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

[Paolo Abeni]

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@xxxxxxxxxx>
> cc: Alexander Duyck <alexander.duyck@xxxxxxxxx>
> cc: Eric Dumazet <edumazet@xxxxxxxxxx>
> cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
> cc: David Ahern <dsahern@xxxxxxxxxx>
> cc: Jakub Kicinski <kuba@xxxxxxxxxx>
> cc: Paolo Abeni <pabeni@xxxxxxxxxx>
> cc: Jens Axboe <axboe@xxxxxxxxx>
> cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> cc: Menglong Dong <imagedong@xxxxxxxxxxx>
> cc: netdev@xxxxxxxxxxxxxxx
> ---
> 
> 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