[RFC 0/2] mapping_gfp_mask from the page fault path

[Michal Hocko <mhocko@suse.cz>]

Hi,
I somehow forgot about these patches. The previous version was
posted here: http://marc.info/?l=linux-mm&m=142668784122763&w=2. The
first attempt was broken but even when fixed it seems like ignoring
mapping_gfp_mask in page_cache_read is too fragile because
filesystems might use locks in their filemap_fault handlers
which could trigger recursion problems as pointed out by Dave
http://marc.info/?l=linux-mm&m=142682332032293&w=2.

The first patch should be straightforward fix to obey mapping_gfp_mask
when allocating for mapping. It can be applied even without the second
one.

The second patch is an attempt to handle mapping_gfp_mask from the
page fault path properly. GFP_IOFS should be safe from he page fault
path in general (we would be quite broken otherwise because there
are places where GFP_KERNEL is used - e.g. pte allocation). MM will
communicate this to the fs layer via struct vm_fault::gfp_mask.
If fs needs to change this allocation context in a fs callback it can
overwrite this mask. If the code flow gets back to MM we will obey this
gfp_mask (e.g. in page_cache_read). This should be more appropriate than
following mapping_gfp_mask blindly. See the patch description for more
details.

I am still not sure this is the right way to go so I am sending this as
an RFC so any comments are highly appreciated.

Thanks!