Re: [PATCH v2] fuse: back uncached readdir buffers with pages

[Matt Ochs <mochs@nvidia.com>]

> On Apr 29, 2026, at 02:27, Miklos Szeredi <miklos@szeredi.hu> wrote:
> 
> On Wed, 29 Apr 2026 at 01:30, Matthew R. Ochs <mochs@nvidia.com> wrote:
> 
>> The larger buffer is also 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 readdir buffer can
>> therefore require a multi-megabyte contiguous atomic allocation and fail
>> with -ENOMEM.
> 
> Shouldn't this be max_read?  Here "read" and "write" refer to
> direction of I/O on the filesystem, not on the fuse device (see
> fuse/file.c)

Thanks, the read/write direction point makes sense.

I tested changing the cap to fc->max_read only, but that reproduces the
original virtiofs failure on the 4K-host/64K-guest setup. The runtime
values for the failing READDIR are:

PAGE_SIZE=65536
fc->max_pages=124
fc->max_read=4294967295
fc->max_write=1048576
max_bufsize=8126464
nr_pages=124

So for this virtiofs mount, fc->max_read is effectively unlimited, while
virtiofsd advertises its 1 MiB MAX_BUFFER_SIZE through max_write and
rejects READDIR sizes above that limit.

Do you prefer handling this locally in fuse_readdir_uncached(), for
example by capping the request with all available limits:

min3_t(size_t, fc->max_pages << PAGE_SHIFT, fc->max_read, fc->max_write)

Or should virtiofs/FUSE instead make fc->max_read reflect this byte-sized
buffer limit before readdir uses it?

I will address the other cleanup comments in v3: drop the cast, keep the
clamp-style sizing, use release_pages(), and remove the nomem double jump.


-matt