Re: implicit AOP_FLAG_NOFS for grab_cache_page_write_begin

[Michal Hocko]

On Fri 17-04-20 00:29:31, Christoph Hellwig wrote:
> On Wed, Apr 15, 2020 at 09:02:28AM +0200, Michal Hocko wrote:
> > Hi,
> > I have just received a bug report about memcg OOM [1]. The underlying
> > issue is memcg specific but the stack trace made me look at the write(2)
> > patch and I have noticed that iomap_write_begin enforces AOP_FLAG_NOFS
> > which means that all the page cache that has to be allocated is
> > GFP_NOFS. What is the reason for this? Do all filesystems really need
> > the reclaim protection? I was hoping that those filesystems which really
> > need NOFS context would be using the scope API
> > (memalloc_nofs_{save,restore}.
> 
> This comes from the historic XFS code, and this commit from Dave
> in particular:
> 
> commit aea1b9532143218f8599ecedbbd6bfbf812385e1
> Author: Dave Chinner <dchinner@xxxxxxxxxx>
> Date:   Tue Jul 20 17:54:12 2010 +1000
> 
>     xfs: use GFP_NOFS for page cache allocation
> 
>     Avoid a lockdep warning by preventing page cache allocation from
>     recursing back into the filesystem during memory reclaim.

Thanks for digging this up! The changelog is not really clear whether
NOFS is to avoid false possitive lockup warnings or real ones. If the
former then we have grown __GFP_NOLOCKDEP flag to workaround the problem
if the later then can we use memalloc_nofs_{save,restore} in the xfs
specific code please?
-- 
Michal Hocko
SUSE Labs