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Received: from willy by casper.infradead.org with local (Exim 4.99.1 #2 (Red Hat Linux)) id 1wnF6Q-00000003Sfq-1Yqa; Fri, 24 Jul 2026 12:34:10 +0000 Date: Fri, 24 Jul 2026 13:34:10 +0100 From: Matthew Wilcox To: Andrew Morton , Jane Chu , linux-mm@kvack.org Cc: Muchun Song , Oscar Salvador , David Hildenbrand , Miaohe Lin , Naoya Horiguchi , Jan Kara , linux-fsdevel@vger.kernel.org, Christian Brauner , Jiaqi Yan Subject: Re: [PATCH v4 02/14] memory-failure: Test the page is hwpoison before taking the mutex Message-ID: References: <20260723143034.175661-1-willy@infradead.org> <20260723143034.175661-3-willy@infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260723143034.175661-3-willy@infradead.org> X-Rspam-User: X-Rspamd-Queue-Id: D63494000E X-Rspamd-Server: rspam01 X-Stat-Signature: 591jzqcbn3u4ni6jgzod36z6fjdigxxp X-HE-Tag: 1784896452-243753 X-HE-Meta: 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 Fef3h/bU 6JDWoa13gRswAD5vQNb5rSpe9FZhF9VYkDtHjcSZ0TZiDq3bDEc/uwuC7gtEyLNwXrHsYhcCz6hKZ1xOkm02KULvaKUWvF9QCN0fcUIchK/6ZJkfG1tTweaRqvGplFIZtTuHSP+2c+DTGlRzP5RVAU4N5KoY96l26QUH1SF3rGAbSfgq/zVrbrZ8MNRHEOBN7U0wLG6Jnb6giKhC0NNeHmI07WiXBxIGvvSbd+SbJsRGx13lBzzMhH7jC9Ln9BNwhVn2Me0iiFn/qJhLQ+r911LzB5RCV/ftfIM8al+QUfPqgxNo4FVujblY9bJO0kwwaysQyL3IftCt9mm5i3iJlCNFUlfgei8gaFNAdMYfvilGOBXv6ojpaOeJDrA== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Sashiko review: > --- Patch [2]: [PATCH v4 02/14] memory-failure: Test the page is hwpoison before taking the mutex --- > commit 1cca344401408425fcfd4ca129165f85229f9227 > Author: Matthew Wilcox (Oracle) > > memory-failure: Test the page is hwpoison before taking the mutex > > This patch moves the is_page_hwpoison() check in unpoison_memory() outside > of mf_mutex. This avoids synchronizing with memory failure just to filter > out pages which are not hwpoisoned. > > > diff --git a/mm/memory-failure.c b/mm/memory-failure.c > > index 944e6e1d49719..3615ced0a11bc 100644 > > --- a/mm/memory-failure.c > > +++ b/mm/memory-failure.c > > @@ -2677,7 +2677,12 @@ int unpoison_memory(unsigned long pfn) > > p = pfn_to_online_page(pfn); > > if (!p) > > return -EIO; > > - folio = page_folio(p); > > + > > + if (!is_page_hwpoison(p)) { > > + unpoison_pr_info("%#lx: page is not poisoned\n", > > + pfn, &unpoison_rs); > > + return -EBUSY; > > + } > > > > mutex_lock(&mf_mutex); > > > > @@ -2688,6 +2693,8 @@ int unpoison_memory(unsigned long pfn) > > goto unlock_mutex; > > } > > > > + folio = page_folio(p); > > + > > if (is_huge_zero_folio(folio)) { > > unpoison_pr_info("%#lx: huge zero page is not supported\n", > > pfn, &unpoison_rs); > > @@ -2695,12 +2702,6 @@ int unpoison_memory(unsigned long pfn) > > goto unlock_mutex; > > } > > > > - if (!is_page_hwpoison(p)) { > > - unpoison_pr_info("%#lx: page is not poisoned\n", > > - pfn, &unpoison_rs); > > - goto unlock_mutex; > > - } > > - > > Does removing this check from inside the mutex introduce a time-of-check to > time-of-use race? > > If two threads call unpoison_memory() concurrently on the same PFN, both could > pass the lockless check. Thread A takes the lock, unpoisons the page, and the > page is returned to the buddy allocator. > > When Thread B acquires the lock, it will now proceed without re-checking > is_page_hwpoison(). If the page was concurrently reallocated as a compound tail > page, the speculative folio_ref_count() or folio_test_slab() checks below > could trigger a VM_BUG_ON_PGFLAGS crash. > > > if (folio_ref_count(folio) > 1) { > > unpoison_pr_info("%#lx: someone grabs the hwpoison page\n", > > pfn, &unpoison_rs); Uh. Yeah. I wasn't thinking about two simultaneous calls to unpoison_memory(). I'll just drop this patch; it wasn't important.