Message-ID: <3918430.kQq0lBPeGt@weasel>
Date: Mon, 15 Dec 2025 12:16:07 +0100
From: Christian Schoenebeck <linux_oss@...debyte.com>
To: Christoph Hellwig <hch@...radead.org>,
 Dominique Martinet <asmadeus@...ewreck.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 Monday, 15 December 2025 08:34:12 CET Dominique Martinet wrote:
> Thanks for having a look
> 
> Christoph Hellwig wrote on Sun, Dec 14, 2025 at 09:55:12PM -0800:
> > > Ok, I don't understand why the current code locks everything down and
> > > wants to use a single scatterlist shared for the whole channel (and
> > > capped to 128 pages?), it should only need to lock around the
> > > virtqueue_add_sg() call, I'll need to play with that some more.
> > 
> > What do you mean with "lock down"?
> 
> Just the odd (to me) use of the chan->lock around basically all of
> p9_virtio_request() and most of p9_virtio_zc_request() -- I'm not pretty
> sure this was just the author trying to avoid an allocation by recycling
> the chan->sg array around though, so ignore this.

The lock protects the channel wide, shared scatterlist while the scatterlist 
is filled from the linear buffers by pack_sg_list(). Then virtqueue_add_sgs() 
pushes scatterlist's segments as virtio descriptors into the virtio FIFOs. 
>From this point it safe to unlock as the scatterlist is no longer needed.

And yes, the assumption probably was that handling the scatterlist as a 
temporary was more expensive due to allocation.

> > > Looking at other virtio drivers I could probably use a sg_table and
> > > have extract_iter_to_sg() do all the work for us...
> > 
> > Looking at the code I'm actually really confused.  Both because I
> > actually though we were talking about the 9fs direct I/O code, but
> > that has actually been removed / converted to netfs a long time ago.
> > 
> > But even more so what the net/9p code is actually doing..  How do
> > we even end up with user addresses here at all?
> 
> FWIW I tried logging and saw ITER_BVEC, ITER_KVEC and ITER_FOLIOQ --
> O_DIRECT writes are seen as BVEC so I guess it's not as direct as I
> expected them to be -- that code could very well be leftovers from
> the switch to iov_iter back in 2015...
> 
> (I'm actually not sure why Christian suggested checking for is_iovec()
> in https://lkml.kernel.org/r/2245723.irdbgypaU6@weasel -- then I
> generalized it to user_backed_iter() and it just worked because checking
> for that moved out bvec and folioq from iov_iter_get_pages_alloc2()
> to... something that obviously should not work in my opinion but
> apparently was enough to not trigger this particular BUG.)

Sorry, I should have explained why I suggested that change: My understanding 
of the p9_get_mapped_pages() code was that the original author intended to go 
down the iov_iter_get_pages_alloc2() path only for user space memory 
exclusively and that he assumed that his preceding !iov_iter_is_kvec(data) 
check would guard this appropriately. But apparently there are several other 
iterator types that are kernel memory as well. So I thought switching the 
check to is_iovec() would be better as these are user space memory, however I 
missed that there is also ITER_UBUF with user space memory, which you realized 
fortunately by suggesting user_backed_iter() check instead.

/Christian

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