Re: __link_block_group uses GFP_KERNEL

[David Sterba]

On Sat, Mar 25, 2017 at 09:48:28AM +0300, Denis Kirjanov wrote:
> On 3/25/17, Jeff Mahoney <jeffm@xxxxxxxx> wrote:
> > On 3/24/17 5:02 AM, Denis Kirjanov wrote:
> >> Hi guys,
> >>
> >> Looks like that current code does GFP_KERNEL allocation inside
> >> __link_block_group.
> >> the function invokes kobject_add and internally creates sysfs files
> >> with the GFP_KERNEL flag set.
> >
> > Yep, that's a bug.
> >
> >> But since do_chunk_alloc executes insides the btrfs transaction it's
> >> not allowed to sleep.
> >
> > It's allowed to sleep but isn't allowed to do reclaim that involves file
> > system writeback.  Michal Hocko's allocation context idea would fix
> > this, but it's not there yet, so we'll need to defer the kobject_add
> > until we can use GFP_KERNEL.
> 
> Ok, I see. Can you point out to the initial patchset?

https://lwn.net/Articles/716323/

Fixing this properly is a lot of work so we might need to add a
temporary workaround, as Jeff suggests, to move calling into sysfs to a
later time.