netdev - Re: [PATCH resend mm,net-next 2/3] mm: Add vm_insert

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Date:   Wed, 12 Feb 2020 18:41:01 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Arjun Roy <arjunroy.kdev@...il.com>
Cc:     davem@...emloft.net, netdev@...r.kernel.org, linux-mm@...ck.org,
        arjunroy@...gle.com, Eric Dumazet <edumazet@...gle.com>,
        Soheil Hassas Yeganeh <soheil@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH resend mm,net-next 2/3] mm: Add vm_insert_pages().

On Mon, 27 Jan 2020 18:59:57 -0800 Arjun Roy <arjunroy.kdev@...il.com> wrote:

> Add the ability to insert multiple pages at once to a user VM with
> lower PTE spinlock operations.
> 
> The intention of this patch-set is to reduce atomic ops for
> tcp zerocopy receives, which normally hits the same spinlock multiple
> times consecutively.

Seems sensible, thanks.  Some other vm_insert_page() callers might want
to know about this, but I can't immediately spot any which appear to be
high bandwidth.

Is there much point in keeping the vm_insert_page() implementation
around?  Replace it with

static inline int
vm_insert_page(struct vm_area_struct *vma, unsigned long addr,
	       struct page *page)
{
	return vm_insert_pages(vma, addr, &page, 1);
}

?

Also, vm_insert_page() does

	if (!page_count(page))
		return -EINVAL;

and this was not carried over into vm_insert_pages().  How come?

I don't know what that test does - it was added by Linus in the
original commit a145dd411eb28c83.  It's only been 15 years so I'm sure
he remembers ;)