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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>
From: Hou Tao <houtao@huaweicloud.com>
Message-ID: <413bd868-a16b-f024-0098-3c70f7808d3c@huaweicloud.com>
Date: Tue, 9 Apr 2024 09:48:08 +0800
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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.
>
>
>> 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