linux-ext4 - Questions about mballoc's stream allocation

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Message-Id: <E1MZPAn-0001Y0-TQ@closure.thunk.org>
Date:	Fri, 07 Aug 2009 09:07:53 -0400
From:	"Theodore Ts'o" <tytso@....edu>
To:	Andreas Dilger <adilger@....com>, Alex Tomas <bzzz@....com>
cc:	linux-ext4@...r.kernel.org
Subject: Questions about mballoc's stream allocation

I've got two questions about mballoc's stream allocation.

First of all, in ext4_mb_regular_allocator(), I'm 99% sure this is a
bug:

	/* if stream allocation is enabled, use global goal */
	size = ac->ac_o_ex.fe_logical + ac->ac_o_ex.fe_len;
	isize = i_size_read(ac->ac_inode) >> bsbits;
	if (size < isize)
		size = isize;

	if (size < sbi->s_mb_stream_request &&
	    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
			(ac->ac_flags & EXT4_MB_HINT_DATA)) {
		/* TBD: may be hot point */
		spin_lock(&sbi->s_md_lock);
		ac->ac_g_ex.fe_group = sbi->s_mb_last_group;
		ac->ac_g_ex.fe_start = sbi->s_mb_last_start;
		spin_unlock(&sbi->s_md_lock);
	}

Shouldn't that be ">=", not "<".  We want to use the values saved in
sbi->s_mb_last_{group,start} only if we are doing a stream allocation,
which would be true only if the file is *larger* than
s_mb_stream_request, no?

The second question I have is with regards to ext4_mb_use_best_found(),
we set sbi->s_mb_last_{group,start} on any data allocation; shouldn't we
only be setting those values only if we were doing a stream allocation
in the first place?

Otherwise, any kind of allocation will end up moving the global goal
block for stream allocations; even if it is a small allocation in the
middle of some block group caused by the flag EXT4_MB_HINT_NO_PREALLOC
being set.

Am I missing anything?

     	   	      	       		     - Ted
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