Re: implicit AOP_FLAG_NOFS for grab_cache_page_write_begin

[Christoph Hellwig]

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.