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 E191D44213B for ; Wed, 8 Jul 2026 16:50:59 +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=1783529463; cv=none; b=obWBEEhYx0moVr4ZGIkxbfSUs5C7EyUxa606c0dvvBig4DXtNHgMsUnYOp25wx4utkOwTiX2iqZnxetZUrqa1cYF5evIOAfLxay/E7Oisny3AcErgfE/j3myQWFKgojS2ttqesOQpzqTz+RjUGvZM1FyBtjnWX3vINz3EhTjaj0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783529463; c=relaxed/simple; bh=YpMzrEioy003Y+OJ41xCvapAjjL4WlzuKOcyDMEbMkM=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=LXPUhKFbooxJF0jKBdneZ/NfFkwqhNEN7XWjpUEa5QjHbRgeRgkqNriOlNz8V8T/HfY5M0mtYImEkSaMK0VdjGuuNoK4E1dfmEZcHBaOKsgDk4P916ejmxIfIwjLpjVgEB0lKqctcWGNGPA7Ep2nGUW53931RZ/sUbRefw2i0G8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=LszKS/nr; 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="LszKS/nr" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 164B71F000E9; Wed, 8 Jul 2026 16:50:52 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1783529456; bh=DimxgE0Cs5OU8BmGADyJ6jo4V5QvvwmBdFh5AA5TCy8=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=LszKS/nrC3Kv6lPXQ5Q+kaRZJwFINNtdH8tjVx9JAC+ct+iR0lDJBMoz4rctUzoqk QICMkIfMzjNG1b53zD5t+37p3cCcCuyeFnhLTdFHLNL0pfPrbQrhgYgQ5Kuwf7F/FJ EX8jiIhpBDsgPtSnin0Zd+t7FI7izcl4WYQ3LRpST/9NP+w/+JNLkLPqtQ6P035TUU hZEODXnepGzSxEMJF7nAWLI4bh0KwbVCWK65djnNjzYUrpVRJZ0v/hO80Fdv6UxM/e yxOl6CPc+Bb/59qCeb5+fABG78zs+n+etnlZ0pHu5DDM4LPn3WCW+RrdHcAmsH+5Hy QSiYKYhM3PX8A== Message-ID: Date: Wed, 8 Jul 2026 18:50:51 +0200 Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH stable v2] mm/khugepaged: write all dirty file folios when collapsing To: Pedro Falcato , Andrew Morton , Lorenzo Stoakes Cc: stable@vger.kernel.org, Alexander Viro , Christian Brauner , Jan Kara , Matthew Wilcox , Song Liu , Eric Hagberg , Zi Yan , Gregg Leventhal , Lance Yang References: <20260708151357.353173-1-pfalcato@suse.de> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 7/8/26 17:13, Pedro Falcato wrote: > [There is no upstream commit, as this code was removed by upstream > commit 044925f9b565 ("mm: fs: remove filemap_nr_thps*() functions and their users")] > > As-is, khugepaged and writable-file opening exclude each other. A file > cannot be open writeable and have THPs (because the filesystem is not aware > of them). khugepaged will never collapse file pages for files that are > opened writeable. On an open(O_RDWR/O_WRONLY), the page cache for that > particular file is dropped. This is fine because nothing could've been > dirtied. > > However, there is an edge-case: collapse_file() might not be able to > coexist with concurrent writers, but it can coexist with dirty folios > (from previous writers). Therefore, the following can happen: > > open(file, O_RDWR) > write(file) > close(file) > madvise(file_mapping, MADV_COLLAPSE, some non-dirty range) > open(file, O_RDWR) > nr_thps > 0 > truncate_inode_pages() > /* THPs are cleared out, but so are the dirty folios */ > > When this edge-case happens, there is data loss, as the dirty folios are > fully discarded. > > Fix it by fully writing back the page cache (and waiting) when collapsing > file THPs. Doing so provides the guarantee that no dirty folio will be > observed while there are active THPs. To fully ensure this is safe, the > invalidate_lock needs to be held while doing the writeout, so that > do_dentry_open()'s page cache truncation excludes this write-and-wait. > > As a side effect, move the nr_thps counter bumping outside the i_pages > lock. This is correct since the counter itself is an atomic_t and the > producer <-> consumer correctness is provided by a full memory barrier: > smp_mb() in collapse_file()/memory barrier implied by full ordering in > get_write_access() -> atomic_inc_unless_negative(). > > Cc: stable@vger.kernel.org > Cc: Alexander Viro > Cc: Christian Brauner > Cc: Jan Kara > Cc: Matthew Wilcox > Cc: Song Liu > Cc: Eric Hagberg > Cc: Zi Yan > Fixes: 99cb0dbd47a1 ("mm,thp: add read-only THP support for (non-shmem) FS") > Reported-by: Gregg Leventhal > Closes: https://lore.kernel.org/linux-mm/CAFN_u7H_0ECF3jixP=T=U7AH5=Q3wQNvJMo8an3VqUDMerQfUw@mail.gmail.com/ > Tested-by: Zi Yan > Tested-by: Lance Yang > Signed-off-by: Pedro Falcato > --- Acked-by: David Hildenbrand (Arm) -- Cheers, David