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Received: from willy by casper.infradead.org with local (Exim 4.97.1 #2 (Red Hat Linux)) id 1s0QI7-00000005lnM-1Tr2; Fri, 26 Apr 2024 18:27:23 +0000 Date: Fri, 26 Apr 2024 19:27:23 +0100 From: Matthew Wilcox To: Sidhartha Kumar Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, akpm@linux-foundation.org, linmiaohe@huawei.com, jane.chu@oracle.com, nao.horiguchi@gmail.com, osalvador@suse.de Subject: Re: [PATCH] mm/memory-failure: remove shake_page() Message-ID: References: <20240426171511.122887-1-sidhartha.kumar@oracle.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Stat-Signature: x4m3j87ztug9rhmimpttpi4ki6wyxx14 X-Rspamd-Queue-Id: BD89AC001C X-Rspam-User: X-Rspamd-Server: rspam12 X-HE-Tag: 1714156048-17725 X-HE-Meta: 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 tW/YSfSL AXxXZilScqmqoTtfOxeojNP79ZIas25mmUl7yFrNgll4vnmHtvLaYaHDxk4fNdG3cRE92+62k/V+Mc8zgsPIe8su3fHdzLbjoIpw8TYHc7iYrS557xPFpzHUDdMUBSGNdtEna1tFoSMc2pWN+JyLYvd79i1Fgfq5X+oqLqle5qpNtoKm7AEvKHNTy5igQu5eNgp23Lf16i8IbovXk+AM6R3KzFkYAyyW+dMG8c5HT8HqbM/LjfqNLA59nkKwnLXXmX3Ev 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: List-Subscribe: List-Unsubscribe: On Fri, Apr 26, 2024 at 10:57:31AM -0700, Sidhartha Kumar wrote: > On 4/26/24 10:34 AM, Matthew Wilcox wrote: > > On Fri, Apr 26, 2024 at 10:15:11AM -0700, Sidhartha Kumar wrote: > > > Use a folio in get_any_page() to save 5 calls to compound head and > > > convert the last user of shake_page() to shake_folio(). This allows us > > > to remove the shake_page() definition. > > > > So I didn't do this before because I wasn't convinced it was safe. > > We don't have a refcount on the folio, so the page might no longer > > be part of this folio by the time we get the refcount on the folio. > > > > I'd really like to see some argumentation for why this is safe. > > If I moved down the folio = page_folio() line to after we verify > __get_hwpoison_page() has returned 1, which indicates the reference count > was successfully incremented via foliO_try_get(), that means the folio > conversion would happen after we have a refcount. In the case we don't call > __get_hwpoison_page(), that means the MF_COUNT_INCREASED flag is set. This > means the page has existing users so that path would be safe as well. So I > think this is safe after moving page_folio() after __get_hwpoison_page(). See if you can find a hole in this chain of reasoning ... memory_failure() p = pfn_to_online_page(pfn); res = try_memory_failure_hugetlb(pfn, flags, &hugetlb); (not a hugetlb) if (TestSetPageHWPoison(p)) { (not already poisoned) if (!(flags & MF_COUNT_INCREASED)) { res = get_hwpoison_page(p, flags); get_hwpoison_page() ret = get_any_page(p, flags); get_any_page() folio = page_folio(page) Because we don't have a reference on the folio at this point (how could we?), the folio might be split, and now we have a pointer to a folio which no longer contains the page (assuming we had a hwerror in what was a tail page at this time).