[v8,0/5] page_owner: print stacks and their outstanding allocations

Message

Oscar Salvador Feb. 12, 2024, 10:30 p.m. UTC

Changes v7 -> v8
     - Rebased on top of -next
     - page_owner maintains its own stack_records list now
     - Kill auxiliary stackdepot function to traverse buckets
     - page_owner_stacks is now a directory with 'show_stacks'
       and 'set_threshold'
     - Update Documentation/mm/page_owner.rst
     - Adressed feedback from Marco

Changes v6 -> v7:
     - Rebased on top of Andrey Konovalov's libstackdepot patchset
     - Reformulated the changelogs

Changes v5 -> v6:
     - Rebase on top of v6.7-rc1
     - Move stack_record struct to the header
     - Addressed feedback from Vlastimil
       (some code tweaks and changelogs suggestions)

Changes v4 -> v5:
     - Addressed feedback from Alexander Potapenko

Changes v3 -> v4:
     - Rebase (long time has passed)
     - Use boolean instead of enum for action by Alexander Potapenko
     - (I left some feedback untouched because it's been long and
        would like to discuss it here now instead of re-vamping
        and old thread)

Changes v2 -> v3:
     - Replace interface in favor of seq operations
       (suggested by Vlastimil)
     - Use debugfs interface to store/read valued (suggested by Ammar)


page_owner is a great debug functionality tool that lets us know
about all pages that have been allocated/freed and their specific
stacktrace.
This comes very handy when debugging memory leaks, since with
some scripting we can see the outstanding allocations, which might point
to a memory leak.

In my experience, that is one of the most useful cases, but it can get
really tedious to screen through all pages and try to reconstruct the
stack <-> allocated/freed relationship, becoming most of the time a
daunting and slow process when we have tons of allocation/free operations. 

This patchset aims to ease that by adding a new functionality into
page_owner.
This functionality creates a new directory called 'page_owner_stacks'
under 'sys/kernel//debug' with a read-only file called 'show_stacks',
which prints out all the stacks followed by their outstanding number
of allocations (being that the times the stacktrace has allocated
but not freed yet).
This gives us a clear and a quick overview of stacks <-> allocated/free.

We take advantage of the new refcount_f field that stack_record struct
gained, and increment/decrement the stack refcount on every
__set_page_owner() (alloc operation) and __reset_page_owner (free operation)
call.

Unfortunately, we cannot use the new stackdepot api
STACK_DEPOT_FLAG_{GET,PUT} because it does not fulfill page_owner needs,
meaning we would have to special case things, at which point
makes more sense for page_owner to do its own {dec,inc}rementing
of the stacks.
E.g: Using STACK_DEPOT_FLAG_PUT, once the refcount reaches 0,
such stack gets evicted, so page_owner would lose information.

This patch also creates a new file called 'set_threshold' within
'page_owner_stacks' directory, and by writing a value to it, the stacks
which refcount is below such value will be filtered out.

A PoC can be found below:

 # cat /sys/kernel/debug/page_owner_stacks/show_stacks > page_owner_full_stacks.txt
 # head -40 page_owner_full_stacks.txt 
  prep_new_page+0xa9/0x120
  get_page_from_freelist+0x801/0x2210
  __alloc_pages+0x18b/0x350
  alloc_pages_mpol+0x91/0x1f0
  folio_alloc+0x14/0x50
  filemap_alloc_folio+0xb2/0x100
  page_cache_ra_unbounded+0x96/0x180
  filemap_get_pages+0xfd/0x590
  filemap_read+0xcc/0x330
  blkdev_read_iter+0xb8/0x150
  vfs_read+0x285/0x320
  ksys_read+0xa5/0xe0
  do_syscall_64+0x80/0x160
  entry_SYSCALL_64_after_hwframe+0x6e/0x76
 stack_count: 521



  prep_new_page+0xa9/0x120
  get_page_from_freelist+0x801/0x2210
  __alloc_pages+0x18b/0x350
  alloc_pages_mpol+0x91/0x1f0
  folio_alloc+0x14/0x50
  filemap_alloc_folio+0xb2/0x100
  __filemap_get_folio+0x14a/0x490
  ext4_write_begin+0xbd/0x4b0 [ext4]
  generic_perform_write+0xc1/0x1e0
  ext4_buffered_write_iter+0x68/0xe0 [ext4]
  ext4_file_write_iter+0x70/0x740 [ext4]
  vfs_write+0x33d/0x420
  ksys_write+0xa5/0xe0
  do_syscall_64+0x80/0x160
  entry_SYSCALL_64_after_hwframe+0x6e/0x76
 stack_count: 4609
...
...

 # echo 5000 > /sys/kernel/debug/page_owner_stacks/set_threshold 
 # cat /sys/kernel/debug/page_owner_stacks/show_stacks > page_owner_full_stacks_5000.txt
 # head -40 page_owner_full_stacks_5000.txt 
  prep_new_page+0xa9/0x120
  get_page_from_freelist+0x801/0x2210
  __alloc_pages+0x18b/0x350
  alloc_pages_mpol+0x91/0x1f0
  folio_alloc+0x14/0x50
  filemap_alloc_folio+0xb2/0x100
  __filemap_get_folio+0x14a/0x490
  ext4_write_begin+0xbd/0x4b0 [ext4]
  generic_perform_write+0xc1/0x1e0
  ext4_buffered_write_iter+0x68/0xe0 [ext4]
  ext4_file_write_iter+0x70/0x740 [ext4]
  vfs_write+0x33d/0x420
  ksys_pwrite64+0x75/0x90
  do_syscall_64+0x80/0x160
  entry_SYSCALL_64_after_hwframe+0x6e/0x76
 stack_count: 6781



  prep_new_page+0xa9/0x120
  get_page_from_freelist+0x801/0x2210
  __alloc_pages+0x18b/0x350
  pcpu_populate_chunk+0xec/0x350
  pcpu_balance_workfn+0x2d1/0x4a0
  process_scheduled_works+0x84/0x380
  worker_thread+0x12a/0x2a0
  kthread+0xe3/0x110
  ret_from_fork+0x30/0x50
  ret_from_fork_asm+0x1b/0x30
 stack_count: 8641

Oscar Salvador (5):
  lib/stackdepot: Move stack_record struct definition into the header
  mm,page_owner: Implement the tracking of the stacks count
  mm,page_owner: Display all stacks and their count
  mm,page_owner: Filter out stacks by a threshold
  mm,page_owner: Update Documentation regarding page_owner_stacks

 Documentation/mm/page_owner.rst |  44 ++++++++
 include/linux/stackdepot.h      |  53 +++++++++
 lib/stackdepot.c                |  51 ++-------
 mm/page_owner.c                 | 190 ++++++++++++++++++++++++++++++++
 4 files changed, 295 insertions(+), 43 deletions(-)