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Miller" , Willem de Bruijn , Eric Dumazet , David Ahern , Jens Axboe , Pavel Begunkov Subject: [RFC v2 04/19] net: optimise page get/free for bvec zc Date: Tue, 21 Dec 2021 15:35:26 +0000 Message-Id: <6bea8a32471c7a4e849d64cf5b6122236b6a38dd.1640029579.git.asml.silence@gmail.com> X-Mailer: git-send-email 2.34.1 In-Reply-To: References: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org get_page() in __zerocopy_sg_from_bvec() and matching put_page()s are expensive. However, we can avoid it if the caller can guarantee that pages stay alive until the corresponding ubuf_info is not released. In particular, it targets io_uring with fixed buffers following the described contract. Assuming that nobody yet uses bvec together with zerocopy, make all calls with bvec iterators follow this model. Signed-off-by: Pavel Begunkov --- include/linux/skbuff.h | 12 ++++++++++-- net/core/datagram.c | 9 +++++++-- net/core/skbuff.c | 14 +++++++++++++- 3 files changed, 30 insertions(+), 5 deletions(-) diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h index b80944a9ce8f..f6a6fd67e1ea 100644 --- a/include/linux/skbuff.h +++ b/include/linux/skbuff.h @@ -461,11 +461,16 @@ enum { SKBFL_PURE_ZEROCOPY = BIT(2), SKBFL_DONT_ORPHAN = BIT(3), + + /* page references are managed by the ubuf_info, so it's safe to + * use frags only up until ubuf_info is released + */ + SKBFL_MANAGED_FRAGS = BIT(4), }; #define SKBFL_ZEROCOPY_FRAG (SKBFL_ZEROCOPY_ENABLE | SKBFL_SHARED_FRAG) #define SKBFL_ALL_ZEROCOPY (SKBFL_ZEROCOPY_FRAG | SKBFL_PURE_ZEROCOPY | \ - SKBFL_DONT_ORPHAN) + SKBFL_DONT_ORPHAN | SKBFL_MANAGED_FRAGS) /* * The callback notifies userspace to release buffers when skb DMA is done in @@ -3155,7 +3160,10 @@ static inline void __skb_frag_unref(skb_frag_t *frag, bool recycle) */ static inline void skb_frag_unref(struct sk_buff *skb, int f) { - __skb_frag_unref(&skb_shinfo(skb)->frags[f], skb->pp_recycle); + struct skb_shared_info *shinfo = skb_shinfo(skb); + + if (!(shinfo->flags & SKBFL_MANAGED_FRAGS)) + __skb_frag_unref(&shinfo->frags[f], skb->pp_recycle); } /** diff --git a/net/core/datagram.c b/net/core/datagram.c index cb1e34fbcd44..46526af40552 100644 --- a/net/core/datagram.c +++ b/net/core/datagram.c @@ -638,7 +638,6 @@ static int __zerocopy_sg_from_bvec(struct sock *sk, struct sk_buff *skb, v = mp_bvec_iter_bvec(from->bvec, bi); copied += v.bv_len; truesize += PAGE_ALIGN(v.bv_len + v.bv_offset); - get_page(v.bv_page); skb_fill_page_desc(skb, frag++, v.bv_page, v.bv_offset, v.bv_len); bvec_iter_advance_single(from->bvec, &bi, v.bv_len); } @@ -667,9 +666,15 @@ int __zerocopy_sg_from_iter(struct sock *sk, struct sk_buff *skb, struct iov_iter *from, size_t length) { int frag = skb_shinfo(skb)->nr_frags; + bool managed = skb_shinfo(skb)->flags & SKBFL_MANAGED_FRAGS; - if (iov_iter_is_bvec(from)) + if (iov_iter_is_bvec(from) && (managed || frag == 0)) { + skb_shinfo(skb)->flags |= SKBFL_MANAGED_FRAGS; return __zerocopy_sg_from_bvec(sk, skb, from, length); + } + + if (managed) + return -EFAULT; while (length && iov_iter_count(from)) { struct page *pages[MAX_SKB_FRAGS]; diff --git a/net/core/skbuff.c b/net/core/skbuff.c index b23db60ea6f9..10cdcb99d34b 100644 --- a/net/core/skbuff.c +++ b/net/core/skbuff.c @@ -666,11 +666,18 @@ static void skb_release_data(struct sk_buff *skb) &shinfo->dataref)) goto exit; - skb_zcopy_clear(skb, true); + if (skb_zcopy(skb)) { + bool skip_unref = shinfo->flags & SKBFL_MANAGED_FRAGS; + + skb_zcopy_clear(skb, true); + if (skip_unref) + goto free_head; + } for (i = 0; i < shinfo->nr_frags; i++) __skb_frag_unref(&shinfo->frags[i], skb->pp_recycle); +free_head: if (shinfo->frag_list) kfree_skb_list(shinfo->frag_list); @@ -1597,6 +1604,7 @@ struct sk_buff *skb_copy(const struct sk_buff *skb, gfp_t gfp_mask) BUG_ON(skb_copy_bits(skb, -headerlen, n->head, headerlen + skb->len)); skb_copy_header(n, skb); + skb_shinfo(n)->flags &= ~SKBFL_MANAGED_FRAGS; return n; } EXPORT_SYMBOL(skb_copy); @@ -1653,6 +1661,7 @@ struct sk_buff *__pskb_copy_fclone(struct sk_buff *skb, int headroom, skb_frag_ref(skb, i); } skb_shinfo(n)->nr_frags = i; + skb_shinfo(n)->flags &= ~SKBFL_MANAGED_FRAGS; } if (skb_has_frag_list(skb)) { @@ -1725,6 +1734,7 @@ int pskb_expand_head(struct sk_buff *skb, int nhead, int ntail, refcount_inc(&skb_uarg(skb)->refcnt); for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) skb_frag_ref(skb, i); + skb_shinfo(skb)->flags &= ~SKBFL_MANAGED_FRAGS; if (skb_has_frag_list(skb)) skb_clone_fraglist(skb); @@ -3788,6 +3798,8 @@ int skb_append_pagefrags(struct sk_buff *skb, struct page *page, if (skb_can_coalesce(skb, i, page, offset)) { skb_frag_size_add(&skb_shinfo(skb)->frags[i - 1], size); } else if (i < MAX_SKB_FRAGS) { + if (skb_shinfo(skb)->flags & SKBFL_MANAGED_FRAGS) + return -EMSGSIZE; get_page(page); skb_fill_page_desc(skb, i, page, offset, size); } else { -- 2.34.1