Re: [PATCH v2 0/6] virtiofs: fix the warning for ITER_KVEC dio

[Bernd Schubert <bernd.schubert@fastmail.fm>]

On 4/22/24 22:06, Michael S. Tsirkin wrote:
> On Tue, Apr 09, 2024 at 09:48:08AM +0800, Hou Tao wrote:
>> Hi,
>>
>> On 4/8/2024 3:45 PM, Michael S. Tsirkin wrote:
>>> On Wed, Feb 28, 2024 at 10:41:20PM +0800, Hou Tao wrote:
>>>> From: Hou Tao <houtao1@huawei.com>
>>>>
>>>> Hi,
>>>>
>>>> The patch set aims to fix the warning related to an abnormal size
>>>> parameter of kmalloc() in virtiofs. The warning occurred when attempting
>>>> to insert a 10MB sized kernel module kept in a virtiofs with cache
>>>> disabled. As analyzed in patch #1, the root cause is that the length of
>>>> the read buffer is no limited, and the read buffer is passed directly to
>>>> virtiofs through out_args[0].value. Therefore patch #1 limits the
>>>> length of the read buffer passed to virtiofs by using max_pages. However
>>>> it is not enough, because now the maximal value of max_pages is 256.
>>>> Consequently, when reading a 10MB-sized kernel module, the length of the
>>>> bounce buffer in virtiofs will be 40 + (256 * 4096), and kmalloc will
>>>> try to allocate 2MB from memory subsystem. The request for 2MB of
>>>> physically contiguous memory significantly stress the memory subsystem
>>>> and may fail indefinitely on hosts with fragmented memory. To address
>>>> this, patch #2~#5 use scattered pages in a bio_vec to replace the
>>>> kmalloc-allocated bounce buffer when the length of the bounce buffer for
>>>> KVEC_ITER dio is larger than PAGE_SIZE. The final issue with the
>>>> allocation of the bounce buffer and sg array in virtiofs is that
>>>> GFP_ATOMIC is used even when the allocation occurs in a kworker context.
>>>> Therefore the last patch uses GFP_NOFS for the allocation of both sg
>>>> array and bounce buffer when initiated by the kworker. For more details,
>>>> please check the individual patches.
>>>>
>>>> As usual, comments are always welcome.
>>>>
>>>> Change Log:
>>> Bernd should I just merge the patchset as is?
>>> It seems to fix a real problem and no one has the
>>> time to work on a better fix .... WDYT?
>>
>> Sorry for the long delay. I am just start to prepare for v3. In v3, I
>> plan to avoid the unnecessary memory copy between fuse args and bio_vec.
>> Will post it before next week.
> 
> Didn't happen before this week apparently.

Hi Michael,

sorry for my later reply, I had been totally busy for the last weeks as
well. Also I can't decide to merge it - I'm not the official fuse
maintainer...
From my point of view, patch 1 is just missing to set the actual limit
and then would be clear and easy back-portable bug fix.
Not promised, I will try it out if I find a bit time tomorrow.


Bernd