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

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Date: Fri, 5 Jan 2024 20:57:55 +0000
From: Matthew Wilcox <willy@...radead.org>
To: Vivek Goyal <vgoyal@...hat.com>
Cc: Hou Tao <houtao@...weicloud.com>, linux-fsdevel@...r.kernel.org,
	Miklos Szeredi <miklos@...redi.hu>,
	Stefan Hajnoczi <stefanha@...hat.com>, linux-kernel@...r.kernel.org,
	virtualization@...ts.linux.dev, houtao1@...wei.com
Subject: Re: [PATCH v2] virtiofs: use GFP_NOFS when enqueuing request through
 kworker

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@...wei.com>
> > > > 
> > > > 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.