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

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Date: Tue, 4 Jun 2024 00:27:06 +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()

>> 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.
> I haven't investigate the origin of the IO
> yet. I suspect the first 2 pages are the superblocks of the raid
> (mdp_superblock_1 and bitmap_super_s) and the rest of the IO is the bitmap.

Well, if these indeed are different origins and just *happen* to be a 
mixture
of slab originated pages and non-slab pages combined together in a 
single bio of a bvec entry,
I'd suspect that it would be more beneficial to split the bvec 
(essentially not allow bio_add_page
to append the page to tail bvec depending on a queue limit (similar to 
how we handle sg gaps).

>
>> Ofir, can you please check which condition in sendpage_ok actually fails?
> It failed because the page has slab, page count is 1. Sorry for not
> clarifying this.
>
> "skbuff: !sendpage_ok - page: 0x54f9f140 (pfn: 120757). is_slab: 1, page_count: 1"
>                                                                   ^
> The print I used:
> pr_info(
>      "!sendpage_ok - page: 0x%p (pfn: %lx). is_slab: %u, page_count: %u\n",
>      (void *)page,
>      page_to_pfn(page),
>      page_address(page),
>      !!PageSlab(page),
>      page_count(page)
> );
>
>