From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 5DD98388393 for ; Fri, 24 Jul 2026 18:48:47 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784918928; cv=none; b=Tz37nrvEdzksyjmdR5DsRtoMCSNkQNkg4UbFBi8qqIiVW3g4euHrEWF8+bjS9iPQBDV/hiDwjEptN/tRV2LTqzQXUl7tioKzIBDk1rPs61bDYSlMv7Yq3y7PYblX91LFrjORs43XQLgL6LqPvND/l+d6k+CPZtTF/tcCz3FggNc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784918928; c=relaxed/simple; bh=vn981Z9KchS2pJbmCmmmAMnTj9mb2rI/7q94gTXhQtA=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=kIj/rvtGwmNl3LLl4NNxzRZR4RcJlMuSaqWRxzyOfaqBXYId4kHQ08pp2rAxJ4Gp0rJIDVgtW/XzYzknER/2JBLCtQ9E+Gq5ArvmAx44bCRN/x6unZU5bXb9e5XZdoPXIJvyVncLDRttNWM1zWoQwXoU6q72MxMM8WmiAKXI0Sk= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=fgzv/HRT; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="fgzv/HRT" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 5B7381F000E9; Fri, 24 Jul 2026 18:48:45 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1784918927; bh=CqEW7l6AG8iqRWq/zb16CXiDHGZlqAaWI/Tl1NSoTUg=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=fgzv/HRTcwYMAihvkw0CrcHkZXL70ubrFGsRZYmMHdlj+0kbOIucftR6RWdHGK1T1 GDhNixpIb6LWRgONJ3vUgxG3l9AmweCzKB2kJJi5M0bxf1wW7xwiUwW675wkkfGgij Sv9m3JIFbws5QKOedwAiI2dynIOupIWQEOechHS+VXK5BU0evODs5KTcwTD9jHShqr QTbxZ9hDc4uJT0e7/Vbl0A0cOa9kXefz13EY9upIWFM8BAGJzKwJgj0tRT9d10WP1s y0C2QtAQue4vsnori8Ir5Pt9THZoxbDHs+1rOWTS3h7adwZTucw/fIdfra9ucSVV35 R35jxkCcPTz2g== Message-ID: <177a1767-c782-4210-b377-44c520c74619@kernel.org> Date: Fri, 24 Jul 2026 20:48:42 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH] mm/thp: order huge zero folio PFN invalidation before removal To: Hengbin Zhang , linux-mm@kvack.org, akpm@linux-foundation.org, ljs@kernel.org Cc: ziy@nvidia.com, linux-kernel@vger.kernel.org References: <20260724100509.2300200-1-uqbarz@gmail.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit Hi, How was this issue identified or observed? On 7/24/26 12:05, Hengbin Zhang wrote: > The nonpersistent huge-zero shrinker removes huge_zero_folio with > xchg() and then invalidates huge_zero_pfn. A concurrent fault can > publish a new folio and its PFN between these operations, after which > the old shrinker invalidates the new generation's PFN identity. You mean, the freeing path does 1) xchg(&huge_zero_folio, NULL); 2) WRITE_ONCE(huge_zero_pfn, ~0UL); Whereby the allocation path does 1) cmpxchg(&huge_zero_folio, NULL, zero_folio) 2) WRITE_ONCE(huge_zero_pfn, folio_pfn(zero_folio)); Sashiko also IMHO correctly complains about a missing barrier when we re-set the refcount. So I wonder whether we should just stop these cmpxch games and just use a spinlock around updating huge_zero_folio+huge_zero_pfn? -- Cheers, David