From: David Howells <dhowells@redhat.com>
To: Al Viro <viro@zeniv.linux.org.uk>, Matthew Wilcox <willy@infradead.org>
Cc: dhowells@redhat.com, netdev@vger.kernel.org,
"David S. Miller" <davem@davemloft.net>,
Eric Dumazet <edumazet@google.com>,
Jakub Kicinski <kuba@kernel.org>, Paolo Abeni <pabeni@redhat.com>,
Willem de Bruijn <willemdebruijn.kernel@gmail.com>,
David Ahern <dsahern@kernel.org>,
Christoph Hellwig <hch@infradead.org>,
Andrew Morton <akpm@linux-foundation.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: How to determine if a page can be spliced into an skbuff, or if it should be copied/rejected?
Date: Thu, 13 Apr 2023 22:26:57 +0100 [thread overview]
Message-ID: <1208134.1681421217@warthog.procyon.org.uk> (raw)
In-Reply-To: <20230413044123.GB3390869@ZenIV>
Al Viro <viro@zeniv.linux.org.uk> wrote:
> On Tue, Apr 11, 2023 at 05:08:50PM +0100, David Howells wrote:
> > Add a function to handle MSG_SPLICE_PAGES being passed internally to
> > sendmsg(). Pages are spliced into the given socket buffer if possible and
> > copied in if not (ie. they're slab pages or have a zero refcount).
>
> That "ie." would better be "e.g." - that condition is *not* enough for
> tell the unsafe ones from the rest.
>
> sendpage_ok() would be better off called "might_be_ok_to_sendpage()".
> If it's false, we'd better not grab a reference to the page and expect the
> sucker to stay safe until the reference is dropped. However, AFAICS
> it might return true on a page that is not safe in that respect.
>
> What rules do you propose for sendpage users? "Pass whatever page reference
> you want, it'll do the right thing"? Anything short of that would better
> be documented as explicitly as possible...
Hmmm... Fair point. Is everything passed through splice guaranteed to be
safe, I wonder? Probably not because vmsplice(). Does that mean the existing
callers of sendpage_ok() are also making unviable assumptions?
So there are the following 'classes' of memory that I can immediately think
of:
- Zero page Splice (no ref?)
- Kernel core data Splice
- Module core data (vmalloc'd) Splice
- Supervisor stack Copy
- Slab objects Copy
- Page frags Splice
- Other skbuff frags Splice
- Arbitrary pages (eg. sunrpc xdr buf) Splice (probably)
- Ordinary pipe buffers Splice
- Spliced tmpfs Splice
- Spliced pagecache (file/block) Splice
- Spliced DIO file/block Splice
- Vmspliced mmap'd anon Splice (with pin?)
- Vmspliced MAP_SHARED pagecache Splice (with pin?)
- Vmspliced MAP_SHARED DAX Splice?
- Vmspliced MAP_SHARED MTD Splice?
- Vmspliced MAP_SHARED other device Reject? (e.g. graphics card mem)
- Vmspliced /dev/{mem,kmem} Reject?
Question is how to tell that we're looking at something that must be copied or
rejected? sendpage_ok() checks the PG_slab bit and the pagecount, for
example.
David
next prev parent reply other threads:[~2023-04-13 21:27 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-04-11 16:08 [PATCH net-next v6 00/18] splice, net: Replace sendpage with sendmsg(MSG_SPLICE_PAGES), part 1 David Howells
2023-04-11 16:08 ` [PATCH net-next v6 01/18] net: Declare MSG_SPLICE_PAGES internal sendmsg() flag David Howells
2023-04-13 0:51 ` Al Viro
2023-04-13 4:29 ` Al Viro
2023-04-13 20:39 ` David Howells
2023-04-13 20:49 ` Al Viro
2023-04-13 21:01 ` Al Viro
2023-04-11 16:08 ` [PATCH net-next v6 02/18] mm: Move the page fragment allocator from page_alloc.c into its own file David Howells
2023-04-11 16:08 ` [PATCH net-next v6 03/18] mm: Make the page_frag_cache allocator use multipage folios David Howells
2023-04-11 16:08 ` [PATCH net-next v6 04/18] mm: Make the page_frag_cache allocator use per-cpu David Howells
2023-04-11 16:55 ` Christoph Hellwig
2023-04-12 15:31 ` Christoph Hellwig
2023-04-12 23:12 ` David Howells
2023-04-11 16:08 ` [PATCH net-next v6 05/18] net: Pass max frags into skb_append_pagefrags() David Howells
2023-04-11 16:08 ` [PATCH net-next v6 06/18] net: Add a function to splice pages into an skbuff for MSG_SPLICE_PAGES David Howells
2023-04-13 4:41 ` Al Viro
2023-04-13 21:26 ` David Howells [this message]
2023-04-11 16:08 ` [PATCH net-next v6 07/18] tcp: Support MSG_SPLICE_PAGES David Howells
2023-04-11 17:09 ` Eric Dumazet
2023-04-11 17:49 ` David Howells
2023-04-11 16:08 ` [PATCH net-next v6 08/18] tcp: Convert do_tcp_sendpages() to use MSG_SPLICE_PAGES David Howells
2023-04-11 16:08 ` [PATCH net-next v6 09/18] tcp_bpf: Inline do_tcp_sendpages as it's now a wrapper around tcp_sendmsg David Howells
2023-04-11 16:08 ` [PATCH net-next v6 10/18] espintcp: Inline do_tcp_sendpages() David Howells
2023-04-11 16:08 ` [PATCH net-next v6 11/18] tls: " David Howells
2023-04-11 16:08 ` [PATCH net-next v6 12/18] siw: " David Howells
2023-04-11 17:22 ` Tom Talpey
2023-04-11 16:08 ` [PATCH net-next v6 13/18] tcp: Fold do_tcp_sendpages() into tcp_sendpage_locked() David Howells
2023-04-11 16:08 ` [PATCH net-next v6 14/18] ip, udp: Support MSG_SPLICE_PAGES David Howells
2023-04-11 16:08 ` [PATCH net-next v6 15/18] ip6, udp6: " David Howells
2023-04-11 16:09 ` [PATCH net-next v6 16/18] udp: Convert udp_sendpage() to use MSG_SPLICE_PAGES David Howells
2023-04-11 16:09 ` [PATCH net-next v6 17/18] ip: Remove ip_append_page() David Howells
2023-04-11 16:09 ` [PATCH net-next v6 18/18] af_unix: Support MSG_SPLICE_PAGES David Howells
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