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Date: Fri, 5 Jan 2024 15:41:48 -0500
From: Vivek Goyal <vgoyal@redhat.com>
To: Matthew Wilcox <willy@infradead.org>
Cc: Hou Tao <houtao@huaweicloud.com>, linux-fsdevel@vger.kernel.org,
	Miklos Szeredi <miklos@szeredi.hu>,
	Stefan Hajnoczi <stefanha@redhat.com>, linux-kernel@vger.kernel.org,
	virtualization@lists.linux.dev, houtao1@huawei.com
Subject: Re: [PATCH v2] virtiofs: use GFP_NOFS when enqueuing request through
 kworker
Message-ID: <ZZhpjEwDwMS_mq-u@redhat.com>
References: <20240105105305.4052672-1-houtao@huaweicloud.com>
 <ZZhjzwnQUEJhNJiq@redhat.com>
 <ZZhkrOdbau2O/B59@casper.infradead.org>
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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@huawei.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