linux-kernel - Re: [RFC] de-asmify the x86-64 system call slowpath

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Message-ID: <CA+55aFysEbwfzJxVbZY6X+eEZ5KSgO5b8-S_a=-nrLj0sQfecA@mail.gmail.com>
Date:	Thu, 6 Feb 2014 17:31:55 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	"Kirill A. Shutemov" <kirill@...temov.name>
Cc:	Peter Anvin <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ning Qu <quning@...gle.com>
Subject: Re: [RFC] de-asmify the x86-64 system call slowpath

On Thu, Feb 6, 2014 at 2:24 PM, Kirill A. Shutemov <kirill@...temov.name> wrote:
>
> If we want to reduce number of page fault with less overhead we probably
> should concentrate on minor page fault -- populate pte around fault
> address which already in page cache. It should cover scripting use-case
> pretty well.

That's what my patch largely does. Except I screwed up and didn't use
FAULT_FLAG_ALLOW_RETRY in fault_around().

Anyway, my patch kind of works, but I'm starting to hate it. I think I
want to try to extend the "->fault()" interface to allow
filemap_fault() to just fill in multiple pages.

We alread have that "vmf->page" thing, we could make it a small array
easily. That would allow proper gang lookup, and much more efficient
"fill in multiple entries in one go" in mm/memory.c.

> Heh! `man 3 mmap' actually suggests MAP_POPULATE | MAP_NONBLOCK.
> What's the story beyond MAP_NONBLOCK?

It does nothing, afaik.

                  Linus
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