netdev - Re: [PATCH 1/4] net: introduce helper sendpages

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Date: Mon, 3 Jun 2024 09:18:59 +0200
From: Hannes Reinecke <hare@...e.de>
To: Ofir Gal <ofir.gal@...umez.com>, davem@...emloft.net,
 linux-block@...r.kernel.org, linux-nvme@...ts.infradead.org,
 netdev@...r.kernel.org, ceph-devel@...r.kernel.org
Cc: dhowells@...hat.com, edumazet@...gle.com, pabeni@...hat.com,
 kbusch@...nel.org, axboe@...nel.dk, hch@....de, sagi@...mberg.me,
 philipp.reisner@...bit.com, lars.ellenberg@...bit.com,
 christoph.boehmwalder@...bit.com, idryomov@...il.com, xiubli@...hat.com
Subject: Re: [PATCH 1/4] net: introduce helper sendpages_ok()

On 5/30/24 15:26, Ofir Gal wrote:
> Network drivers are using sendpage_ok() to check the first page of an
> iterator in order to disable MSG_SPLICE_PAGES. The iterator can
> represent list of contiguous pages.
> 
> When MSG_SPLICE_PAGES is enabled skb_splice_from_iter() is being used,
> it requires all pages in the iterator to be sendable. Therefore it needs
> to check that each page is sendable.
> 
> The patch introduces a helper sendpages_ok(), it returns true if all the
> contiguous pages are sendable.
> 
> Drivers who want to send contiguous pages with MSG_SPLICE_PAGES may use
> this helper to check whether the page list is OK. If the helper does not
> return true, the driver should remove MSG_SPLICE_PAGES flag.
> 
> Signed-off-by: Ofir Gal <ofir.gal@...umez.com>
> ---
>   include/linux/net.h | 20 ++++++++++++++++++++
>   1 file changed, 20 insertions(+)
> 
> diff --git a/include/linux/net.h b/include/linux/net.h
> index 688320b79fcc..b33bdc3e2031 100644
> --- a/include/linux/net.h
> +++ b/include/linux/net.h
> @@ -322,6 +322,26 @@ static inline bool sendpage_ok(struct page *page)
>   	return !PageSlab(page) && page_count(page) >= 1;
>   }
>   
> +/*
> + * Check sendpage_ok on contiguous pages.
> + */
> +static inline bool sendpages_ok(struct page *page, size_t len, size_t offset)
> +{
> +	unsigned int pagecount;
> +	size_t page_offset;
> +	int k;
> +
> +	page = page + offset / PAGE_SIZE;
> +	page_offset = offset % PAGE_SIZE;
> +	pagecount = DIV_ROUND_UP(len + page_offset, PAGE_SIZE);
> +
Don't we miss the first page for offset > PAGE_SIZE?
I'd rather check for all pages from 'page' up to (offset + len), just
to be on the safe side.

> +	for (k = 0; k < pagecount; k++)
> +		if (!sendpage_ok(page + k))
> +			return false;
> +
> +	return true;
> +}
> +
>   int kernel_sendmsg(struct socket *sock, struct msghdr *msg, struct kvec *vec,
>   		   size_t num, size_t len);
>   int kernel_sendmsg_locked(struct sock *sk, struct msghdr *msg,

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                  Kernel Storage Architect
hare@...e.de                                +49 911 74053 688
SUSE Software Solutions GmbH, Frankenstr. 146, 90461 Nürnberg
HRB 36809 (AG Nürnberg), GF: I. Totev, A. McDonald, W. Knoblich