Re: [PATCH 0/2] btrfs: fortification for GFP_NOFS allocations

[Michal Hocko <mhocko@kernel.org>]

On Wed 09-09-15 18:13:39, Vlastimil Babka wrote:
> On 08/19/2015 08:17 PM, Chris Mason wrote:
> >On Wed, Aug 19, 2015 at 02:17:39PM +0200, mhocko@kernel.org wrote:
> >>Hi,
> >>these two patches were sent as a part of a larger RFC which aims at
> >>allowing GFP_NOFS allocations to fail to help sort out memory reclaim
> >>issues bound to the current behavior
> >>(http://marc.info/?l=linux-mm&m=143876830616538&w=2).
> >>
> >>It is clear that move to the GFP_NOFS behavior change is a long term
> >>plan but these patches should be good enough even with that change in
> >>place. It also seems that Chris wasn't opposed and would be willing to
> >>take them http://marc.info/?l=linux-mm&m=143991792427165&w=2 so here we
> >>come. I have rephrased the changeslogs to not refer to the patch which
> >>changes the NOFS behavior.
> >>
> >>Just to clarify. These two patches allowed my particular testcase
> >>(mentioned in the cover referenced above) to survive it doesn't mean
> >>that the failing GFP_NOFS are OK now. I have seen some other places
> >>where GFP_NOFS allocation is followed by BUG_ON(ALLOC_FAILED). I have
> >>not encountered them though.
> >>
> >>Let me know if you would prefer other changes.
> >
> >My plan is to start with these two and take more as required.
> 
> I've previously noticed in __set_extent_bit() things like:
> 
>         if (!prealloc && (mask & __GFP_WAIT)) {
>                 prealloc = alloc_extent_state(mask);
>                 BUG_ON(!prealloc);
>         }
> 
> and later:
> 
>                 prealloc = alloc_extent_state_atomic(prealloc);
>                 BUG_ON(!prealloc);

Yes. I have noticed also many other places:
$ git grep "BUG_ON.*ENOMEM" -- fs/btrfs/ | wc -l
47

I have talked to David Sterba and he said this is on his todo list.
So this will likely take some more time but it is definitely good to
sort out.
-- 
Michal Hocko
SUSE Labs