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

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From: Hou Tao <houtao1@xxxxxxxxxx>


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:

  * 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(-)


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