From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-we0-f179.google.com (mail-we0-f179.google.com [74.125.82.179]) by kanga.kvack.org (Postfix) with ESMTP id 6DE6C6B0038 for ; Fri, 20 Mar 2015 09:14:57 -0400 (EDT) Received: by wegp1 with SMTP id p1so81846456weg.1 for ; Fri, 20 Mar 2015 06:14:57 -0700 (PDT) Received: from mx2.suse.de (cantor2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id he10si6963659wjc.188.2015.03.20.06.14.55 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Fri, 20 Mar 2015 06:14:55 -0700 (PDT) Date: Fri, 20 Mar 2015 14:14:53 +0100 From: Michal Hocko Subject: Re: [PATCH] mm: Use GFP_KERNEL allocation for the page cache in page_cache_read Message-ID: <20150320131453.GA4821@dhcp22.suse.cz> References: <1426687766-518-1-git-send-email-mhocko@suse.cz> <55098F3B.7070000@redhat.com> <20150318145528.GK17241@dhcp22.suse.cz> <20150319071439.GE28621@dastard> <20150319124441.GC12466@dhcp22.suse.cz> <20150320034820.GH28621@dastard> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150320034820.GH28621@dastard> Sender: owner-linux-mm@kvack.org List-ID: To: Dave Chinner Cc: Rik van Riel , Andrew Morton , Al Viro , Johannes Weiner , Mel Gorman , Neil Brown , Tetsuo Handa , Sage Weil , Mark Fasheh , linux-mm@kvack.org, LKML On Fri 20-03-15 14:48:20, Dave Chinner wrote: > On Thu, Mar 19, 2015 at 01:44:41PM +0100, Michal Hocko wrote: > > On Thu 19-03-15 18:14:39, Dave Chinner wrote: > > > On Wed, Mar 18, 2015 at 03:55:28PM +0100, Michal Hocko wrote: > > > > On Wed 18-03-15 10:44:11, Rik van Riel wrote: > > > > > On 03/18/2015 10:09 AM, Michal Hocko wrote: > > > > > > page_cache_read has been historically using page_cache_alloc_cold to > > > > > > allocate a new page. This means that mapping_gfp_mask is used as the > > > > > > base for the gfp_mask. Many filesystems are setting this mask to > > > > > > GFP_NOFS to prevent from fs recursion issues. page_cache_read is, > > > > > > however, not called from the fs layer > > > > > > > > > > Is that true for filesystems that have directories in > > > > > the page cache? > > > > > > > > I haven't found any explicit callers of filemap_fault except for ocfs2 > > > > and ceph and those seem OK to me. Which filesystems you have in mind? > > > > > > Just about every major filesystem calls filemap_fault through the > > > .fault callout. > > > > That is right but the callback is called from the VM layer where we > > obviously do not take any fs locks (we are holding only mmap_sem > > for reading). > > Those who call filemap_fault directly (ocfs2 and ceph) and those > > who call the callback directly: qxl_ttm_fault, radeon_ttm_fault, > > kernfs_vma_fault, shm_fault seem to be safe from the reclaim recursion > > POV. radeon_ttm_fault takes a lock for reading but that one doesn't seem > > to be used from the reclaim context. > > > > Or did I miss your point? Are you concerned about some fs overloading > > filemap_fault and do some locking before delegating to filemap_fault? > > The latter: > > https://git.kernel.org/cgit/linux/kernel/git/dgc/linux-xfs.git/commit/?h=xfs-mmap-lock&id=de0e8c20ba3a65b0f15040aabbefdc1999876e6b I will have a look at the code to see what we can do about it. > > > GFP_KERNEL allocation for mappings is simply wrong. All mapping > > > allocations where the caller cannot pass a gfp_mask need to obey > > > the mapping_gfp_mask that is set by the mapping owner.... > > > > Hmm, I thought this is true only when the function might be called from > > the fs path. > > How do you know in, say, mpage_readpages, you aren't being called > from a fs path that holds locks? e.g. we can get there from ext4 > doing readdir, so it is holding an i_mutex lock at that point. > > Many other paths into mpages_readpages don't hold locks, but there > are some that do, and those that do need functionals like this to > obey the mapping_gfp_mask because it is set appropriately for the > allocation context of the inode that owns the mapping.... What about the following? ---