Re: [PATCH net-next v3 01/18] net: Copy slab data for sendmsg(MSG_SPLICE_PAGES)

Paolo Abeni
2023-06-23T08:10:52Z

Hi,

First thing first, I'm sorry for the delayed feedback. I was traveling.

On Tue, 2023-06-20 at 15:53 +0100, David Howells wrote:
> If sendmsg() is passed MSG_SPLICE_PAGES and is given a buffer that contains
> some data that's resident in the slab, copy it rather than returning EIO.
> This can be made use of by a number of drivers in the kernel, including:
> iwarp, ceph/rds, dlm, nvme, ocfs2, drdb. It could also be used by iscsi,
> rxrpc, sunrpc, cifs and probably others.
>
> skb_splice_from_iter() is given it's own fragment allocator as
> page_frag_alloc_align() can't be used because it does no locking to prevent
> parallel callers from racing. alloc_skb_frag() uses a separate folio for
> each cpu and locks to the cpu whilst allocating, reenabling cpu migration
> around folio allocation.
>
> This could allocate a whole page instead for each fragment to be copied, as
> alloc_skb_with_frags() would do instead, but that would waste a lot of
> space (most of the fragments look like they're going to be small).
>
> This allows an entire message that consists of, say, a protocol header or
> two, a number of pages of data and a protocol footer to be sent using a
> single call to sock_sendmsg().
>
> The callers could be made to copy the data into fragments before calling
> sendmsg(), but that then penalises them if MSG_SPLICE_PAGES gets ignored.
>
> Signed-off-by: David Howells <dhowells@redhat.com>
> cc: Alexander Duyck <alexander.duyck@gmail.com>
> cc: Eric Dumazet <edumazet@google.com>
> cc: "David S. Miller" <davem@davemloft.net>
> cc: David Ahern <dsahern@kernel.org>
> cc: Jakub Kicinski <kuba@kernel.org>
> cc: Paolo Abeni <pabeni@redhat.com>
> cc: Jens Axboe <axboe@kernel.dk>
> cc: Matthew Wilcox <willy@infradead.org>
> cc: Menglong Dong <imagedong@tencent.com>
> cc: netdev@vger.kernel.org
> ---
>
> Notes:
> ver #3)
> - Remove duplicate decl of skb_splice_from_iter().
>
> ver #2)
> - Fix parameter to put_cpu_ptr() to have an '&'.
>
> include/linux/skbuff.h | 2 +
> net/core/skbuff.c | 171 ++++++++++++++++++++++++++++++++++++++++-
> 2 files changed, 170 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index 91ed66952580..5f53bd5d375d 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -5037,6 +5037,8 @@ static inline void skb_mark_for_recycle(struct sk_buff *skb)
> #endif
> }
>
> +void *alloc_skb_frag(size_t fragsz, gfp_t gfp);
> +void *copy_skb_frag(const void *s, size_t len, gfp_t gfp);
> ssize_t skb_splice_from_iter(struct sk_buff *skb, struct iov_iter *iter,
> ssize_t maxsize, gfp_t gfp);
>
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index fee2b1c105fe..d962c93a429d 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -6755,6 +6755,145 @@ nodefer: __kfree_skb(skb);
> smp_call_function_single_async(cpu, &sd->defer_csd);
> }
>
> +struct skb_splice_frag_cache {
> + struct folio *folio;
> + void *virt;
> + unsigned int offset;
> + /* we maintain a pagecount bias, so that we dont dirty cache line
> + * containing page->_refcount every time we allocate a fragment.
> + */
> + unsigned int pagecnt_bias;
> + bool pfmemalloc;
> +};
> +
> +static DEFINE_PER_CPU(struct skb_splice_frag_cache, skb_splice_frag_cache);
> +
> +/**
> + * alloc_skb_frag - Allocate a page fragment for using in a socket
> + * @fragsz: The size of fragment required
> + * @gfp: Allocation flags
> + */
> +void *alloc_skb_frag(size_t fragsz, gfp_t gfp)
> +{
> + struct skb_splice_frag_cache *cache;
> + struct folio *folio, *spare = NULL;
> + size_t offset, fsize;
> + void *p;
> +
> + if (WARN_ON_ONCE(fragsz == 0))
> + fragsz = 1;
> +
> + cache = get_cpu_ptr(&skb_splice_frag_cache);
> +reload:
> + folio = cache->folio;
> + offset = cache->offset;
> +try_again:
> + if (fragsz > offset)
> + goto insufficient_space;
> +
> + /* Make the allocation. */
> + cache->pagecnt_bias--;
> + offset = ALIGN_DOWN(offset - fragsz, SMP_CACHE_BYTES);
> + cache->offset = offset;
> + p = cache->virt + offset;
> + put_cpu_ptr(&skb_splice_frag_cache);
> + if (spare)
> + folio_put(spare);
> + return p;
> +
> +insufficient_space:
> + /* See if we can refurbish the current folio. */
> + if (!folio || !folio_ref_sub_and_test(folio, cache->pagecnt_bias))
> + goto get_new_folio;
> + if (unlikely(cache->pfmemalloc)) {
> + __folio_put(folio);
> + goto get_new_folio;
> + }
> +
> + fsize = folio_size(folio);
> + if (unlikely(fragsz > fsize))
> + goto frag_too_big;
> +
> + /* OK, page count is 0, we can safely set it */
> + folio_set_count(folio, PAGE_FRAG_CACHE_MAX_SIZE + 1);
> +
> + /* Reset page count bias and offset to start of new frag */
> + cache->pagecnt_bias = PAGE_FRAG_CACHE_MAX_SIZE + 1;
> + offset = fsize;
> + goto try_again;

IMHO this function uses a bit too much labels and would be more easy to
read, e.g. moving the above chunk of code in conditional branch.

Even without such change, I think the above 'goto try_again;'
introduces an unneeded conditional, as at this point we know 'fragsz <=
fsize'.

> +
> +get_new_folio:
> + if (!spare) {
> + cache->folio = NULL;
> + put_cpu_ptr(&skb_splice_frag_cache);
> +
> +#if PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE
> + spare = folio_alloc(gfp | __GFP_NOWARN | __GFP_NORETRY |
> + __GFP_NOMEMALLOC,
> + PAGE_FRAG_CACHE_MAX_ORDER);
> + if (!spare)
> +#endif
> + spare = folio_alloc(gfp, 0);
> + if (!spare)
> + return NULL;
> +
> + cache = get_cpu_ptr(&skb_splice_frag_cache);
> + /* We may now be on a different cpu and/or someone else may
> + * have refilled it
> + */
> + cache->pfmemalloc = folio_is_pfmemalloc(spare);
> + if (cache->folio)
> + goto reload;

I think there is some problem with the above.

If cache->folio is != NULL, and cache->folio was not pfmemalloc-ed
while the spare one is, it looks like the wrong policy will be used.
And should be even worse if folio was pfmemalloc-ed while spare is not.

I think moving 'cache->pfmemalloc' initialization...

> + }
> +

... here should fix the above.

> + cache->folio = spare;
> + cache->virt = folio_address(spare);
> + folio = spare;
> + spare = NULL;
> +
> + /* Even if we own the page, we do not use atomic_set(). This would
> + * break get_page_unless_zero() users.
> + */
> + folio_ref_add(folio, PAGE_FRAG_CACHE_MAX_SIZE);
> +
> + /* Reset page count bias and offset to start of new frag */
> + cache->pagecnt_bias = PAGE_FRAG_CACHE_MAX_SIZE + 1;
> + offset = folio_size(folio);
> + goto try_again;

What if fragsz > PAGE_SIZE, we are consistently unable to allocate an
high order page, but order-0, pfmemalloc-ed page allocation is
successful? It looks like this could become an unbounded loop?

> +
> +frag_too_big:
> + /* The caller is trying to allocate a fragment with fragsz > PAGE_SIZE
> + * but the cache isn't big enough to satisfy the request, this may
> + * happen in low memory conditions. We don't release the cache page
> + * because it could make memory pressure worse so we simply return NULL
> + * here.
> + */
> + cache->offset = offset;
> + put_cpu_ptr(&skb_splice_frag_cache);
> + if (spare)
> + folio_put(spare);
> + return NULL;
> +}
> +EXPORT_SYMBOL(alloc_skb_frag);
> +
> +/**
> + * copy_skb_frag - Copy data into a page fragment.
> + * @s: The data to copy
> + * @len: The size of the data
> + * @gfp: Allocation flags
> + */
> +void *copy_skb_frag(const void *s, size_t len, gfp_t gfp)
> +{
> + void *p;
> +
> + p = alloc_skb_frag(len, gfp);
> + if (!p)
> + return NULL;
> +
> + return memcpy(p, s, len);
> +}
> +EXPORT_SYMBOL(copy_skb_frag);
> +
> static void skb_splice_csum_page(struct sk_buff *skb, struct page *page,
> size_t offset, size_t len)
> {
> @@ -6808,17 +6947,43 @@ 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 (WARN_ON_ONCE(!sendpage_ok(page)))
> + bool put = false;
> +
> + if (PageSlab(page)) {

I'm a bit concerned from the above. If I read correctly, tcp 0-copy
will go through that for every page, even if the expected use-case is
always !PageSlub(page). compound_head() could be costly if the head
page is not hot on cache and I'm not sure if that could be the case for
tcp 0-copy. The bottom line is that I fear a possible regression here.

Given all the above, and the late stage of the current devel cycle,
would you consider slicing down this series to just kill
MSG_SENDPAGE_NOTLAST, as Jakub suggested?

Thanks!

Paolo