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Message-ID: <3477bf65-64dc-7854-6720-589f7fcdac07@nvidia.com>
Date: Fri, 7 Feb 2020 13:05:52 -0800
From: John Hubbard <jhubbard@...dia.com>
To: Matthew Wilcox <willy@...radead.org>
CC: Andrew Morton <akpm@...ux-foundation.org>,
Al Viro <viro@...iv.linux.org.uk>,
Christoph Hellwig <hch@...radead.org>,
Dan Williams <dan.j.williams@...el.com>,
Dave Chinner <david@...morbit.com>,
Ira Weiny <ira.weiny@...el.com>, Jan Kara <jack@...e.cz>,
Jason Gunthorpe <jgg@...pe.ca>,
Jonathan Corbet <corbet@....net>,
Jérôme Glisse <jglisse@...hat.com>,
"Kirill A . Shutemov" <kirill@...temov.name>,
Michal Hocko <mhocko@...e.com>,
Mike Kravetz <mike.kravetz@...cle.com>,
Shuah Khan <shuah@...nel.org>,
Vlastimil Babka <vbabka@...e.cz>, <linux-doc@...r.kernel.org>,
<linux-fsdevel@...r.kernel.org>, <linux-kselftest@...r.kernel.org>,
<linux-rdma@...r.kernel.org>, <linux-mm@...ck.org>,
LKML <linux-kernel@...r.kernel.org>,
"Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH v5 01/12] mm: dump_page(): better diagnostics for compound
pages
On 2/7/20 9:27 AM, Matthew Wilcox wrote:
...
>
> A definite improvement, but I think we could do better. For example,
> you've changed PageCompound to PageTail here, whereas we really do want
> to dump some more information for PageHead pages than the plain vanilla
> order-0 page has. Another thing is that page_mapping() calls compound_head(),
> so if the page is corrupted, we're going to get a funky pointer dereference.
>
> I spent a bit of time on this reimplementation ... what do you think?
>
It looks fine to me. I gave it a quick spin, here's the output for a normal
and a huge page, and it has everything we want to see:
page:ffffea0010f0d640 refcount:1025 mapcount:1 mapping:0000000021857089 index:0xed
anon flags: 0x17ffe0000080036(referenced|uptodate|lru|active|swapbacked)
raw: 017ffe0000080036 ffffea0011731f08 ffffea0011730008 ffff8884777272c1
raw: 00000000000000ed 0000000000000000 0000040100000000 0000000000000000
page dumped because: testing dump_page()
page:ffffea0010ef1b80 head:ffffea0010ef0000 refcount:0 mapcount:1 mapping:00000000a8e1c7fa index:0xed order:9 compound_mapcount: 1
anon flags: 0x17ffe0000000000()
raw: 017ffe0000000000 ffffea0010ef0001 ffffea0010ef1b88 dead000000000400
raw: 0000000000000000 0000000000000000 00000000ffffffff 0000000000000000
head: 017ffe0000090036 ffffea0011734548 ffffea0010ef8008 ffff8884777271b9
head: 000000000000007f 0000000000000000 00000201ffffffff 0000000000000000
page dumped because: testing dump_page()
> - Print the mapping pointer using %p insted of %px. The actual value of
> the pointer can be read out of the raw page dump and using %p gives a
> chance to correlate it to earlier printk of the mapping pointer.
> - Add the order of the page for compound pages
> - Dump the raw head page as well as the raw page being dumped
>
> diff --git a/mm/debug.c b/mm/debug.c
> index ecccd9f17801..0564d4cb8233 100644
> --- a/mm/debug.c
> +++ b/mm/debug.c
> @@ -44,8 +44,10 @@ const struct trace_print_flags vmaflag_names[] = {
>
> void __dump_page(struct page *page, const char *reason)
> {
> + struct page *head = compound_head(page);
> struct address_space *mapping;
> bool page_poisoned = PagePoisoned(page);
> + bool compound = PageCompound(page);
> /*
> * Accessing the pageblock without the zone lock. It could change to
> * "isolate" again in the meantime, but since we are just dumping the
> @@ -66,25 +68,32 @@ void __dump_page(struct page *page, const char *reason)
> goto hex_only;
> }
>
> - mapping = page_mapping(page);
> + if (page < head || (page >= head + MAX_ORDER_NR_PAGES)) {
> + /* Corrupt page, cannot call page_mapping */
> + mapping = page->mapping;
> + head = page;
> + compound = false;
> + } else {
> + mapping = page_mapping(page);
> + }
>
> /*
> * Avoid VM_BUG_ON() in page_mapcount().
> * page->_mapcount space in struct page is used by sl[aou]b pages to
> * encode own info.
> */
> - mapcount = PageSlab(page) ? 0 : page_mapcount(page);
> + mapcount = PageSlab(head) ? 0 : page_mapcount(head);
>
> - 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,
> + if (compound)
> + pr_warn("page:%px head:%px refcount:%d mapcount:%d mapping:%p "
> + "index:%#lx order:%u compound_mapcount: %d\n",
> + page, head, page_ref_count(page), mapcount,
> page->mapping, page_to_pgoff(page),
> - compound_mapcount(page));
> + compound_order(head), compound_mapcount(page));
> else
> - pr_warn("page:%px refcount:%d mapcount:%d mapping:%px index:%#lx\n",
> + pr_warn("page:%px refcount:%d mapcount:%d mapping:%p index:%#lx\n",
> page, page_ref_count(page), mapcount,
> - page->mapping, page_to_pgoff(page));
> + mapping, page_to_pgoff(page));
> if (PageKsm(page))
> type = "ksm ";
> else if (PageAnon(page))
> @@ -106,6 +115,10 @@ void __dump_page(struct page *page, const char *reason)
> print_hex_dump(KERN_WARNING, "raw: ", DUMP_PREFIX_NONE, 32,
> sizeof(unsigned long), page,
> sizeof(struct page), false);
> + if (!page_poisoned && compound)
> + print_hex_dump(KERN_WARNING, "head: ", DUMP_PREFIX_NONE, 32,
> + sizeof(unsigned long), head,
> + sizeof(struct page), false);
Good thought to get the hex dump of the head page in this case, yes.
>
> if (reason)
> pr_warn("page dumped because: %s\n", reason);
>
Seeing as how I want to further enhance dump_page() slightly for this series (to
include the 3rd struct page's hpage_pincount), would you care to send this as a
formal patch that I could insert into this series, to replace patch 5?
thanks,
--
John Hubbard
NVIDIA
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