On Fri 16-12-16 17:47:25, Chris Mason wrote: > On 12/16/2016 05:14 PM, Michal Hocko wrote: > > On Fri 16-12-16 13:15:18, Chris Mason wrote: > > > On 12/16/2016 02:39 AM, Michal Hocko wrote: > > [...] > > > > I believe the right way to go around this is to pursue what I've started > > > > in [1]. I will try to prepare something for testing today for you. Stay > > > > tuned. But I would be really happy if somebody from the btrfs camp could > > > > check the NOFS aspect of this allocation. We have already seen > > > > allocation stalls from this path quite recently > > > > > > Just double checking, are you asking why we're using GFP_NOFS to avoid going > > > into btrfs from the btrfs writepages call, or are you asking why we aren't > > > allowing highmem? > > > > I am more interested in the NOFS part. Why cannot this be a full > > GFP_KERNEL context? What kind of locks we would lock up when recursing > > to the fs via slab shrinkers? > > > > Since this is our writepages call, any jump into direct reclaim would go to > writepage, which would end up calling the same set of code to read metadata > blocks, which would do a GFP_KERNEL allocation and end up back in writepage > again. But we are not doing pageout on the page cache from the direct reclaim for a long time. So basically the only way to recurse back to the fs code is via slab ([di]cache) shrinkers. Are those a problem as well? -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>