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Shutemov" , Yang Shi , David Hildenbrand , Hugh Dickins , linux-kernel@vger.kernel.org Subject: Re: [PATCH v2] mm/khugepaged: skip shmem with userfaultfd Message-ID: References: <20230206112856.1802547-1-stevensd@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Queue-Id: 9A375180002 X-Stat-Signature: kmsfxqfoiw8kyxfkza5q7c5x83mj1j8j X-Rspam-User: X-Rspamd-Server: rspam08 X-HE-Tag: 1675736989-299550 X-HE-Meta: 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 w8Q8Etqo DEdJKcU1PNMLFO5yz7TK5b7y0F/kHdiYxlvk01nbg2EZyVY0pP+SRnq9PEXQmWpwxuMz4hXcjHl2kElPTfq3d7yrlVQ1mYjlPeR0t5mlgfGmYVzoZXe51y9vG8kugziyp0oBS7UkQOyR6GrXevlA9rWGGAU9pB3NMYuZS/uOeawuwyLTq1Oes2MvvesUQUnDrG8M0Ne9HRfzOTmU1bjYJsRZb0fKMhg3xEoLbomyjp3aoeaIZV1PIUn+WLQ== 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 Tue, Feb 07, 2023 at 10:37:06AM +0900, David Stevens wrote: > On Tue, Feb 7, 2023 at 6:50 AM Matthew Wilcox wrote: > > On Mon, Feb 06, 2023 at 03:52:19PM -0500, Peter Xu wrote: > > > The problem is khugepaged will release pgtable lock during collapsing, so > > > AFAICT there can be a race where some other thread tries to insert pages > > > into page cache in parallel with khugepaged right after khugepaged released > > > the page cache lock. > > > > > > For example, it seems to me new page cache can be inserted when khugepaged > > > is copying small page content to the new hpage. > > This particular race can't happen with either patch, since the missing > page cache entries are filled when we create the multi-index entry for > hpage. Can too. for (index = start; index < end; index++) { ... if (xa_is_value(page) || !PageUptodate(page)) { xas_unlock_irq(&xas); /* swap in or instantiate fallocated page */ if (shmem_get_folio(mapping->host, index, &folio, SGP_NOALLOC)) { result = SCAN_FAIL; goto xa_unlocked; } ... So we start the iteration, and then a page fault happens in one of the indices we've already examined, but we don't have the page on the list. It's a nice wide race too because we're bringing the page in from swap.