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From:   David Howells <dhowells@redhat.com>
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To:     Paolo Abeni <pabeni@redhat.com>
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        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
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        Matthew Wilcox <willy@infradead.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Christoph Hellwig <hch@infradead.org>,
        Jens Axboe <axboe@kernel.dk>, Jeff Layton <jlayton@kernel.org>,
        Christian Brauner <brauner@kernel.org>,
        Chuck Lever III <chuck.lever@oracle.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org
Subject: Re: [PATCH net-next v7 03/16] net: Add a function to splice pages into an skbuff for MSG_SPLICE_PAGES
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Date:   Thu, 18 May 2023 10:53:28 +0100
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Paolo Abeni <pabeni@redhat.com> wrote:

> Minor nit: please respect the reverse x-mas tree order (there are a few
> other occurrences around)

I hadn't come across that.  Normally I only apply that to the types so tha=
t
the names aren't all over the place.  But whatever.

> > +		if (space =3D=3D 0 &&
> > +		    !skb_can_coalesce(skb, skb_shinfo(skb)->nr_frags,
> > +				      pages[0], off)) {
> > +			iov_iter_revert(iter, len);
> > +			break;
> > +		}
> =

> It looks like the above condition/checks duplicate what the later
> skb_append_pagefrags() will perform below. I guess the above chunk
> could be removed?

Good point.  There used to be an allocation between in the case sendpage_o=
k()
failed and we wanted to copy the data.  I've removed that for the moment.

> > +			ret =3D -EIO;
> > +			if (!sendpage_ok(page))
> > +				goto out;
> =

> My (limited) understanding is that the current sendpage code assumes
> that the caller provides/uses pages suitable for such use. The existing
> sendpage_ok() check is in place as way to try to catch possible code
> bug - via the WARN_ONCE().
>
> I think the same could be done here?

Yeah.

Okay, I made the attached changes to this patch.

David
---
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 56d629ea2f3d..f4a5b51aed22 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -6923,10 +6923,10 @@ static void skb_splice_csum_page(struct sk_buff *s=
kb, struct page *page,
 ssize_t skb_splice_from_iter(struct sk_buff *skb, struct iov_iter *iter,
 			     ssize_t maxsize, gfp_t gfp)
 {
+	size_t frag_limit =3D READ_ONCE(sysctl_max_skb_frags);
 	struct page *pages[8], **ppages =3D pages;
-	unsigned int i;
 	ssize_t spliced =3D 0, ret =3D 0;
-	size_t frag_limit =3D READ_ONCE(sysctl_max_skb_frags);
+	unsigned int i;
 =

 	while (iter->count > 0) {
 		ssize_t space, nr;
@@ -6946,20 +6946,13 @@ ssize_t skb_splice_from_iter(struct sk_buff *skb, =
struct iov_iter *iter,
 			break;
 		}
 =

-		if (space =3D=3D 0 &&
-		    !skb_can_coalesce(skb, skb_shinfo(skb)->nr_frags,
-				      pages[0], off)) {
-			iov_iter_revert(iter, len);
-			break;
-		}
-
 		i =3D 0;
 		do {
 			struct page *page =3D pages[i++];
 			size_t part =3D min_t(size_t, PAGE_SIZE - off, len);
 =

 			ret =3D -EIO;
-			if (!sendpage_ok(page))
+			if (WARN_ON_ONCE(!sendpage_ok(page)))
 				goto out;
 =

 			ret =3D skb_append_pagefrags(skb, page, off, part,