netdev - Re: [PATCH v2 1/4] net: introduce helper sendpages

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Date: Fri, 31 May 2024 11:51:04 +0300
From: Sagi Grimberg <sagi@...mberg.me>
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, philipp.reisner@...bit.com,
 lars.ellenberg@...bit.com, christoph.boehmwalder@...bit.com,
 idryomov@...il.com, xiubli@...hat.com
Subject: Re: [PATCH v2 1/4] net: introduce helper sendpages_ok()



On 30/05/2024 17:24, 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;

lets not modify the input page variable.

p = page + offset >> PAGE_SHIFT;
poffset = offset & PAGE_MASK;

> +	pagecount = DIV_ROUND_UP(len + page_offset, PAGE_SIZE);
> +
> +	for (k = 0; k < pagecount; k++)
> +		if (!sendpage_ok(page + k))
> +			return false;

perhaps instead of doing a costly DIV_ROUND_UP for every network send we 
can do:

         count = 0;
         while (count < len) {
                 if (!sendpage_ok(p))
                         return false;
                 page++;
                 count += PAGE_SIZE;
         }

And we can lose page_offset.

It can be done in a number of ways, but we should be able to do it
without the DIV_ROUND_UP...

I still don't understand how a page in the middle of a contiguous range ends
up coming from the slab while others don't.

Ofir, can you please check which condition in sendpage_ok actually fails?