Re: [PATCH] mm: dump_page: add debugfs file for dumping page state by pfn

[Konstantin Khlebnikov <khlebnikov@yandex-team.ru>]

On 25/05/2020 18.35, Vlastimil Babka wrote:
> On 5/25/20 4:19 PM, Konstantin Khlebnikov wrote:
>> Tool 'page-types' could list pages mapped by process or file cache pages,
>> but it shows only limited amount of state exported via procfs.
>>
>> Let's employ existing helper dump_page() to reach remaining information:
>> writing pfn into /sys/kernel/debug/dump_page dumps state into kernel log.
> 
> Yeah that's indeed useful, however I'm less sure if kernel log is the proper way
> to extract the data. For example IIRC with the page_owner file can "seek to pfn"
> to dump it, although that makes it somewhat harder to use.
> 
> Or we could write pfn to one file and read the dump from another one? But that's
> not atomic.
> 
> Perhaps if we could do something like "cat /sys/kernel/debug/dump_page/<pfn>"
> without all the pfns being actually listed in the dump_page directory with "ls"?
> Is that possible?

Too much code for me. =)

This could be kind of ftrace tracer which iterates over pages and dumps them,
but anyway looks ridiculously overengineered.

This one hack connects existing 'pagemap' with existing 'dump_page', so almost free.

For complicated cases there is gdb and special tool drgn https://github.com/osandov/drgn

Writing script which parses all that stuff from kernel log isn't big deal either.
I have one with 100+ lines regexp for all kinds of kernel splats.
Will publish when find time for polishing.

> 
>> # echo 0x37c43c > /sys/kernel/debug/dump_page
>> # dmesg | tail -6
>>   page:ffffcb0b0df10f00 refcount:1 mapcount:0 mapping:000000007755d3d9 index:0x30
>>   0xffffffffae4239e0 name:"libGeoIP.so.1.6.9"
>>   flags: 0x200000000020014(uptodate|lru|mappedtodisk)
>>   raw: 0200000000020014 ffffcb0b187fd288 ffffcb0b189e6248 ffff9528a04afe10
>>   raw: 0000000000000030 0000000000000000 00000001ffffffff 0000000000000000
>>   page dumped because: debugfs request
>>
>> With CONFIG_PAGE_OWNER=y shows also stacks for last page alloc and free:
>>
>>   page:ffffea0018fff480 refcount:1 mapcount:1 mapping:0000000000000000 index:0x7f9f28f62
>>   anon flags: 0x100000000080034(uptodate|lru|active|swapbacked)
>>   raw: 0100000000080034 ffffea00184140c8 ffffea0018517d88 ffff8886076ba161
>>   raw: 00000007f9f28f62 0000000000000000 0000000100000000 ffff888bfc79f000
>>   page dumped because: debugfs request
>>   page->mem_cgroup:ffff888bfc79f000
>>   page_owner tracks the page as allocated
>>   page last allocated via order 0, migratetype Movable, gfp_mask 0x100dca(GFP_HIGHUSER_MOVABLE|__GFP_ZERO)
>>    prep_new_page+0x139/0x1a0
>>    get_page_from_freelist+0xde9/0x14e0
>>    __alloc_pages_nodemask+0x18b/0x360
>>    alloc_pages_vma+0x7c/0x270
>>    __handle_mm_fault+0xd40/0x12b0
>>    handle_mm_fault+0xe7/0x1e0
>>    do_page_fault+0x2d5/0x610
>>    page_fault+0x2f/0x40
>>   page last free stack trace:
>>    free_pcp_prepare+0x11e/0x1c0
>>    free_unref_page_list+0x71/0x180
>>    release_pages+0x31e/0x480
>>    tlb_flush_mmu+0x44/0x150
>>    tlb_finish_mmu+0x3d/0x70
>>    exit_mmap+0xdd/0x1a0
>>    mmput+0x70/0x140
>>    do_exit+0x33f/0xc40
>>    do_group_exit+0x3a/0xa0
>>    __x64_sys_exit_group+0x14/0x20
>>    do_syscall_64+0x48/0x130
>>    entry_SYSCALL_64_after_hwframe+0x44/0xa9
>>
>> Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
>> ---
>>   Documentation/admin-guide/mm/pagemap.rst |    3 +++
>>   Documentation/vm/page_owner.rst          |   10 ++++++++++
>>   mm/debug.c                               |   27 +++++++++++++++++++++++++++
>>   3 files changed, 40 insertions(+)
>>
>> diff --git a/Documentation/admin-guide/mm/pagemap.rst b/Documentation/admin-guide/mm/pagemap.rst
>> index 340a5aee9b80..663ad5490d72 100644
>> --- a/Documentation/admin-guide/mm/pagemap.rst
>> +++ b/Documentation/admin-guide/mm/pagemap.rst
>> @@ -205,3 +205,6 @@ Before Linux 3.11 pagemap bits 55-60 were used for "page-shift" (which is
>>   always 12 at most architectures). Since Linux 3.11 their meaning changes
>>   after first clear of soft-dirty bits. Since Linux 4.2 they are used for
>>   flags unconditionally.
>> +
>> +Page state could be dumped into kernel log by writing pfn in text form
>> +into /sys/kernel/debug/dump_page.
>> diff --git a/Documentation/vm/page_owner.rst b/Documentation/vm/page_owner.rst
>> index 0ed5ab8c7ab4..d4d4dc64c19d 100644
>> --- a/Documentation/vm/page_owner.rst
>> +++ b/Documentation/vm/page_owner.rst
>> @@ -88,3 +88,13 @@ Usage
>>   
>>      See the result about who allocated each page
>>      in the ``sorted_page_owner.txt``.
>> +
>> +Notes
>> +=====
>> +
>> +To lookup pages in file cache or mapped in process you could use interface
>> +pagemap documented in Documentation/admin-guide/mm/pagemap.rst or tool
>> +page-types in the tools/vm directory.
>> +
>> +Page state could be dumped into kernel log by writing pfn in text form
>> +into /sys/kernel/debug/dump_page.
>> diff --git a/mm/debug.c b/mm/debug.c
>> index 2189357f0987..5803f2b63d95 100644
>> --- a/mm/debug.c
>> +++ b/mm/debug.c
>> @@ -14,6 +14,7 @@
>>   #include <linux/migrate.h>
>>   #include <linux/page_owner.h>
>>   #include <linux/ctype.h>
>> +#include <linux/debugfs.h>
>>   
>>   #include "internal.h"
>>   
>> @@ -147,6 +148,32 @@ void dump_page(struct page *page, const char *reason)
>>   }
>>   EXPORT_SYMBOL(dump_page);
>>   
>> +#ifdef CONFIG_DEBUG_FS
>> +static int dump_page_set(void *data, u64 pfn)
>> +{
>> +	struct page *page;
>> +
>> +	if (!capable(CAP_SYS_ADMIN))
>> +		return -EPERM;
>> +
>> +	page = pfn_to_online_page(pfn);
>> +	if (!page)
>> +		return -ENXIO;
>> +
>> +	dump_page(page, "debugfs request");
>> +	return 0;
>> +}
>> +DEFINE_DEBUGFS_ATTRIBUTE(dump_page_fops, NULL, dump_page_set, "%llx\n");
>> +
>> +static int __init dump_page_debugfs(void)
>> +{
>> +	debugfs_create_file_unsafe("dump_page", 0200, NULL, NULL,
>> +				   &dump_page_fops);
>> +	return 0;
>> +}
>> +late_initcall(dump_page_debugfs);
>> +#endif /* CONFIG_DEBUG_FS */
>> +
>>   #ifdef CONFIG_DEBUG_VM
>>   
>>   void dump_vma(const struct vm_area_struct *vma)
>>
>