Re: [PATCH v2] virtiofs: use GFP_NOFS when enqueuing request through kworker

[Matthew Wilcox]

On Fri, Jan 05, 2024 at 03:41:48PM -0500, Vivek Goyal wrote:
> On Fri, Jan 05, 2024 at 08:21:00PM +0000, Matthew Wilcox wrote:
> > On Fri, Jan 05, 2024 at 03:17:19PM -0500, Vivek Goyal wrote:
> > > On Fri, Jan 05, 2024 at 06:53:05PM +0800, Hou Tao wrote:
> > > > From: Hou Tao <houtao1@xxxxxxxxxx>
> > > > 
> > > > When invoking virtio_fs_enqueue_req() through kworker, both the
> > > > allocation of the sg array and the bounce buffer still use GFP_ATOMIC.
> > > > Considering the size of both the sg array and the bounce buffer may be
> > > > greater than PAGE_SIZE, use GFP_NOFS instead of GFP_ATOMIC to lower the
> > > > possibility of memory allocation failure.
> > > > 
> > > 
> > > What's the practical benefit of this patch. Looks like if memory
> > > allocation fails, we keep retrying at interval of 1ms and don't
> > > return error to user space.
> > 
> > You don't deplete the atomic reserves unnecessarily?
> 
> Sounds reasonable. 
> 
> With GFP_NOFS specificed, can we still get -ENOMEM? Or this will block
> indefinitely till memory can be allocated. 

If you need the "loop indefinitely" behaviour, that's
GFP_NOFS | __GFP_NOFAIL.  If you're actually doing something yourself
which can free up memory, this is a bad choice.  If you're just sleeping
and retrying, you might as well have the MM do that for you.