linux-kernel - Re: [PATCH v2 1/6] fuse: limit the length of ITER_KVEC dio by max

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Date: Sat, 9 Mar 2024 12:26:52 +0800
From: Hou Tao <houtao@...weicloud.com>
To: Miklos Szeredi <miklos@...redi.hu>
Cc: linux-fsdevel@...r.kernel.org, Vivek Goyal <vgoyal@...hat.com>,
 Stefan Hajnoczi <stefanha@...hat.com>,
 Bernd Schubert <bernd.schubert@...tmail.fm>,
 "Michael S . Tsirkin" <mst@...hat.com>, Matthew Wilcox
 <willy@...radead.org>, Benjamin Coddington <bcodding@...hat.com>,
 linux-kernel@...r.kernel.org, virtualization@...ts.linux.dev,
 houtao1@...wei.com
Subject: Re: [PATCH v2 1/6] fuse: limit the length of ITER_KVEC dio by
 max_pages

Hi,

On 3/1/2024 9:42 PM, Miklos Szeredi wrote:
> On Wed, 28 Feb 2024 at 15:40, Hou Tao <houtao@...weicloud.com> wrote:
>
>> So instead of limiting both the values of max_read and max_write in
>> kernel, capping the maximal length of kvec iter IO by using max_pages in
>> fuse_direct_io() just like it does for ubuf/iovec iter IO. Now the max
>> value for max_pages is 256, so on host with 4KB page size, the maximal
>> size passed to kmalloc() in copy_args_to_argbuf() is about 1MB+40B. The
>> allocation of 2MB of physically contiguous memory will still incur
>> significant stress on the memory subsystem, but the warning is fixed.
>> Additionally, the requirement for huge physically contiguous memory will
>> be removed in the following patch.
> So the issue will be fixed properly by following patches?
>
> In that case this patch could be omitted, right?

Sorry for the late reply. Being busy with off-site workshop these days.

No, this patch is still necessary and it is used to limit the number of
scatterlist used for fuse request and reply in virtio-fs. If the length
of out_args[0].size is not limited, the number of scatterlist used to
map the fuse request may be greater than the queue size of virtio-queue
and the fuse request may hang forever.

>
> Thanks,
> Miklos