On Sat, 2007-10-06 at 21:43 +0100, Hugh Dickins wrote:
> Building in a filesystem on a loop device on a tmpfs file can hang when
> swapping, the loop thread caught in that infamous throttle_vm_writeout.
> 
> In theory this is a long standing problem, which I've either never seen
> in practice, or long ago suppressed the recollection, after discounting
> my load and my tmpfs size as unrealistically high.  But now, with the
> new aops, it has become easy to hang on one machine.
> 
> Loop used to grab_cache_page before the old prepare_write to tmpfs,
> which seems to have been enough to free up some memory for any swapin
> needed; but the new write_begin lets tmpfs find or allocate the page
> (much nicer, since grab_cache_page missed tmpfs pages in swapcache).
> 
> When allocating a fresh page, tmpfs respects loop's mapping_gfp_mask,
> which has __GFP_IO|__GFP_FS stripped off, and throttle_vm_writeout is
> designed to break out when __GFP_IO or GFP_FS is unset; but when tmfps
> swaps in, read_swap_cache_async allocates with GFP_HIGHUSER_MOVABLE
> regardless of the mapping_gfp_mask - hence the hang.
> 
> So, pass gfp_mask down the line from shmem_getpage to shmem_swapin
> to swapin_readahead to read_swap_cache_async to add_to_swap_cache.
> 
> Signed-off-by: Hugh Dickins <hugh@veritas.com>

Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>