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Date: Thu, 19 Mar 2015 14:55:58 +0100
From: Michal Hocko <mhocko@suse.cz>
Subject: Re: [PATCH] mm: Use GFP_KERNEL allocation for the page cache in
 page_cache_read
Message-ID: <20150319135558.GD12466@dhcp22.suse.cz>
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 <20150318154540.GN17241@dhcp22.suse.cz>
 <20150319083835.2115ba11@notabene.brown>
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To: NeilBrown <neilb@suse.de>
Cc: Andrew Morton <akpm@linux-foundation.org>, Al Viro <viro@zeniv.linux.org.uk>, Johannes Weiner <hannes@cmpxchg.org>, Mel Gorman <mgorman@suse.de>, Rik van Riel <riel@redhat.com>, Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>, Sage Weil <sage@inktank.com>, Mark Fasheh <mfasheh@suse.com>, linux-mm@kvack.org, LKML <linux-kernel@vger.kernel.org>

On Thu 19-03-15 08:38:35, Neil Brown wrote:
[...]
> Nearly half the places in the kernel which call mapping_gfp_mask() remove the
> __GFP_FS bit.
> 
> That suggests to me that it might make sense to have
>    mapping_gfp_mask_fs()
> and
>    mapping_gfp_mask_nofs()
>
> and let the presence of __GFP_FS (and __GFP_IO) be determined by the
> call-site rather than the filesystem.

Sounds reasonable to me but filesystems tend to use this in a very
different ways.
- xfs drops GFP_FS in xfs_setup_inode so all page cache allocations are
  NOFS.
- reiserfs drops GFP_FS only before calling read_mapping_page in
  reiserfs_get_page and never restores the original mask.
- btrfs doesn't seem to rely on mapping_gfp_mask for anything other than
  btree_inode (unless it gets inherrited in a way I haven't noticed).
- ext* doesn't seem to rely on the mapping gfp mask at all.

So it is not clear to me how we should change that into callsites. But I
guess we can change at least the page fault path like the following. I
like it much more than the previous way which is too hackish.
---