Re: [PATCH RFC] ext4: use __GFP_NOFAIL in ext4_free_blocks()

From: Michal Hocko
Date: Thu Feb 25 2016 - 04:08:50 EST

On Thu 25-02-16 11:01:32, Nikolay Borisov wrote:
> On 02/24/2016 07:09 PM, Konstantin Khlebnikov wrote:
> > This might be unexpected but pages allocated for sbi->s_buddy_cache are
> > charged to current memory cgroup. So, GFP_NOFS allocation could fail if
> > current task has been killed by OOM or if current memory cgroup has no
> > free memory left. Block allocator cannot handle such failures here yet.
> >
> > Signed-off-by: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
> Adding new users of GFP_NOFAIL is deprecated.

This is not true. GFP_NOFAIL should be used where the allocation failure
is no tolleratable and it is much more preferrable to doing an opencoded
endless loop over page allocator.

> Where exactly does the
> block allocator fail, I skimmed the code and failing ext4_mb_load_buddy
> seems to be handled at all call sites. There are some BUG_ONs but from
> the comments there I guess they should occur when we try to find a page
> and not allocate a new one?

I have posted a similar patch last year:
because I could see emergency reboots when GFP_NOFS allocations were
allowed to fail.

Michal Hocko