linux-kernel - Re: [PATCHv3 0/2] mm: map few pages around fault address if they are in page cache

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Date:	Fri, 28 Feb 2014 02:10:39 +0200
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Andi Kleen <ak@...ux.intel.com>,
	Matthew Wilcox <matthew.r.wilcox@...el.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Dave Chinner <david@...morbit.com>, Ning Qu <quning@...il.com>,
	linux-mm <linux-mm@...ck.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCHv3 0/2] mm: map few pages around fault address if they are
 in page cache

On Thu, Feb 27, 2014 at 01:28:22PM -0800, Linus Torvalds wrote:
> On Thu, Feb 27, 2014 at 11:53 AM, Kirill A. Shutemov
> <kirill.shutemov@...ux.intel.com> wrote:
> > Here's new version of faultaround patchset. It took a while to tune it and
> > collect performance data.
> 
> Andrew, mind taking this into -mm with my acks? It's based on top of
> Kirill's cleanup patches that I think are also in your tree.
> 
> Kirill - no complaints from me. I do have two minor issues that you
> might satisfy, but I think the patch is fine as-is.
> 
> The issues/questions are:
> 
>  (a) could you test this on a couple of different architectures? Even
> if you just have access to intel machines, testing it across a couple
> of generations of microarchitectures would be good. The reason I say
> that is that from my profiles, it *looks* like the page fault costs
> are relatively higher on Ivybridge/Haswell than on some earlier
> uarchs.

These numbers were from Ivy Bridge.
I'll bring some numbers for Westmere and Haswell.

>  (b) I suspect we should try to strongly discourage filesystems from
> actually using map_pages unless they use the standard
> filemap_map_pages function as-is. Even with the fairly clean
> interface, and forcing people to use "do_set_pte()", I think the docs
> might want to try to more explicitly discourage people from using this
> to do their own hacks..

We would need ->map_pages() at least for shmem/tmpfs. It should be
benefitial there.

Also Matthew noticed that some drivers do ugly hacks like fault in whole
VMA on first page fault. IIUC, it's for performance reasons. See
psbfb_vm_fault() or ttm_bo_vm_fault().

I thought it could be reasonable to have ->map_pages() there and do VMA
population get_user_pages() on mmap() instead.

What do you think?

-- 
 Kirill A. Shutemov
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