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

[Michael S. Tsirkin]

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@xxxxxxxxxx>
> >>
> >> 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.

> >
> >
> >> 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@xxxxxxxxxxxxxxx/
> >>
> >> [1]: https://lore.kernel.org/linux-fsdevel/20240105105305.4052672-1-houtao@xxxxxxxxxxxxxxx/
> >>
> >> 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