linux-kernel - Re: PAGE_SIZE Availability Inconsistency

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Message-ID: <46009CEA.1050206@zytor.com>
Date:	Tue, 20 Mar 2007 19:48:10 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Anton Blanchard <anton@...ba.org>
CC:	David Miller <davem@...emloft.net>, hch@...radead.org,
	arjan@...radead.org, zippel@...ux-m68k.org, dmlb2000@...il.com,
	linux-kernel@...r.kernel.org, gdb@...rceware.org
Subject: Re: PAGE_SIZE Availability Inconsistency

Anton Blanchard wrote:
> Hi,
> 
>> The advantage would be that it wouldn't require a v3 for platforms for 
>> which MIN_PAGE_SIZE == PAGE_SIZE, which accounts for a very large 
>> percentage of systems.
>>
>> You still have to look for the darn magic in two places, so there is no 
>> reason for it to be different.
> 
> The problem is if you can hit in two places then what PAGE_SIZE should
> you use to size the contents of the swap header while remaining backward
> compatible.
> 
> Im leaning towards Dave suggestion of creating a clean v3 swap header.
> 

Changing the header format doesn't make *ANY* difference whatsoever.

You have to write two copies of the swap header, and the kernel should 
check for a header at MIN_PAGE_SIZE first and then at PAGE_SIZE.

If there are fields (other than position) in the v2 swap header that are 
dependent on PAGE_SIZE, then the copy at MIN_PAGE_SIZE should be sized 
using MIN_PAGE_SIZE, and the copy at PAGE_SIZE should be sized at 
PAGE_SIZE.  It's that simple.

Creating a new format will not help that one iota, and will create 
gratuitous incompatiblity for the very common case of PAGE_SIZE == 
MIN_PAGE_SIZE.

	-hpa

	-hpa

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