Re: [PATCH v2 1/8] mm: dump_page: print head page's refcount, for compound pages

John Hubbard
2020-01-30T06:26:24Z

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