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

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Date: Fri, 5 Jan 2024 15:41:48 -0500
From: Vivek Goyal <vgoyal@...hat.com>
To: Matthew Wilcox <willy@...radead.org>
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 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. 

I am trying to figure out with GFP_NOFS, do we still need to check for
-ENOMEM while requeuing the req and asking worker thread to retry after
1ms. 

Thanks
Vivek