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Subject: Re: [PATCH v2 0/6] virtiofs: fix the warning for ITER_KVEC dio
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: linux-fsdevel@vger.kernel.org, Miklos Szeredi <miklos@szeredi.hu>,
 Vivek Goyal <vgoyal@redhat.com>, Stefan Hajnoczi <stefanha@redhat.com>,
 Bernd Schubert <bernd.schubert@fastmail.fm>,
 Matthew Wilcox <willy@infradead.org>,
 Benjamin Coddington <bcodding@redhat.com>, linux-kernel@vger.kernel.org,
 virtualization@lists.linux.dev, houtao1@huawei.com
References: <20240228144126.2864064-1-houtao@huaweicloud.com>
 <20240408034514-mutt-send-email-mst@kernel.org>
 <413bd868-a16b-f024-0098-3c70f7808d3c@huaweicloud.com>
 <20240422160615-mutt-send-email-mst@kernel.org>
From: Hou Tao <houtao@huaweicloud.com>
Message-ID: <3a287e9b-55ad-dda9-5f53-c12536bad31d@huaweicloud.com>
Date: Tue, 23 Apr 2024 21:25:33 +0800
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On 4/23/2024 4:06 AM, 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.

Sorry for failing to make it this week. Being busy these two weeks. Hope
to send v3 out before the end of April.
>
>>>
>>>> v2:
>>>>   * limit the length of ITER_KVEC dio by max_pages instead of the
>>>>     newly-introduced max_nopage_rw. Using max_pages make the ITER_KVEC
>>>>     dio being consistent with other rw operations.
>>>>   * replace kmalloc-allocated bounce buffer by using a bounce buffer
>>>>     backed by scattered pages when the length of the bounce buffer for
>>>>     KVEC_ITER dio is larger than PAG_SIZE, so even on hosts with
>>>>     fragmented memory, the KVEC_ITER dio can be handled normally by
>>>>     virtiofs. (Bernd Schubert)
>>>>   * merge the GFP_NOFS patch [1] into this patch-set and use
>>>>     memalloc_nofs_{save|restore}+GFP_KERNEL instead of GFP_NOFS
>>>>     (Benjamin Coddington)
>>>>
>>>> v1: https://lore.kernel.org/linux-fsdevel/20240103105929.1902658-1-houtao@huaweicloud.com/
>>>>
>>>> [1]: https://lore.kernel.org/linux-fsdevel/20240105105305.4052672-1-houtao@huaweicloud.com/
>>>>
>>>> Hou Tao (6):
>>>>   fuse: limit the length of ITER_KVEC dio by max_pages
>>>>   virtiofs: move alloc/free of argbuf into separated helpers
>>>>   virtiofs: factor out more common methods for argbuf
>>>>   virtiofs: support bounce buffer backed by scattered pages
>>>>   virtiofs: use scattered bounce buffer for ITER_KVEC dio
>>>>   virtiofs: use GFP_NOFS when enqueuing request through kworker
>>>>
>>>>  fs/fuse/file.c      |  12 +-
>>>>  fs/fuse/virtio_fs.c | 336 +++++++++++++++++++++++++++++++++++++-------
>>>>  2 files changed, 296 insertions(+), 52 deletions(-)
>>>>
>>>> -- 
>>>> 2.29.2