From: John Hubbard <jhubbard@nvidia.com>
To: Matthew Wilcox <willy@infradead.org>
Cc: "Kirill A. Shutemov" <kirill@shutemov.name>,
"Andrew Morton" <akpm@linux-foundation.org>,
"Al Viro" <viro@zeniv.linux.org.uk>,
"Christoph Hellwig" <hch@infradead.org>,
"Dan Williams" <dan.j.williams@intel.com>,
"Dave Chinner" <david@fromorbit.com>,
"Ira Weiny" <ira.weiny@intel.com>, "Jan Kara" <jack@suse.cz>,
"Jason Gunthorpe" <jgg@ziepe.ca>,
"Jonathan Corbet" <corbet@lwn.net>,
"Jérôme Glisse" <jglisse@redhat.com>,
"Michal Hocko" <mhocko@suse.com>,
"Mike Kravetz" <mike.kravetz@oracle.com>,
"Shuah Khan" <shuah@kernel.org>,
"Vlastimil Babka" <vbabka@suse.cz>,
linux-doc@vger.kernel.org, linux-fsdevel@vger.kernel.org,
linux-kselftest@vger.kernel.org, linux-rdma@vger.kernel.org,
linux-mm@kvack.org, LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2 1/8] mm: dump_page: print head page's refcount, for compound pages
Date: Wed, 29 Jan 2020 22:23:10 -0800 [thread overview]
Message-ID: <a0d66400-96a2-f94e-311d-a94f75e72d65@nvidia.com> (raw)
In-Reply-To: <20200129225957.GH6615@bombadil.infradead.org>
On 1/29/20 2:59 PM, Matthew Wilcox wrote:
...
> I have a hunk in my current tree which looks like this:
>
> @@ -77,6 +77,11 @@ void __dump_page(struct page *page, const char *reason)
> pr_warn("page:%px refcount:%d mapcount:%d mapping:%px index:%#lx
> \n",
> page, page_ref_count(page), mapcount,
> page->mapping, page_to_pgoff(page));
> + if (PageTail(page)) {
> + struct page *head = compound_head(page);
> + pr_warn("head:%px mapping:%px index:%#lx\n",
> + head, head->mapping, page_to_pgoff(head));
> + }
> if (PageKsm(page))
> pr_warn("ksm flags: %#lx(%pGp)\n", page->flags, &page->flags);
> else if (PageAnon(page))
>
> I wonder if we can combine these two patches in some more useful way?
>
> I also think we probably want a sanity check that 'head' and 'page'
> are within a sane range of each other (ie head < page and head +
> MAX_ORDER_NR_PAGES > page) to protect against a struct page that contains
> complete garbage.
>
OK, here's a go at combining those. I like the observation, implicit in your
diffs, that PageTail rather than PageCompound is the key differentiator in
deciding what to print. How's this look:
diff --git a/mm/debug.c b/mm/debug.c
index a90da5337c14..944652843e7b 100644
--- a/mm/debug.c
+++ b/mm/debug.c
@@ -75,12 +75,31 @@ void __dump_page(struct page *page, const char *reason)
*/
mapcount = PageSlab(page) ? 0 : page_mapcount(page);
- if (PageCompound(page))
- pr_warn("page:%px refcount:%d mapcount:%d mapping:%px "
- "index:%#lx compound_mapcount: %d\n",
- page, page_ref_count(page), mapcount,
- page->mapping, page_to_pgoff(page),
- compound_mapcount(page));
+ if (PageTail(page)) {
+ struct page *head = compound_head(page);
+
+ if ((page < head) || (page >= head + MAX_ORDER_NR_PAGES)) {
+ /*
+ * Page is hopelessly corrupted, so limit any reporting
+ * to information about the page itself. Do not attempt
+ * to look at the head page.
+ */
+ pr_warn("page:%px refcount:%d mapcount:%d mapping:%px "
+ "index:%#lx (corrupted tail page case)\n",
+ page, page_ref_count(page), mapcount,
+ page->mapping, page_to_pgoff(page));
+ } else {
+ pr_warn("page:%px compound refcount:%d mapcount:%d "
+ "mapping:%px index:%#lx compound_mapcount:%d\n",
+ page, page_ref_count(head),
+ mapcount, head->mapping, page_to_pgoff(head),
+ compound_mapcount(page));
+
+ if (page_ref_count(page) != 0)
+ pr_warn("page:%px PROBLEM: non-zero refcount (==%d) on "
+ "this tail page\n", page, page_ref_count(page));
+ }
+ }
else
pr_warn("page:%px refcount:%d mapcount:%d mapping:%px index:%#lx\n",
page, page_ref_count(page), mapcount,
?
Here's sample output for a normal page, a tail page, and a tail page with a bad
(non-zero) refcount:
============
Normal page:
============
[ 38.572084] page:ffffea0011465880 refcount:2 mapcount:1 mapping:ffff888454d99001 index:0xb2
[ 38.579256] anon flags: 0x17ffe0000080036(referenced|uptodate|lru|active|swapbacked)
[ 38.585799] raw: 017ffe0000080036 ffffea0011460fc8 ffffea0011466d08 ffff888454d99001
[ 38.592350] raw: 00000000000000b2 0000000000000000 0000000200000000 0000000000000000
[ 38.598885] page dumped because: test dump page
==========
Tail page:
==========
[ 38.436384] page:ffffea0010aa0280 compound refcount:503 mapcount:1 mapping:ffff888455fb3399 index:0xa8 compound_mapcount:1
[ 38.446350] anon flags: 0x17ffe0000000000()
[ 38.449661] raw: 017ffe0000000000 ffffea0010aa0001 ffffea0010aa0288 dead000000000400
[ 38.456228] raw: 0000000000000000 0000000000000000 00000000ffffffff 0000000000000000
[ 38.462794] page dumped because: test dump page
============================
Tail page with bad refcount:
============================
[ 38.466088] page:ffffea0010aa0b40 compound refcount:468 mapcount:1 mapping:ffff888455fb3399 index:0xa8 compound_mapcount:1
[ 38.475967] page:ffffea0010aa0b40 PROBLEM: non-zero refcount (==2) on this tail page
[ 38.482490] anon flags: 0x17ffe0000000000()
[ 38.485432] raw: 017ffe0000000000 ffffea0010aa0001 ffffea0010aa0b48 dead000000000400
[ 38.491996] raw: 0000000000000000 0000000000000000 00000002ffffffff 0000000000000000
[ 38.498532] page dumped because: test bad tail page refcount
thanks,
--
John Hubbard
NVIDIA
next prev parent reply other threads:[~2020-01-30 6:26 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-29 3:24 [PATCH v2 0/8] mm/gup: track FOLL_PIN pages (follow on from v12) John Hubbard
2020-01-29 3:24 ` [PATCH v2 1/8] mm: dump_page: print head page's refcount, for compound pages John Hubbard
2020-01-29 11:25 ` Kirill A. Shutemov
2020-01-29 22:26 ` John Hubbard
2020-01-29 22:59 ` Matthew Wilcox
2020-01-30 6:23 ` John Hubbard [this message]
2020-01-30 6:30 ` John Hubbard
2020-01-29 3:24 ` [PATCH v2 2/8] mm/gup: split get_user_pages_remote() into two routines John Hubbard
2020-01-29 3:24 ` [PATCH v2 3/8] mm/gup: pass a flags arg to __gup_device_* functions John Hubbard
2020-01-29 3:24 ` [PATCH v2 4/8] mm/gup: track FOLL_PIN pages John Hubbard
2020-01-29 13:51 ` Kirill A. Shutemov
2020-01-30 6:44 ` John Hubbard
2020-01-30 11:31 ` Kirill A. Shutemov
2020-01-31 3:19 ` John Hubbard
2020-01-29 3:24 ` [PATCH v2 5/8] mm/gup: page->hpage_pinned_refcount: exact pin counts for huge pages John Hubbard
2020-01-29 3:24 ` [PATCH v2 6/8] mm/gup: /proc/vmstat: pin_user_pages (FOLL_PIN) reporting John Hubbard
2020-01-29 3:24 ` [PATCH v2 7/8] mm/gup_benchmark: support pin_user_pages() and related calls John Hubbard
2020-01-29 3:24 ` [PATCH v2 8/8] selftests/vm: run_vmtests: invoke gup_benchmark with basic FOLL_PIN coverage John Hubbard
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