.vm.mem_profiling=0. This cause ambiguity as we 
don't know what 0B actually means in the output. It can mean either 
memory allocation profiling is temporary disabled or the allocation 
at that position is actually 0. Such ambiguity will make further 
parsing harder as we cannot differentiate between two case.

The second issue is that multiple entities can call show_mem() 
which messed up the allocation info in dmesg. We saw outputs like this:  
```
    327 MiB    83635 mm/compaction.c:1880 func:compaction_alloc
   48.4 GiB 12684937 mm/memory.c:1061 func:folio_prealloc
   7.48 GiB    10899 mm/huge_memory.c:1159 func:vma_alloc_anon_folio_pmd
    298 MiB    95216 kernel/fork.c:318 func:alloc_thread_stack_node
    250 MiB    63901 mm/zsmalloc.c:987 func:alloc_zspage
    1.42 GiB   372527 mm/memory.c:1063 func:folio_prealloc
    1.17 GiB    95693 mm/slub.c:2424 func:alloc_slab_page
     651 MiB   166732 mm/readahead.c:270 func:page_cache_ra_unbounded
     419 MiB   107261 net/core/page_pool.c:572 func:__page_pool_alloc_pages_slow
     404 MiB   103425 arch/x86/mm/pgtable.c:25 func:pte_alloc_one
```
The above example is because one kthread invokes show_mem() 
from __alloc_pages_slowpath while kernel itself calls 
oom_kill_process()

Revision History
=================
Changes from v2 [2]
- Merge status dump with "Memory allocations:" starting line
- Move the definition of spinlock within ifdef 

Changes from v1 [1]
- Dump status of memory allocation profiling instead of disabling 
the output following Vishal's advise.
- Move lock from file scope to within __show_mem() and replace mutex 
with spinlock following Andrew, Vlastimil and Shakeel's advice.

[1] https://lore.kernel.org/linux-mm/cover.1756318426.git.pyyjason@gmail.com/
[2] https://lore.kernel.org/linux-mm/cover.1756827906.git.pyyjason@gmail.com/

Yueyang Pan (2):
  mm/show_mem: Dump the status of the mem alloc profiling  before
    printing
  mm/show_mem: Add trylock while printing alloc info

 mm/show_mem.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

-- 
2.47.3