linux-ext4 - Re: [RFC PATCH -v2 7/9] ext4: don't use the block freed but not yet committed during buddy initialization

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Message-ID: <20081104171515.GL30291@mit.edu>
Date:	Tue, 4 Nov 2008 12:15:15 -0500
From:	Theodore Tso <tytso@....edu>
To:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
Cc:	cmm@...ibm.com, sandeen@...hat.com, linux-ext4@...r.kernel.org
Subject: Re: [RFC PATCH -v2 7/9] ext4: don't use the block freed but not
	yet committed during buddy initialization

On Mon, Nov 03, 2008 at 11:06:07PM +0530, Aneesh Kumar K.V wrote:
> +static void ext4_mb_generate_from_freelist(struct super_block *sb, void *bitmap,
> +					ext4_group_t group,
> +					struct ext4_free_data *entry)
> +{
	...
> +	if (n->rb_left) {
> +		new_entry = rb_entry(n->rb_left, struct ext4_free_data, node);
> +		ext4_mb_generate_from_freelist(sb, bitmap, group, new_entry);
> +	}
> +	if (n->rb_right) {
> +		new_entry = rb_entry(n->rb_right, struct ext4_free_data, node);
> +		ext4_mb_generate_from_freelist(sb, bitmap, group, new_entry);
> +	}

ext4_mb_generate_from_freelist() is recursively calling itself, which
could easily blow the stack if there are a large number of items on
the free list (remember, this can include data blocks if
!ext4_should_writeback_data()).

You should probably use rb_first and rb_next in a loop rather than a
recursive descent.  I also remain concerned that
ext4_mb_generate_from_freelist() is could burn a large amount of CPU
in some cases, and as I said on the conference call, if there is a way
to avoid it, that would be a Good Thing.

						- Ted
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