linux-kernel - Re: [PATCH] 9p/virtio: restrict page pinning to user_backed

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Message-ID: <3385064.aeNJFYEL58@weasel>
Date: Wed, 10 Dec 2025 14:33:56 +0100
From: Christian Schoenebeck <linux_oss@...debyte.com>
To: asmadeus@...ewreck.org, Christoph Hellwig <hch@...radead.org>
Cc: Eric Van Hensbergen <ericvh@...nel.org>,
 Latchesar Ionkov <lucho@...kov.net>, v9fs@...ts.linux.dev,
 linux-kernel@...r.kernel.org, David Howells <dhowells@...hat.com>,
 Matthew Wilcox <willy@...radead.org>, linux-fsdevel@...r.kernel.org,
 Chris Arges <carges@...udflare.com>
Subject:
 Re: [PATCH] 9p/virtio: restrict page pinning to user_backed_iter() iovec

On Wednesday, 10 December 2025 07:04:30 CET Christoph Hellwig wrote:
> On Wed, Dec 10, 2025 at 06:04:23AM +0900, Dominique Martinet via B4 Relay
> wrote:
[...]
> > 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.

But couldn't this patch be used as a preliminary solution for this issue 
before switching to iov_iter_extract_pages(), as the latter does not look like 
a trivial change?

Maybe I'm still missing something important here, not sure.

/Christian