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 F216E64AA4; Mon, 3 Aug 2026 23:55:20 +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=1785801322; cv=none; b=ECKfxYx4RXsum2dPCL9uGBTaHjQNsrZ6AM5O5wYJZo8bMH2a883pEcjDgyv1gCXorUU696MefihRp3AXleeYKA5duUH/LEdqcAN9l45gcLHWltGgv7XGjh2MZg76lIo3jgLpZzhK0YQsySXJVUry2Nqadu3r5u3NiolTWXJTy2c= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785801322; c=relaxed/simple; bh=dWHgX6/LNshsXdY0gfcQLUIkK6De6AUM9ZeE8zjIEso=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=eaHbDVjlPNdZMWUrNSD4hRkm3VHp+Txwn6wGBEoQSCem7T/gzIE3jsTbaI/D/GkAip9U8pBaNMv3FNKVE22ojOxYmViTnEvsnvPhfhC3GGvdzWAA8J1CWcqZ8a+MwWR9BwksmjuAMR0uNNtD/YssA5tX/oTHkAJti/xA8mHa6fI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=RtGEZUNf; 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="RtGEZUNf" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 77A071F000E9; Mon, 3 Aug 2026 23:55:13 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1785801320; bh=QvcmvXr4acqUWBRh7hziDDp8JEZdMtVWzfIqcampLcw=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=RtGEZUNfVk1jOgNOJwApSJ93O6cVC45ktiN2j42C43XLae4gT7K0HxtErE329zu6Y 4q1mEBzTDnzUbmn1OURLkSbBTmQJewI+Y6tf5wHTwUYU05Z3Meug4Fm+oc4/qqZqXE xSaSxhssLrPHFanyR2w62mPUk2uHFQDsY+APGSl/m/zgFlQfzvQqSXOELjBmVyK5Z2 ZH2j1I/wSxiSXllvvgkwdz24/JoVF76jb877bCjAEig9Bko1o5oI3KFWoKE4hg+/Cl /ZTzrxndYjiGyPujGg8KW0p1CK5WBj4Vxu8QsyZXgGXwSUpyd7LTOBAIHjESgRcrXJ skL+Bc38weOCA== Date: Tue, 4 Aug 2026 07:55:09 +0800 From: Gao Xiang To: Zi Yan Cc: David Hildenbrand , "Matthew Wilcox (Oracle)" , Andrew Morton , Muchun Song , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Baolin Wang , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , Gregory Price , Ying Huang , Alistair Popple , Johannes Weiner , Qi Zheng , Shakeel Butt , Kairui Song , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Gao Xiang , Chao Yu , Jan Kara , Yue Hu , Jeffle Xu , Sandeep Dhavale , Hongbo Li , Chunhai Guo , linux-erofs@lists.ozlabs.org, linux-fsdevel@vger.kernel.org Subject: Re: [PATCH RFC 07/14] fs/erofs: mm/pagemap: add readahead_folio_reverse() to avoid folio->private Message-ID: Mail-Followup-To: Zi Yan , David Hildenbrand , "Matthew Wilcox (Oracle)" , Andrew Morton , Muchun Song , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Baolin Wang , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , Gregory Price , Ying Huang , Alistair Popple , Johannes Weiner , Qi Zheng , Shakeel Butt , Kairui Song , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Gao Xiang , Chao Yu , Jan Kara , Yue Hu , Jeffle Xu , Sandeep Dhavale , Hongbo Li , Chunhai Guo , linux-erofs@lists.ozlabs.org, linux-fsdevel@vger.kernel.org References: <20260731-remove-pg_private-v1-0-142c97ba3562@nvidia.com> <20260731-remove-pg_private-v1-7-142c97ba3562@nvidia.com> Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20260731-remove-pg_private-v1-7-142c97ba3562@nvidia.com> On Fri, Jul 31, 2026 at 10:13:30PM -0400, Zi Yan wrote: > erofs needs to traverse readahead folios in reverse order to achieve > maximum performance by > 1. reading all folios from readahead_folio(); > 2. storing the prior folio pointer in folio->private; > 3. traverse from the last folio to the first one. > > Add readahead_folio_reverse() to achieve the same function without using > folio->private. > > It prepares for a future commit that replaces PG_private checks with > !folio->private checks. After switching the checks, erofs's use of > folio->private without bumping folio refcount can cause unexpected > outcomes, e.g., in filemap_release_folio(), try_to_free_buffers() becomes > reachable. > > No funtional change intended. > > Assisted-by: Claude:claude-opus-4-8 > Assisted-by: Codex:gpt-5 > Signed-off-by: Zi Yan > To: Gao Xiang > To: Chao Yu > To: "Matthew Wilcox (Oracle)" > To: Jan Kara > Cc: Yue Hu > Cc: Jeffle Xu > Cc: Sandeep Dhavale > Cc: Hongbo Li > Cc: Chunhai Guo > Cc: linux-erofs@lists.ozlabs.org > Cc: linux-kernel@vger.kernel.org > Cc: linux-fsdevel@vger.kernel.org > Cc: linux-mm@kvack.org > --- > fs/erofs/zdata.c | 11 ++--------- > include/linux/pagemap.h | 31 +++++++++++++++++++++++++++++++ > 2 files changed, 33 insertions(+), 9 deletions(-) > > diff --git a/fs/erofs/zdata.c b/fs/erofs/zdata.c > index 74520e9102596..b59f2745a8e72 100644 > --- a/fs/erofs/zdata.c > +++ b/fs/erofs/zdata.c > @@ -1902,21 +1902,14 @@ static void z_erofs_readahead(struct readahead_control *rac) > struct inode *realinode = erofs_real_inode(sharedinode, &need_iput); > Z_EROFS_DEFINE_FRONTEND(f, realinode, sharedinode, readahead_pos(rac)); > unsigned int nrpages = readahead_count(rac); > - struct folio *head = NULL, *folio; > + struct folio *folio; > int err; > > trace_erofs_readahead(realinode, readahead_index(rac), nrpages, false); > z_erofs_pcluster_readmore(&f, rac, true); > - while ((folio = readahead_folio(rac))) { > - folio->private = head; > - head = folio; > - } > > /* traverse in reverse order for best metadata I/O performance */ > - while (head) { > - folio = head; > - head = folio_get_private(folio); > - > + while ((folio = readahead_folio_reverse(rac))) { Yes, it's needed due to EROFS compression metadata design and on-demand partial decompression, the last extent in the readahead request can be parsed as a partial extent (means from the starting logical offset of extents to the necessary offset.). Since there may be many extents in a single readahead request, so it needs to iterate backwards here; but the actual compressed data I/Os will be issued forwards. Previously I tend to avoid touching core-mm so it uses folio->private but if MM folks can provide a new helper, that would be very helpful (one more words: all folios are locked in the forward order previously, so it won't have any deadlock risk). Thanks, Gao Xiang