Re: [PATCH v4] fuse: back uncached readdir buffers with pages

[Matt Ochs]

> On Jun 11, 2026, at 08:16, Miklos Szeredi <miklos@xxxxxxxxxx> wrote:
> On Tue, 26 May 2026 at 17:20, Matthew R. Ochs <mochs@xxxxxxxxxx> wrote:
>> 
>> 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.
>> 
>> Fixes: dabb90391028 ("fuse: increase readdir buffer size")
>> Cc: stable@xxxxxxxxxxxxxxx
>> Signed-off-by: Matthew R. Ochs <mochs@xxxxxxxxxx>
> 
> Applied with cleanups.  Please verify:
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/mszeredi/fuse.git/commit/?h=for-next&id=2fcb1dd15faba5657b825cc5d54423251c70e79a

Thanks Miklos.

I have verified this patch.


-matt