Re: [PATCH] mm: Use GFP_KERNEL allocation for the page cache in page_cache_read

From: Dave Chinner
Date: Thu Mar 19 2015 - 03:15:43 EST


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.

C symbol: filemap_fault

File Function Line
0 9p/vfs_file.c <global> 831 .fault = filemap_fault,
1 9p/vfs_file.c <global> 838 .fault = filemap_fault,
2 btrfs/file.c <global> 2081 .fault = filemap_fault,
3 cifs/file.c <global> 3242 .fault = filemap_fault,
4 ext4/file.c <global> 215 .fault = filemap_fault,
5 f2fs/file.c <global> 93 .fault = filemap_fault,
6 fuse/file.c <global> 2062 .fault = filemap_fault,
7 gfs2/file.c <global> 498 .fault = filemap_fault,
8 nfs/file.c <global> 653 .fault = filemap_fault,
9 nilfs2/file.c <global> 128 .fault = filemap_fault,
a ubifs/file.c <global> 1536 .fault = filemap_fault,
b xfs/xfs_file.c <global> 1420 .fault = filemap_fault,


> Btw. how would that work as we already have GFP_KERNEL allocation few
> lines below?

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....

Cheers,

Dave.
--
Dave Chinner
david@xxxxxxxxxxxxx
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