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From: Sukrit Bhatnagar <Sukrit.Bhatnagar@sony.com>
To: Petr Mladek <pmladek@suse.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
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	Pekka Enberg <penberg@kernel.org>,
	David Rientjes <rientjes@google.com>,
	Joonsoo Kim <iamjoonsoo.kim@lge.com>,
	Andrew Morton <akpm@linux-foundation.org>,
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	Roman Gushchin <roman.gushchin@linux.dev>,
	Hyeonggon Yoo <42.hyeyoo@gmail.com>,
	Masami Hiramatsu <mhiramat@kernel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	"Matthew Wilcox (Oracle)" <willy@infradead.org>
Cc: linux-doc@vger.kernel.org,
	linux-kernel@vger.kernel.org,
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	Sukrit.Bhatnagar@sony.com
Subject: [PATCH 0/2] Improve dump_page() output for slab pages
Date: Wed, 22 May 2024 16:46:27 +0900
Message-Id: <20240522074629.2420423-1-Sukrit.Bhatnagar@sony.com>
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While using dump_page() on a range of pages, I noticed that there were some
PG_slab pages that were also showing as PG_anon pages, according to the
function output.

[    7.071985] page: refcount:1 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x102768
[    7.072602] head: order:3 entire_mapcount:0 nr_pages_mapped:0 pincount:0
[    7.073085] anon flags: 0x8000000000000840(slab|head|zone=2)
[    7.073777] raw: 8000000000000840 ffff8881000419c0 0000000000000000 dead000000000001

It was also printing the "page_type" field for slab pages, but that was fixed in
a very recent commit:
    8f790d0c7cfe (mm: improve dumping of mapcount and page_type)

Given that the slab pages cannot be mapped to userspace, this output seems
misleading.

In dump_page(), folio_test_anon() is used, which checks the "mapping" field.
But the struct slab was separated from struct page.
So accessing the mapping field through a struct page pointer, which actually
points to a struct slab, will result in garbage memory access and the PG_anon
test can return true.

It seems that other parts of the kernel MM make the check for slab before
checking for anon, but dump_page() is not doing that.

On the other hand, the struct slab has kmem_cache which maintains another set
of flags. It would be nice to have these flags added as a part of the debug
output, and to have a convenient way to print them.

(The long chain of pointer dereferences for cache flags looks messy, but I
assume it should be fine for a debug function.)

Sukrit Bhatnagar (2):
  mm: printk: introduce new format %pGs for slab flags
  mm: debug: print correct information for slab folios

 Documentation/core-api/printk-formats.rst |  2 +
 include/linux/slab.h                      |  5 ++
 include/trace/events/mmflags.h            | 67 +++++++++++++++++++++++
 lib/test_printf.c                         | 13 +++++
 lib/vsprintf.c                            | 22 ++++++++
 mm/debug.c                                | 12 +++-
 mm/internal.h                             |  1 +
 7 files changed, 121 insertions(+), 1 deletion(-)

-- 
2.34.1