linux-kernel - Re: [PATCH 05/14] vrange: Add new vrange(2) system call

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Date:	Mon, 07 Oct 2013 16:26:41 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	John Stultz <john.stultz@...aro.org>
CC:	LKML <linux-kernel@...r.kernel.org>,
	Minchan Kim <minchan@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Android Kernel Team <kernel-team@...roid.com>,
	Robert Love <rlove@...gle.com>, Mel Gorman <mel@....ul.ie>,
	Hugh Dickins <hughd@...gle.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Rik van Riel <riel@...hat.com>,
	Dmitry Adamushko <dmitry.adamushko@...il.com>,
	Dave Chinner <david@...morbit.com>, Neil Brown <neilb@...e.de>,
	Andrea Righi <andrea@...terlinux.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	Mike Hommey <mh@...ndium.org>, Taras Glek <tglek@...illa.com>,
	Dhaval Giani <dhaval.giani@...il.com>, Jan Kara <jack@...e.cz>,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	Michel Lespinasse <walken@...gle.com>,
	Rob Clark <robdclark@...il.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH 05/14] vrange: Add new vrange(2) system call

On 10/07/2013 04:14 PM, John Stultz wrote:
>>
>> I see from the change history of the patch that this was an madvise() at
>> some point, but was changed into a separate system call at some point,
>> does anyone remember why that was?  A quick look through my LKML
>> archives doesn't really make it clear.
> 
> The reason we can't use madvise, is that to properly handle error cases
> and report the pruge state, we need an extra argument.
> 
> In much earlier versions, we just returned an error when setting
> NONVOLATILE if the data was purged. However, since we have to possibly
> do allocations when marking a range as non-volatile, we needed a way to
> properly handle that allocation failing. We can't just return ENOMEM, as
> we may have already marked purged memory as non-volatile.
> 
> Thus, that's why with vrange, we return the number of bytes modified,
> along with the purge state. That way, if an error does occur we can
> return the purge state of the bytes successfully modified, and only
> return an error if nothing was changed, much like when a write fails.
> 

I am not clear at all what the "purge state" is in this case.

	-hpa


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