Date: Fri, 16 Feb 2024 00:37:03 +0100
From: Andrey Konovalov <andreyknvl@...il.com>
To: Oscar Salvador <osalvador@...e.de>
Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-kernel@...r.kernel.org, 
	linux-mm@...ck.org, Michal Hocko <mhocko@...e.com>, Vlastimil Babka <vbabka@...e.cz>, 
	Marco Elver <elver@...gle.com>, Alexander Potapenko <glider@...gle.com>
Subject: Re: [PATCH v10 0/7] page_owner: print stacks and their outstanding allocations

On Thu, Feb 15, 2024 at 10:57 PM Oscar Salvador <osalvador@...e.de> wrote:
>
> Changes v9 -> v10
>      - Fix unwanted change in patch#2
>      - Collect Acked-by and Reviewed-by from Marco and Vlastimil
>        for the missing patches
>      - Fix stack_record count by substracting 1 in stack_print by Vlastimil
>
> Changes v8 -> v9
>      - Fix handle-0 for the very first stack_record entry
>      - Collect Acked-by and Reviewed-by from Marco and Vlastimil
>      - Adressed feedback from Marco and Vlastimil
>      - stack_print() no longer allocates a memory buffer, prints directly
>        using seq_printf: by Vlastimil
>      - Added two static struct stack for dummy_handle and faiure_handle
>      - add_stack_record_to_list() now filters out the gfp_mask the same way
>        stackdepot does, for consistency
>      - Rename set_threshold to count_threshold
>
> 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 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 (7):
>   lib/stackdepot: Fix first entry having a 0-handle
>   lib/stackdepot: Move stack_record struct definition into the header
>   mm,page_owner: Maintain own list of stack_records structs
>   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 |  45 +++++++
>  include/linux/stackdepot.h      |  58 +++++++++
>  lib/stackdepot.c                |  65 +++--------
>  mm/page_owner.c                 | 200 +++++++++++++++++++++++++++++++-
>  4 files changed, 318 insertions(+), 50 deletions(-)
>
> --
> 2.43.0
>

Acked-by: Andrey Konovalov <andreyknvl@...il.com>

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