linux-kernel - Re: New helper to free highmem pages in larger chunks

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Date:	Tue, 6 Oct 2015 05:35:57 +0000
From:	Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	"arc-linux-dev@...opsys.com" <arc-linux-dev@...opsys.com>,
	Robin Holt <robin.m.holt@...il.com>,
	Nathan Zimmer <nzimmer@....com>, Jiang Liu <liuj97@...il.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	lkml <linux-kernel@...r.kernel.org>, Mel Gorman <mgorman@...e.de>
Subject: Re: New helper to free highmem pages in larger chunks

On Tuesday 06 October 2015 03:40 AM, Andrew Morton wrote:
> On Sat, 3 Oct 2015 18:25:13 +0530 Vineet Gupta <Vineet.Gupta1@...opsys.com> wrote:
>
>> Hi,
>>
>> I noticed increased boot time when enabling highmem for ARC. Turns out that
>> freeing highmem pages into buddy allocator is done page at a time, while it is
>> batched for low mem pages. Below is call flow.
>>
>> I'm thinking of writing free_highmem_pages() which takes start and end pfn and
>> want to solicit some ideas whether to write it from scratch or preferably call
>> existing __free_pages_memory() to reuse the logic to convert a pfn range into
>> {pfn, order} tuples.
>>
>> For latter however there are semantical differences as you can see below which I'm
>> not sure of:
>>   -highmem page->count is set to 1, while 0 for low mem
> That would be weird.
>
> Look more closely at __free_pages_boot_core() - it uses
> set_page_refcounted() to set the page's refcount to 1.  Those
> set_page_count() calls look superfluous to me.

If you closer still, set_page_refcounted() is called outside the loop for the
first page only. For all pages, loop iterator sets them to 1. Turns out there's
more fun here....

I ran this under a debugger and much earlier in boot process, there's existing
setting of page count to 1 for *all* pages of *all* zones (include highmem pages).
See call flow below.

free_area_init_node
    free_area_init_core
        loops thru all zones
            memmap_init_zone
               loops thru all pages of zones
               __init_single_page

This means the subsequent setting of page count to 0 (or 1 for the special first
page) is superfluous - actually buggy at best. I will send a patch to fix that. I
hope I don't break some obscure init path which doesn't hit the above init.


>
>>   -atomic clearing of page reserved flag vs. non atomic
> I doubt if the atomic is needed - who else can be looking at this page
> at this time?

I'll send another one to separately fix that as well. Seems like boot mem setup is
a relatively neglect part of kernel.

-Vineet
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