Re: Propagating GFP_NOFS inside __vmalloc()

[Dave Chinner <david@fromorbit.com>]

On Wed, Nov 10, 2010 at 10:35:55PM +0100, Ricardo M. Correia wrote:
> On Wed, 2010-11-10 at 21:42 +0100, Ricardo M. Correia wrote:
> > Hi,
> > 
> > As part of Lustre filesystem development, we are running into a
> > situation where we (sporadically) need to call into __vmalloc() from a
> > thread that processes I/Os to disk (it's a long story).
> > 
> > In general, this would be fine as long as we pass GFP_NOFS to
> > __vmalloc(), but the problem is that even if we pass this flag, vmalloc
> > itself sometimes allocates memory with GFP_KERNEL.
> 
> By the way, it seems that existing users in Linus' tree may be
> vulnerable to the same bug that we experienced:
> 
> In GFS:
>     8   1253  fs/gfs2/dir.c <<gfs2_alloc_sort_buffer>>
>              ptr = __vmalloc(size, GFP_NOFS, PAGE_KERNEL);
> 
> The Ceph filesystem:
>   20     22  net/ceph/buffer.c <<ceph_buffer_new>>
>              b->vec.iov_base = __vmalloc(len, gfp, PAGE_KERNEL);
> .. which can be called from:
>    3    560  fs/ceph/inode.c <<fill_inode>>
>              xattr_blob = ceph_buffer_new(iinfo->xattr_len, GFP_NOFS);
> 
> In the MM code:
>   18   5184  mm/page_alloc.c <<alloc_large_system_hash>>
>              table = __vmalloc(size, GFP_ATOMIC, PAGE_KERNEL);
> 
> All of these seem to be vulnerable to GFP_KERNEL allocations from within
> __vmalloc(), at least on x86-64 (as I've detailed below).

Hmmm. I'd say there's a definite possibility that vm_map_ram() as
called from in fs/xfs/linux-2.6/xfs_buf.c needs to use GFP_NOFS
allocation, too. Currently vm_map_ram() just uses GFP_KERNEL
internally, but is certainly being called in contexts where we don't
allow recursion (e.g. in a transaction) so probably should allow a
gfp mask to be passed in....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com