linux-ext4 - Re: [PATCH 0/2] new API to allocate buffer-cache for superblock in non-movable area

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Date:	Wed, 30 Jul 2014 16:56:27 +0900
From:	Gioh Kim <gioh.kim@....com>
To:	Theodore Ts'o <tytso@....edu>,
	Peter Zijlstra <peterz@...radead.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Jan Kara <jack@...e.cz>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	linux-ext4@...r.kernel.org, Joonsoo Kim <js1304@...il.com>,
	Minchan Kim <minchan@...nel.org>
Subject: Re: [PATCH 0/2] new API to allocate buffer-cache for superblock in
 non-movable area

2014-07-27 오전 10:01, Theodore Ts'o 쓴 글:
> Gioh,
>
> As follow up, if you want some further discussions about why these
> patches should be accepted, it would be good to get some hard data
> about why the keeping the ext4 superblock pinned is causing such a
> problem for page migation.  Can you give us more details about what
> the impact is of not having these patches?  And how it compres to
> other data structures which are currently allocated in the moveable
> area and tend to be pinned effectively indefinitely?
>
> Thanks,
>
> 						- Ted
>

I am very sorry to be late. I couldn't access the network for a week.

sb_bread() allocates page from movable area but the reference count of the buffer-head
that manages page should be zero to migrate the page.
Therefore brelase() should be called immediately after sb_bread() such like fat_fill_super().
But ext4 called brelse() when unmount the superblock.
The page cannot be movable until unmount.
CMA/HOTPLUG memory try to move the page but it fails.

If ext4 needs to keep buffer-cache of superblock until unmount,
it should allocated the page from non-movable area (because it can be a long time).
This patch try to do it.

I also sent an email to Jan Kara. Please refer it.

Thank you for your kindness.
Please inform me if you need any information.
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