linux-ext4 - RE: [PATCH 2/3] ext4: Context support

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Date:	Fri, 22 Jun 2012 14:07:29 +0000
From:	"Luca Porzio (lporzio)" <lporzio@...ron.com>
To:	Artem Bityutskiy <dedekind1@...il.com>,
	Arnd Bergmann <arnd.bergmann@...aro.org>
CC:	"Ted Ts'o" <tytso@....edu>,
	Alex Lemberg <Alex.Lemberg@...disk.com>,
	HYOJIN JEONG <syr.jeong@...sung.com>,
	Saugata Das <saugata.das@...aro.org>,
	Saugata Das <saugata.das@...ricsson.com>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-mmc@...r.kernel.org" <linux-mmc@...r.kernel.org>,
	"patches@...aro.org" <patches@...aro.org>,
	"venkat@...aro.org" <venkat@...aro.org>
Subject: RE: [PATCH 2/3] ext4: Context support

Hallo,

> -----Original Message-----
> From: linux-mmc-owner@...r.kernel.org [mailto:linux-mmc-owner@...r.kernel.org]
> On Behalf Of Artem Bityutskiy
> Sent: Friday, June 22, 2012 3:29 PM
> To: Arnd Bergmann
> Cc: Ted Ts'o; Alex Lemberg; HYOJIN JEONG; Saugata Das; Saugata Das; linux-
> ext4@...r.kernel.org; linux-fsdevel@...r.kernel.org; linux-
> mmc@...r.kernel.org; patches@...aro.org; venkat@...aro.org; Luca Porzio
> (lporzio)
> Subject: Re: [PATCH 2/3] ext4: Context support
> 
> On Wed, 2012-06-13 at 19:44 +0000, Arnd Bergmann wrote:
> > I think using the inode number is a reasonable fit. Using the inode
> > number of the parent directory might be more appropriate but it breaks
> > with hard links and cross-directory renames (we must not use the same
> > LBA with conflicting context numbers, or flush the old context
> > inbetween).
> 
> I would put it this way.
> 
> 1. contex = inode number for data blocks.
> 2. context = parent directory's inode number for stat data (times,
> permissions, etc) blocks and directory entry blocks. Should help things like
> readdir and readdir + stat. Besides, this stuff tend to change more often than
> the data, so mixing it with the data in the same eraseblock is not smart.
> 3. context = parent inode number for all the stuff belonging to xattrs.
> 
> We do something similar in UBIFS.
> 

Doesn't this end up using too many contexts?
Opening one contexts per inode would end up in opening more contexts than available.
The eMMC spec forbids more than 15 contexts for the whole device.

> --
> Best Regards,
> Artem Bityutskiy

Cheers,
    Luca