Re: [PATCH] 9p/virtio: restrict page pinning to user_backed_iter() iovec

[Christoph Hellwig <hch@infradead.org>]

On Wed, Dec 10, 2025 at 06:04:23AM +0900, Dominique Martinet via B4 Relay wrote:
> From: Dominique Martinet <asmadeus@codewreck.org>
> 
> When doing a loop mount of a filesystem over 9p, read requests can come
> from unexpected places and blow up as reported by Chris Arges with this
> reproducer:
> ```
> dd if=/dev/zero of=./xfs.img bs=1M count=300
> yes | mkfs.xfs -b size=8192 ./xfs.img
> rm -rf ./mount && mkdir -p ./mount
> mount -o loop ./xfs.img ./mount

We should really wire this up to xfstests so that all file systems
see the pattern of kmalloc allocations passed into the block layer
and then on to the direct I/O code.

> The problem is that iov_iter_get_pages_alloc2() apparently cannot be
> called on folios (as illustrated by the backtrace below), so limit what
> iov we can pin from !iov_iter_is_kvec() to user_backed_iter()

As willy pointed out this is a kmalloc.

And 9p (just like NFS) really needs to switch away from
iov_iter_get_pages_alloc2 to iov_iter_extract_pages, which handles not
just this perfectly fine but also fixes various other issues.

Note that the networking code still wants special treatment for kmalloc
pages, so you might have more work there.