linux-ext4 - Re: ditching e4b->alloc

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Message-ID: <AANLkTikW04brZ549=29N1Yan=GH29BN4WSVrA7erR_F6@mail.gmail.com>
Date:	Sun, 6 Mar 2011 22:15:41 +0200
From:	Amir Goldstein <amir73il@...il.com>
To:	"Ted Ts'o" <tytso@....edu>
Cc:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Re: ditching e4b->alloc_semp

On Sat, Mar 5, 2011 at 6:06 PM, Ted Ts'o <tytso@....edu> wrote:
> On Mon, Feb 21, 2011 at 10:02:44PM +0200, Amir Goldstein wrote:
>>
>> 1. (AKA easy lane) use a single page (or more) per block group.
>> this will increase the memory usage for 1K blocks fs and for 2K block fs
>> on 8K page system, but are these use cases really that common?
>
> The most common use cases will be 4k block file system on 16k page
> systems, which show up on PowerPC and Itanium systems.

OK. no easy way out.

>
>> 2. (AKA hard lane) attach buffer heads to buddy page and use
>> buffer_uptodate() and buffer_lock() instead of PageUptodate() and lock_page()
>> to initialize buddy cache of groups that share the same page.
>
> How about this; use lock_page() to guarantee exclusive access to the
> shared buddy bitmap, and then define a new bit in
> ext4_group_info->bb_state to indicate whether or not a particular
> block group has been initialized.  If the page has gotten flushed from
> memory, so that it is not present at all (i.e., find_get_page returns
> NULL), then iterate over all of the groups to clear the
> EXT4_GROUP_INFO_BUDDY_INIT bit.
>
> If the page is returned by find_get_page(), then all you need to do is
> check the EXT4_GROUP_INFO_BUDDY_INIT bit to discover whether or not or
> not the buddy bitmap needs to be initialized.
>

That sounds about right, but why do I need a new bit?
Why can't I use EXT4_GROUP_INFO_NEED_INIT_BIT to tell me the exact same thing?
The only difference from current implementation is that I would have
to test the bit again
after taking page_lock().

I didn't have time to check the implementation into details yet.
I will try to get to it during this week and send a tryout patch.

Amir.
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