[PATCH v7 0/4] page_owner: print stacks and their outstanding allocations

Oscar Salvador
2024-02-08T23:45:14Z

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 read-only file called "page_owner_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 'page_owner_threshold'.
By writing a value to it, the stacks which refcount is below such
value will be filtered out.

In order to better exercise the path in stack_depot_get_next_stack(),
I artificially filled the buckets with more than one stack, making sure
I was getting all of then when reading from it.

On a side note, stack_depot_get_next_stack() could be somehow reconstructed
to be in page_owner code, but we would have to move stack_table
into the header, so page_owner can access it.
I can do that if that's preferred, so stackdepot.c would not get "poluted".

A PoC can be found below:

# cat /sys/kernel/debug/page_owner_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_threshold
# cat /sys/kernel/debug/page_owner_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 (4):
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

include/linux/stackdepot.h | 72 ++++++++++++++++++++
lib/stackdepot.c | 97 ++++++++++++++------------
mm/page_owner.c | 136 +++++++++++++++++++++++++++++++++++++
3 files changed, 262 insertions(+), 43 deletions(-)

--
2.43.0