fuse: back uncached readdir buffers with pages

Commit dabb90391028 ("fuse: increase readdir buffer size") changed
fuse_readdir_uncached() to size its temporary buffer from ctx->count.
This is useful for overlayfs and other in-kernel callers that use
INT_MAX to indicate an unlimited directory read.

The larger buffer is currently supplied as a kvec output argument. For
virtiofs, kvec arguments are copied through req->argbuf, which is
allocated with kmalloc(..., GFP_ATOMIC). A large uncached readdir buffer
can therefore require a multi-megabyte contiguous atomic allocation
before the request is queued.

Avoid the large bounce-buffer allocation by backing uncached readdir
output with pages and setting out_pages. Transports such as virtiofs can
then pass the pages as scatter-gather entries instead of copying the
output through argbuf.

Map the pages with vm_map_ram() only while parsing the returned dirents.
The existing parser can then continue to use a linear kernel mapping.

[SzM: separate allocation of pages into a helper function]

Fixes: dabb90391028 ("fuse: increase readdir buffer size")
Cc: stable@vger.kernel.org
Signed-off-by: Matthew R. Ochs <mochs@nvidia.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

0 files changed, 0 insertions, 0 deletions