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Date: Mon, 8 Apr 2024 03:45:51 -0400
From: "Michael S. Tsirkin" <mst@redhat.com>
To: Hou Tao <houtao@huaweicloud.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
Subject: Re: [PATCH v2 0/6] virtiofs: fix the warning for ITER_KVEC dio
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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?


> 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