Re: ditching e4b->alloc_semp

[Amir Goldstein <amir73il@gmail.com>]

On Mon, Mar 7, 2011 at 6:08 AM, Ted Ts'o <tytso@mit.edu> wrote:
> On Sun, Mar 06, 2011 at 10:15:41PM +0200, Amir Goldstein wrote:
>>
>> 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 current meaning of NEED_INIT_BIT is that it indicates that the
> group has been initialized once since the file system has been
> mounted.  It is used by ext4_mb_good_group() to know whether it can
> rely on ext4_group_info->bb_free, ext4_group_info->bb_fragments,
> ext4_group_info->bb_largest_free_order, et. al, without needing to
> reload the buddy bitmap.
>
> We added this so that even if memory pressure has forced the buddy
> bitmap and block allocation bitmaps out of memory, we have enough
> information in the ext4_group_info summary array that we can quickly
> decide whether or not a group is a likely good candidate to be
> examined more closely to have the necessary free blocks.  Without this
> (relatively recent) change, the mballoc code might potentially need to
> read in tens if not hundreds of block allocation bitmaps only to find
> that it didn't have enough contiguous blocks, and then the memory
> pressure would push the block bitmap out of memory again.... and file
> system performance would go into the toilet.
>

Right... we need it.
I also wanted to examine if clearing the NEED_INIT_BIT on add_group_blocks
is really necessary.
Couldn't the buddy bitmap of partial group be initialized with all
blocks at the end
"used", similar to the block bitmap itself?
Then add_group_blocks() could just "free" the extra added blocks.

>
> So we don't want to disturb the meaning of this particular bit.  If we
> zap the NEED_INIT_BIT whenever we discover that the group's buddy
> bitmap page has been pushed out of memory, then we will once again
> need to read in massive numbers of block bitmaps because clearing the
> bit effectively marks the summary information stored ext4_group_info
> structure as invalid.
>
>                                                        - Ted
>
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