linux-kernel - Re: [PATCHv2, RFC 08/30] thp, mm: rewrite add_to_page_cache

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Date:	Fri, 22 Mar 2013 07:51:31 -0700
From:	Dave Hansen <dave@...1.net>
To:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
CC:	Andrea Arcangeli <aarcange@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Hugh Dickins <hughd@...gle.com>,
	Wu Fengguang <fengguang.wu@...el.com>, Jan Kara <jack@...e.cz>,
	Mel Gorman <mgorman@...e.de>, linux-mm@...ck.org,
	Andi Kleen <ak@...ux.intel.com>,
	Matthew Wilcox <matthew.r.wilcox@...el.com>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Hillf Danton <dhillf@...il.com>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCHv2, RFC 08/30] thp, mm: rewrite add_to_page_cache_locked()
 to support huge pages

On 03/22/2013 03:34 AM, Kirill A. Shutemov wrote:
> Dave Hansen wrote:
>> On 03/14/2013 10:50 AM, Kirill A. Shutemov wrote:
>>> +			error = radix_tree_insert(&mapping->page_tree,
>>> +					offset + i, page + i);
>>> +			if (error) {
>>> +				page_cache_release(page + i);
>>> +				break;
>>> +			}
>>>  		}
>>
>> Throughout all this new code, I'd really challenge you to try as much as
>> possible to minimize the code stuck under "if (PageTransHuge(page))".
> 
> I put thp-related code under the 'if' intentionally to be able to optimize
> it out if !CONFIG_TRANSPARENT_HUGEPAGE. The config option is disabled by
> default.

You've gotta give the compiler some credit! :)  In this function's case,
the compiler should be able to realize that nr=1 is constant if
!TRANSPARENT_HUGEPAGE.  It'll realize that all your for loops are:

	for (i = 0; i < 1; i++) {

and will end up generating _very_ similar code to what you get with the
explicit #ifdefs.  You already _have_ #ifdefs, but they're just up in
the headers around PageTransHuge()'s definition.

The advantages for you are *huge* since it means that folks will have a
harder time inadvertently breaking your CONFIG_TRANSPARENT_HUGEPAGE code.

>> Does the cgroup code know how to handle these large pages internally
>> somehow?  It looks like the charge/uncharge is only being done for the
>> head page.
> 
> It can. We only need to remove PageCompound() check there. Patch is in
> git.

OK, cool.  This _might_ deserve a comment or something here.  Again, it
looks asymmetric and fishy to someone just reading the code.

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