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Shutemov" To: John Hubbard Cc: Matthew Wilcox , linux-mm@kvack.org, "Kirill A . Shutemov" Subject: Re: [PATCH] mm: Improve dump_page() for compound pages Message-ID: <20200210212125.f24n5javxkfv6uvi@box> References: <20200208044415.30012-1-willy@infradead.org> <20200210124225.56cjwblfa7njgp5o@box> <92d69132-6664-3fc3-4743-11109df0b079@nvidia.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <92d69132-6664-3fc3-4743-11109df0b079@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 Mon, Feb 10, 2020 at 11:50:21AM -0800, John Hubbard wrote: > On 2/10/20 4:42 AM, Kirill A. Shutemov wrote: > ... > >> @@ -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); > > > > This is wrong. We want to see mapcount for the tail page, not head. > > > > I see what you mean: page_mapcount(page) sums up both the page's and the > head page's mapcount in some cases. The function doesn't seem to work > correctly unless it is fed the tail page. What makes you think this? It has to be called on the page user provided. Head or tail. > Here, even though the "head" variable's meaning is overloaded (=="head page, > unless the tail page was corrupted, in which case, tail page"), it would still be > accurate to change that line back to the original line, so that it once again > reads: > > mapcount = PageSlab(page) ? 0 : page_mapcount(page); PageSlab() can be called for the head page. > > Matthew? > > (Also, I see that __page_mapcount is EXPORT-ed, which is odd: nothing uses it > other than page_mapcount. ... and page_mapcount() can be inlined anywhere. -- Kirill A. Shutemov