linux-kernel - Re: [RFC, PATCHv2 0/2] mm: map few pages around fault address if they are in page cache

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Message-ID: <CA+55aFwEAYhhUijNUf1dRppzh=+5QfXTAdGQe8D_mJH77tPHug@mail.gmail.com>
Date:	Tue, 18 Feb 2014 10:28:11 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:	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>,
	linux-mm <linux-mm@...ck.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC, PATCHv2 0/2] mm: map few pages around fault address if they
 are in page cache

On Tue, Feb 18, 2014 at 10:07 AM, Kirill A. Shutemov
<kirill.shutemov@...ux.intel.com> wrote:
>
> Patch is wrong. Correct one is below.

Hmm. I don't hate this. Looking through it, it's fairly simple
conceptually, and the code isn't that complex either. I can live with
this.

I think it's a bit odd how you pass both "max_pgoff" and "nr_pages" to
the fault-around function, though. In fact, I'd consider that a bug.
Passing in "FAULT_AROUND_PAGES" is just wrong, since the code cannot -
and in fact *must* not - actually fault in that many pages, since the
starting/ending address can be limited by other things.

So I think that part of the code is bogus. You need to remove
nr_pages, because any use of it is just incorrect. I don't think it
can actually matter, since the max_pgoff checks are more restrictive,
but if you think it can matter please explain how and why it wouldn't
be a major bug?

Apart from that, I'd really like to see numbers for different ranges
of FAULT_AROUND_ORDER, because I think 5 is pretty high, but on the
whole I don't find this horrible, and you still lock the page so it
doesn't involve any new rules. I'm not hugely happy with another raw
radix-tree user, but it's not horrible.

Btw, is the "radix_tree_deref_retry(page) -> goto restart" really
necessary? I'd be almost more inclined to just make it just do a
"break;" to break out of the loop and stop doing anything clever at
all.

IOW, from a quick look there's a couple of small details I don't like
that look odd, but ..

                Linus
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