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Subject: Re: [PATCH v3 2/2] virtiofs: use GFP_NOFS when enqueuing request
 through kworker
To: Miklos Szeredi <miklos@szeredi.hu>
Cc: linux-fsdevel@vger.kernel.org, Vivek Goyal <vgoyal@redhat.com>,
 Stefan Hajnoczi <stefanha@redhat.com>,
 Bernd Schubert <bernd.schubert@fastmail.fm>,
 "Michael S . Tsirkin" <mst@redhat.com>, Matthew Wilcox
 <willy@infradead.org>, Benjamin Coddington <bcodding@redhat.com>,
 linux-kernel@vger.kernel.org, virtualization@lists.linux.dev,
 houtao1@huawei.com
References: <20240426143903.1305919-1-houtao@huaweicloud.com>
 <20240426143903.1305919-3-houtao@huaweicloud.com>
 <CAJfpegv5GCiF6PDguR7FyCJ_9osytFuy1UDtJnqwu6WDUCU+jg@mail.gmail.com>
From: Hou Tao <houtao@huaweicloud.com>
Message-ID: <5253544f-9855-af61-1d67-69b599ddefb9@huaweicloud.com>
Date: Sat, 11 May 2024 09:09:53 +0800
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Hi,

On 5/10/2024 7:19 PM, Miklos Szeredi wrote:
> On Fri, 26 Apr 2024 at 16:38, Hou Tao <houtao@huaweicloud.com> 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 the sg array may be greater than PAGE_SIZE, use
>> GFP_NOFS instead of GFP_ATOMIC to lower the possibility of memory
>> allocation failure and to avoid unnecessarily depleting the atomic
>> reserves. GFP_NOFS is not passed to virtio_fs_enqueue_req() directly,
>> GFP_KERNEL and memalloc_nofs_{save|restore} helpers are used instead.
> Makes sense.
>
> However, I don't understand why the GFP_NOFS behavior is optional. It
> should work when queuing the request for the first time as well, no?

No. fuse_request_queue_background() may call queue_request_and_unlock()
with fc->bg_lock being held and bg_lock is a spin-lock, so as for now it
is bad to call kmalloc(GFP_NOFS) with a spin-lock being held. The
acquisition of fc->bg_lock in  fuse_request_queue_background() may could
be optimized, but I will leave it for future work.
> Thanks,
> Miklos
> .