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Shutemov" To: John Hubbard Cc: Andrew Morton , Al Viro , Christoph Hellwig , Dan Williams , Dave Chinner , Ira Weiny , Jan Kara , Jason Gunthorpe , Jonathan Corbet , =?utf-8?B?SsOpcsO0bWU=?= Glisse , Michal Hocko , Mike Kravetz , Shuah Khan , Vlastimil Babka , Matthew Wilcox , 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 , "Kirill A . Shutemov" Subject: Re: [PATCH v3 01/12] mm: dump_page(): better diagnostics for compound pages Message-ID: <20200203131649.vptndo5emkzlaiew@box> References: <20200201034029.4063170-1-jhubbard@nvidia.com> <20200201034029.4063170-2-jhubbard@nvidia.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200201034029.4063170-2-jhubbard@nvidia.com> X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Fri, Jan 31, 2020 at 07:40:18PM -0800, John Hubbard wrote: > A compound page collects the refcount in the head page, while leaving > the refcount of each tail page at zero. Therefore, when debugging a > problem that involves compound pages, it's best to have diagnostics that > reflect that situation. However, dump_page() is oblivious to these > points. > > Change dump_page() as follows: > > 1) For tail pages, print relevant head page information: refcount, in > particular. But only do this if the page is not corrupted so badly > that the pointer to the head page is all wrong. > > 2) Do a separate check to catch any (rare) cases of the tail page's > refcount being non-zero, and issue a separate, clear pr_warn() if > that ever happens. > > Suggested-by: Matthew Wilcox > Suggested-by: Kirill A. Shutemov > Signed-off-by: John Hubbard Acked-by: Kirill A. Shutemov Few nit-picks below. > --- > mm/debug.c | 34 ++++++++++++++++++++++++++++------ > 1 file changed, 28 insertions(+), 6 deletions(-) > > diff --git a/mm/debug.c b/mm/debug.c > index ecccd9f17801..beb1c59d784b 100644 > --- a/mm/debug.c > +++ b/mm/debug.c > @@ -42,6 +42,32 @@ const struct trace_print_flags vmaflag_names[] = { > {0, NULL} > }; > > +static void __dump_tail_page(struct page *page, int mapcount) > +{ > + struct page *head = compound_head(page); > + > + if ((page < head) || (page >= head + MAX_ORDER_NR_PAGES)) { I'm not sure if we want to use compound_nr() here instead of MAX_ORDER_NR_PAGES. Do you have any reasonaing about it? > + /* > + * 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)); Wrap into {}, please. > +} > + > void __dump_page(struct page *page, const char *reason) > { > struct address_space *mapping; > @@ -75,12 +101,8 @@ 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)) > + __dump_tail_page(page, mapcount); > else > pr_warn("page:%px refcount:%d mapcount:%d mapping:%px index:%#lx\n", > page, page_ref_count(page), mapcount, > -- > 2.25.0 > -- Kirill A. Shutemov