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Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1pK3Zn-004Vl0-3t; Mon, 23 Jan 2023 20:37:59 +0000 Date: Mon, 23 Jan 2023 20:37:59 +0000 From: Matthew Wilcox To: Sidhartha Kumar Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, akpm@linux-foundation.org, david@redhat.com, osalvador@suse.de Subject: Re: [PATCH 1/2] mm/memory_hotplug: remove head page reference in do_migrate_range Message-ID: References: <20230123202347.317065-1-sidhartha.kumar@oracle.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230123202347.317065-1-sidhartha.kumar@oracle.com> X-Rspamd-Queue-Id: 2121D4001C X-Rspamd-Server: rspam09 X-Rspam-User: X-Stat-Signature: 17yw8j6if791dfck5zfdh84sbh8xfirb X-HE-Tag: 1674506285-384021 X-HE-Meta: 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 K7jyxZrx 88H2k8bSexMZBSORO8SAOFiZsou4purbx5wtLFS2MKZXnkX7orHBY48nPCQzZp7g6U621dt0K5CJXQTx7YGfNVSqp4aMfHBXG1FPdhTxennxlev2q3gFVLM5cRdLaHn7QUA94Umou/hzK6Xgr7yq+m+IGozBsX21IaEVUD0dF0l8EQEpdpCMwxoz3KokE25udgaKHZbh0lLOklMI= 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, Jan 23, 2023 at 12:23:46PM -0800, Sidhartha Kumar wrote: > @@ -1637,14 +1637,13 @@ do_migrate_range(unsigned long start_pfn, unsigned long end_pfn) > continue; > page = pfn_to_page(pfn); > folio = page_folio(page); > - head = &folio->page; > > - if (PageHuge(page)) { > - pfn = page_to_pfn(head) + compound_nr(head) - 1; > + if (folio_test_hugetlb(folio)) { > + pfn = folio_pfn(folio) + folio_nr_pages(folio) - 1; > isolate_hugetlb(folio, &source); > continue; > - } else if (PageTransHuge(page)) > - pfn = page_to_pfn(head) + thp_nr_pages(page) - 1; > + } else if (folio_test_transhuge(folio)) > + pfn = folio_pfn(folio) + thp_nr_pages(page) - 1; I'm pretty sure those two lines should be... } else if (folio_test_large(folio)) pfn = folio_pfn(folio) + folio_nr_pages(folio) - 1; But, erm ... we're doing this before we have a refcount on the page, right? So this is unsafe because the page might change which folio it is in. And the folio we found earlier might become a tail page of a different folio. (As the comment below explains, HWPoison pages won't, so it's not unsafe for them). Also, thp_nr_pages(page) is going to return 1 for tail pages. So this is a noop, unless page is a head page. It's all a bit confusing, and being memory-hotplug, it's not well tested. More thought needed.