mm,page_owner: update Documentation regarding page_owner_stacks

Update page_owner documentation including the new page_owner_stacks
feature to show how it can be used.

Link: https://lkml.kernel.org/r/20240215215907.20121-8-osalvador@suse.deSigned-off-by: Oscar Salvador <osalvador@suse.de>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Marco Elver <elver@google.com>
Acked-by: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Documentation/mm/page_owner.rst

@@ -24,6 +24,11 @@ fragmentation statistics can be obtained through gfp flag information of
 each page. It is already implemented and activated if page owner is
 enabled. Other usages are more than welcome.
 
+It can also be used to show all the stacks and their outstanding
+allocations, which gives us a quick overview of where the memory is going
+without the need to screen through all the pages and match the allocation
+and free operation.
+
 page owner is disabled by default. So, if you'd like to use it, you need
 to add "page_owner=on" to your boot cmdline. If the kernel is built
 with page owner and page owner is disabled in runtime due to not enabling
@@ -68,6 +73,46 @@ Usage
 
 4) Analyze information from page owner::
 
+	cat /sys/kernel/debug/page_owner_stacks/show_stacks > stacks.txt
+	cat stacks.txt
+	 prep_new_page+0xa9/0x120
+	 get_page_from_freelist+0x7e6/0x2140
+	 __alloc_pages+0x18a/0x370
+	 new_slab+0xc8/0x580
+	 ___slab_alloc+0x1f2/0xaf0
+	 __slab_alloc.isra.86+0x22/0x40
+	 kmem_cache_alloc+0x31b/0x350
+	 __khugepaged_enter+0x39/0x100
+	 dup_mmap+0x1c7/0x5ce
+	 copy_process+0x1afe/0x1c90
+	 kernel_clone+0x9a/0x3c0
+	 __do_sys_clone+0x66/0x90
+	 do_syscall_64+0x7f/0x160
+	 entry_SYSCALL_64_after_hwframe+0x6c/0x74
+	stack_count: 234
+	...
+	...
+	echo 7000 > /sys/kernel/debug/page_owner_stacks/count_threshold
+	cat /sys/kernel/debug/page_owner_stacks/show_stacks> stacks_7000.txt
+	cat stacks_7000.txt
+	 prep_new_page+0xa9/0x120
+	 get_page_from_freelist+0x7e6/0x2140
+	 __alloc_pages+0x18a/0x370
+	 alloc_pages_mpol+0xdf/0x1e0
+	 folio_alloc+0x14/0x50
+	 filemap_alloc_folio+0xb0/0x100
+	 page_cache_ra_unbounded+0x97/0x180
+	 filemap_fault+0x4b4/0x1200
+	 __do_fault+0x2d/0x110
+	 do_pte_missing+0x4b0/0xa30
+	 __handle_mm_fault+0x7fa/0xb70
+	 handle_mm_fault+0x125/0x300
+	 do_user_addr_fault+0x3c9/0x840
+	 exc_page_fault+0x68/0x150
+	 asm_exc_page_fault+0x22/0x30
+	stack_count: 8248
+	...
+
 	cat /sys/kernel/debug/page_owner > page_owner_full.txt
 	./page_owner_sort page_owner_full.txt sorted_page_owner.txt