netdev - Re: [PATCH 0/5] Beginnings of skb_frag -> bio

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Message-ID: <20190501094437.GA3698@bombadil.infradead.org>
Date:   Wed, 1 May 2019 02:44:37 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     davem@...emloft.net, hch@....de, netdev@...r.kernel.org
Subject: Re: [PATCH 0/5] Beginnings of skb_frag -> bio_vec conversion

On Wed, May 01, 2019 at 04:14:41AM -0400, Eric Dumazet wrote:
> On 4/30/19 9:17 PM, Matthew Wilcox wrote:
> > It turns out there's a lot of accessors for the skb_frag, which would
> > make this conversion really easy if some drivers didn't bypass them.
> > This is what I've done so far; my laptop's not really beefy enough to
> > cope with changing skbuff.h too often ;-)
> 
> I guess the missing part here is the "why" all this is done ?
> 
> 32 bit hosts will have bigger skb_shared_info and this impacts sk_rcvbuf and sk_sndbuf limits.
> 
> 17 * 4 are 68 extra bytes per skb.

Right.  The plan is to replace get_user_pages() with get_user_bvec().  If
userspace has physically consecutive pages (and often it will, even when
not using THP), we can reduce the number of elements in the array at the
start.  So each skb_frag_t is larger, but you'll have fewer of them for a
large I/O.  Obviously this particularly benefits THP.