Re: [PATCH 1/2] nfsd: don't replace page in rq_pages if it's a continuation of last page

[Jeff Layton <jlayton@kernel.org>]

On Fri, 2023-03-17 at 14:16 +0000, Chuck Lever III wrote:
> 
> > On Mar 17, 2023, at 9:06 AM, Jeff Layton <jlayton@kernel.org> wrote:
> > 
> > On Fri, 2023-03-17 at 06:56 -0400, Jeff Layton wrote:
> > > The splice read calls nfsd_splice_actor to put the pages containing file
> > > data into the svc_rqst->rq_pages array. It's possible however to get a
> > > splice result that only has a partial page at the end, if (e.g.) the
> > > filesystem hands back a short read that doesn't cover the whole page.
> > > 
> > > nfsd_splice_actor will plop the partial page into its rq_pages array and
> > > return. Then later, when nfsd_splice_actor is called again, the
> > > remainder of the page may end up being filled out. At this point,
> > > nfsd_splice_actor will put the page into the array _again_ corrupting
> > > the reply. If this is done enough times, rq_next_page will overrun the
> > > array and corrupt the trailing fields -- the rq_respages and
> > > rq_next_page pointers themselves.
> > > 
> > > If we've already added the page to the array in the last pass, don't add
> > > it to the array a second time when dealing with a splice continuation.
> > > This was originally handled properly in nfsd_splice_actor, but commit
> > > 91e23b1c3982 removed the check for it, and started universally replacing
> > > pages.
> > > 
> > > Fixes: 91e23b1c3982 ("NFSD: Clean up nfsd_splice_actor()")
> > > Reported-by: Dario Lesca <d.lesca@solinos.it>
> > > Tested-by: David Critch <dcritch@redhat.com>
> > > Link: https://bugzilla.redhat.com/show_bug.cgi?id=2150630
> > > Signed-off-by: Jeff Layton <jlayton@kernel.org>
> > > ---
> > > fs/nfsd/vfs.c | 7 +++++--
> > > 1 file changed, 5 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
> > > index 502e1b7742db..3709ef57d96e 100644
> > > --- a/fs/nfsd/vfs.c
> > > +++ b/fs/nfsd/vfs.c
> > > @@ -941,8 +941,11 @@ nfsd_splice_actor(struct pipe_inode_info *pipe, struct pipe_buffer *buf,
> > > 	struct page *last_page;
> > > 
> > > 	last_page = page + (offset + sd->len - 1) / PAGE_SIZE;
> > > -	for (page += offset / PAGE_SIZE; page <= last_page; page++)
> > > -		svc_rqst_replace_page(rqstp, page);
> > > +	for (page += offset / PAGE_SIZE; page <= last_page; page++) {
> > > +		/* Only replace page if we haven't already done so */
> > 
> > Note that I think that this was probably the real rationale for the pp[-
> > 1] check that 91e23b1c3982 removed. Given that, maybe we should flesh
> > this comment out a bit more for posterity?
> > 
> > 		/*
> > 		 * When we're splicing from a pipe, it's possible that
> > 		 * we'll get an incomplete page that may be updated on
> > 		 * a later call. Only splice it into rq_pages once.
> > 		 */
> 
> The "real" bug here is that the API contract for pipe splicing
> isn't well defined, so I agree that it's very likely the pp[-1]
> check was because a splice can call the actor repeatedly for the
> same page. No one could remember why that check was there.
> 

The whole splice API is a minefield.

> To be clear, if the passed-in page matches the current page in
> the rqst, we're "extending the current page" rather than avoiding
> replacement... maybe:
> 
> 	/*
> 	 * Skip page replacement when extending the contents
> 	 * of the current page.
> 	 */
> 

Sure, sounds good.

> In the patch description, would you mention that this case
> arises if the READ request is not page-aligned?
> 

Does it though? I'm not sure that page alignment has that much to do
with it. I imagine you can hit this even with aligned I/Os.

My guess is the bigger issue is when your storage is doing sub-page-size
I/Os under the hood. We end up filling up part of a page from storage
and the kernel submits what it has to the pipe and then the next bit
comes in and the page is updated for the next actor call.

> If you resend this patch, please Cc: viro@ . Thanks for chasing
> this down!
> 

Will do.

> 
> > > +		if (page != *(rqstp->rq_next_page - 1))
> > > +			svc_rqst_replace_page(rqstp, page);
> > > +	}
> > > 	if (rqstp->rq_res.page_len == 0)	// first call
> > > 		rqstp->rq_res.page_base = offset % PAGE_SIZE;
> > > 	rqstp->rq_res.page_len += sd->len;
> > 
> > -- 
> > Jeff Layton <jlayton@kernel.org>
> 
> --
> Chuck Lever
> 
> 

-- 
Jeff Layton <jlayton@kernel.org>