From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=91.218.175.246 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1794129805; b=TF9xHbeZUqt2a5CuFwQ/M7VvxfmoS6jnDIn6GqV9xwSVFtaO+PFgyPNzhyCvnFxic8v7 6PtzKQ7vW32ew6nTfi6ne92vJMm7nMi23m3aHIFvN7O8tFVuPE24bigTGe4O4MmBplI/v pnV8ninqhkav4qrLHo2tbHFqPeW12hN519sJsqfej/6m2tAXHwB3yKCK9fwjO8nEhPIDu lFg4mrEr3vGxlRa2o8OPveOi3nJZh3VIeKGOA0BdEVv+iVdCETaW8XBV9f830XC2beefm SXOLnFIbIHdvfhz2sbP/44d6GPxmgqF1sq1SwN8Jp2mEZOtvBGToLdJhWM1svhT4b8w== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1794129805; h=DMARC-Filter:X-Envelope-To:DKIM-Signature:X-Envelope-To:Received: X-Mizu-Trace-ID:X-Migadu-Flow:From:To:Cc:Subject:Date:Message-ID: X-Mailer:In-Reply-To:References:MIME-Version:Content-Type: Content-Transfer-Encoding; bh=5XXct/mS8QSJCzfaXHvfDvD/GjABqbPTOqaOMdXOtqQ=; b=E3qbvucfkI6/DX6cqzv5hVoponi5ZIPm83OXZlOFeUQD8iQsKwCVkCRuiu0/rdIh6g5l kFnVVICEvjQcuMh0R9UoSaPm9SQlrIhvl74h9nVz5GJIiqYFCJBoY5LZAUoBGcWkLA7+O 5/8B5tm0SrK2Crb567bwKeYgQUFBHjTvg08u/I1cqPRSpUQJ7CA9qAsfTqAuucAnBNbeq ZMxA5Jzk2Wutu01bO40KVzkwDpwniKD0N9WefmU7eG08FRRAfVr6g+b9MD3YgOm+DZTJQ CKeJWn+rVFNmIOnNS1L44ir0K52htS6Az3OpvCut3puNQO2UtBJ2nVeH26kQiplsBxg== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass header.d=linux.dev header.i=@linux.dev header.a=rsa-sha256 header.s=key1 header.b=K82No4gD; arc=none smtp.remote-ip=91.218.175.246 Received: from mta0.migadu.com (out-246.mta0.migadu.com [91.218.175.246]) (using TLSv1.3 with cipher TLS_AES_128_GCM_SHA256 (128/128 bits) key-exchange X25519 server-signature ECDSA (prime256v1) server-digest SHA256) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 483B3606A320 for ; Sun, 8 Nov 2026 09:23:23 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.2 smtp.subspace.kernel.org 483B3606A320 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key, unprotected) header.d=linux.dev header.i=@linux.dev header.a=rsa-sha256 header.s=key1 header.b=K82No4gD X-Envelope-To: stable@vger.kernel.org DKIM-Signature: a=rsa-sha256; bh=5XXct/mS8QSJCzfaXHvfDvD/GjABqbPTOqaOMdXOtqQ=; c=simple/simple; d=linux.dev; h=from:to:subject:date:message-id:mime-version:content-type; s=key1; t=1794129802; v=1; x=1794734602; b=K82No4gD/U0EDL8G5b0U3E/LJ/m2ZvtIurq4uACX0LgP99VkFG0/tl/mLp9yB/4+LZc8Oqyx y91WHpBUYPqB+EC56RlWaLqSTWEwxtt9cN3p4O9UVS6Ss8cvT+MH9gSuujZ94cxLHGxpsZP3usU bGtGN4oEaj5k9niDGR5r6uZ0= X-Envelope-To: stable@vger.kernel.org Received: by smtp.migadu.com with ESMTPS id fe3ad667d8daeff1; Sun, 08 Nov 2026 09:23:22 +0000 X-Mizu-Trace-ID: fe3ad667d8daeff1 X-Migadu-Flow: FLOW_OUT From: Lance Yang To: david@kernel.org Cc: akpm@linux-foundation.org, ziy@nvidia.com, baolin.wang@linux.alibaba.com, liam@infradead.org, nico.pache@linux.dev, ryan.roberts@arm.com, dev.jain@arm.com, baohua@kernel.org, lance.yang@linux.dev, usama.arif@linux.dev, kas@kernel.org, qi.zheng@linux.dev, hughd@google.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org Subject: Re: [PATCH v2 2/2] mm: don't allocate a page table under RCU in do_set_pmd() Date: Sun, 8 Nov 2026 17:23:15 +0800 Message-ID: <20261108092315.89639-1-lance.yang@linux.dev> X-Mailer: git-send-email 2.49.0 In-Reply-To: <20261106-khugepaged_set_pmd-v2-2-d2457b5e377a@kernel.org> References: <20261106-khugepaged_set_pmd-v2-2-d2457b5e377a@kernel.org> X-Mailing-List: stable@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On Fri, Nov 06, 2026 at 01:36:13PM +0100, David Hildenbrand (Arm) wrote: >do_fault_around() skips PTE table preallocation when the PMD already points >to a PTE table. A concurrent collapse can then retract that table and >clear the PMD before filemap_map_pages() locks the new PMD-sized folio. Right, having a PTE table when we check in do_fault_around() doesn't mean it'll still be there later ;) >filemap_map_pmd() can then call do_set_pmd() with an empty PMD and no >preallocated table. On PowerPC hash MMU, do_set_pmd() then tries >allocating a PTE table to deposit. But filemap_map_pmd() calls >do_set_pmd() under RCU, so allocating from that context is not good. > >This race became possible when retract_page_tables() stopped taking the >mmap and VMA locks. Previously, those locks excluded retraction while >the fault was in progress. > >Add a may_alloc argument to do_set_pmd() and pass false from >filemap_map_pmd(), so we can fail with VM_FAULT_FALLBACK instead of >trying to allocate. YES, just fall back and let the regular fault handling take over after leaving RCU. >This issue should be fairly hard to hit in practice. > >Fixes: 1d65b771bc08 ("mm/khugepaged: retract_page_tables() without mmap or vma lock") >Cc: stable@vger.kernel.org >Signed-off-by: David Hildenbrand (Arm) >--- LGTM1 Reviewed-by: Lance Yang Cheers, Lance