netdev - Re: [PATCH net-next v7 03/16] net: Add a function to splice pages into an skbuff for MSG_SPLICE

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Date: Thu, 18 May 2023 10:53:28 +0100
From: David Howells <dhowells@...hat.com>
To: Paolo Abeni <pabeni@...hat.com>
Cc: dhowells@...hat.com, netdev@...r.kernel.org,
    "David S. Miller" <davem@...emloft.net>,
    Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
    Willem de Bruijn <willemdebruijn.kernel@...il.com>,
    David Ahern <dsahern@...nel.org>,
    Matthew Wilcox <willy@...radead.org>,
    Al Viro <viro@...iv.linux.org.uk>,
    Christoph Hellwig <hch@...radead.org>, Jens Axboe <axboe@...nel.dk>,
    Jeff Layton <jlayton@...nel.org>,
    Christian Brauner <brauner@...nel.org>,
    Chuck Lever III <chuck.lever@...cle.com>,
    Linus Torvalds <torvalds@...ux-foundation.org>,
    linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
    linux-mm@...ck.org
Subject: Re: [PATCH net-next v7 03/16] net: Add a function to splice pages into an skbuff for MSG_SPLICE_PAGES

Paolo Abeni <pabeni@...hat.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 that
the names aren't all over the place.  But whatever.

> > +		if (space == 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_ok()
failed and we wanted to copy the data.  I've removed that for the moment.

> > +			ret = -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 *skb, 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 = READ_ONCE(sysctl_max_skb_frags);
 	struct page *pages[8], **ppages = pages;
-	unsigned int i;
 	ssize_t spliced = 0, ret = 0;
-	size_t frag_limit = 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 == 0 &&
-		    !skb_can_coalesce(skb, skb_shinfo(skb)->nr_frags,
-				      pages[0], off)) {
-			iov_iter_revert(iter, len);
-			break;
-		}
-
 		i = 0;
 		do {
 			struct page *page = pages[i++];
 			size_t part = min_t(size_t, PAGE_SIZE - off, len);
 
 			ret = -EIO;
-			if (!sendpage_ok(page))
+			if (WARN_ON_ONCE(!sendpage_ok(page)))
 				goto out;
 
 			ret = skb_append_pagefrags(skb, page, off, part,