linux-kernel - Re: [PATCH v2 3/3] pagemap: switch to the new format and do some cleanup

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Date:	Tue, 12 May 2015 13:54:31 +0300
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
Cc:	linux-mm@...ck.org, Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mark Williamson <mwilliamson@...o-software.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	linux-api@...r.kernel.org, Andy Lutomirski <luto@...capital.net>,
	Vlastimil Babka <vbabka@...e.cz>, Pavel Machek <pavel@....cz>,
	Mark Seaborn <mseaborn@...omium.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Daniel James <djames@...o-software.com>,
	Finn Grimwood <fgrimwood@...o-software.com>
Subject: Re: [PATCH v2 3/3] pagemap: switch to the new format and do some
 cleanup

On Tue, May 12, 2015 at 12:43:06PM +0300, Konstantin Khlebnikov wrote:
> This patch removes page-shift bits (scheduled to remove since 3.11) and
> completes migration to the new bit layout. Also it cleans messy macro.
> 
> Signed-off-by: Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
> ---
>  fs/proc/task_mmu.c    |  152 ++++++++++++++++---------------------------------
>  tools/vm/page-types.c |   29 +++------
>  2 files changed, 58 insertions(+), 123 deletions(-)
> 
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index 0b7a8ffec95f..66bc7207ce90 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -710,23 +710,6 @@ const struct file_operations proc_tid_smaps_operations = {
>  	.release	= proc_map_release,
>  };
>  
> -/*
> - * We do not want to have constant page-shift bits sitting in
> - * pagemap entries and are about to reuse them some time soon.
> - *
> - * Here's the "migration strategy":
> - * 1. when the system boots these bits remain what they are,
> - *    but a warning about future change is printed in log;
> - * 2. once anyone clears soft-dirty bits via clear_refs file,
> - *    these flag is set to denote, that user is aware of the
> - *    new API and those page-shift bits change their meaning.
> - *    The respective warning is printed in dmesg;
> - * 3. In a couple of releases we will remove all the mentions
> - *    of page-shift in pagemap entries.
> - */

Wouldn't it be better to just have v2=1 by default for couple releases to
see if anything breaks? This way we can revert easily if regression reported.
I guess someone could miss this change coming if he didn't touch clear_refs.

-- 
 Kirill A. Shutemov
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