linux-ext4 - Re: [PATCH] e2fsprogs: Limit number of reserved gdt blocks on small fs

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Date:	Mon, 02 Mar 2015 11:27:11 -0600
From:	Eric Sandeen <sandeen@...hat.com>
To:	Lukáš Czerner <lczerner@...hat.com>
CC:	linux-ext4@...r.kernel.org
Subject: Re: [PATCH] e2fsprogs: Limit number of reserved gdt blocks on small
 fs

On 3/2/15 11:08 AM, Lukáš Czerner wrote:
> On Mon, 2 Mar 2015, Eric Sandeen wrote:

...

>> Some extra "making sure" in the above sentence.  ;)
> 
> Right, I'll fix that.
> 
>>
>>>                    reserved_gdt_blocks is small enough. If the user
>>> +	 * specifies non-default values he can still run into this issue
>>> +	 * but we do not want to make that mistake by default.
>>> +	 *
>>> +	 * Magic numbers:
>>> +	 * - At block count smaller than 32768, journal will be 1024 block
>>> +	 *   blocks long which would not be enough by default.
>>> +	 * - 64 reserved gtd blocks is maximum number we can fit into said
>>
>> gtd?  (gdt?  gdb?)
> 
> Oh yes...
> 
> I'll try to use it consistently.

thanks :)

>>
>>> +	 *   journal by default (flex_bg_count * 4 + rsv_gdb)
>>
>> This is a little confusing; "flex_bg_count" isn't a thing, and it seems to
>> say that the reserved blocks depend on the number of reserved blocks?
> 
> Does it ? It says that number of journal credits depend on the
> number of reserved gdt blocks, or maybe I am missing something.
> 
> flex_bg_count is a number of groups in flex group, I'll try to come
> up with a better name.

It just looks like a variable name; it's one that doesn't exist, so it's
not clear what you're talking about.

>>
>> I'm curious, why does this matter only for 1k blocks, and not, say 2k blocks?
> 
> That's because of the way we compute the number of reserved GDT
> blocks. The number of GDT blocks depends on the number of block
> groups, which depends on the number of blocks in the file system.
> And with 1k file system we have much more blocks. In fact we have:
> 
> - twice as many blocks as with 2k file system
> - four times the block groups
> 
> And we can also fit only half of the number of group descriptors
> into a single block which means that we need twice as many blocks
> per block group.
> 
> Which means that with 1k file system we need 8 times as many gdt
> blocks as with 2k fs and 2048 times more gdt blocks as with 4k with
> the same setup.
> 
>>
>>> +	 */
>>> +	if ((fs->blocksize == 1024) && (ext2fs_blocks_count(sb) < 32768) &&
>>> +	    (rsv_gdb > 64))
>>> +		rsv_gdb = 64;
>>> +
>>>  	if (rsv_gdb > EXT2_ADDR_PER_BLOCK(sb))
>>>  		rsv_gdb = EXT2_ADDR_PER_BLOCK(sb);
>>>  #ifdef RES_GDT_DEBUG
>>> diff --git a/misc/mke2fs.c b/misc/mke2fs.c
>>> index aeb852f..e9edd3b 100644
>>> --- a/misc/mke2fs.c
>>> +++ b/misc/mke2fs.c
>>> @@ -3076,6 +3076,34 @@ int main (int argc, char *argv[])
>>>  		}
>>>  		if (!quiet)
>>>  			printf("%s", _("done\n"));
>>> +
>>> +		/*
>>> +		 * In online resize we require a huge number of journal
>>> +		 * credits mainly because of the reserved_gdt_blocks. The
>>> +		 * exact number of journal credits needed is:
>>> +		 * flex_gd->count * 4 + reserved_gdb
>>> +		 *
>>> +		 * Warn user if we will not have enough credits to resize
>>> +		 * the file system online.
>>> +		 */
>>> +		if (fs_param.s_feature_compat & EXT2_FEATURE_COMPAT_RESIZE_INODE)
>>> +		{
>>> +			unsigned int credits;
>>> +
>>> +			/*
>>> +			* This is the amount we're allowed to use for a
>>> +			* single handle in a transaction
>>> +			*/
>>> +			journal_blocks /= 8;
>>
>> This seems a little dangerous; if someone decides to use journal_blocks
>> later in the function, it might be unexpectedly smaller.
>>
>>> +			credits = (1 << fs_param.s_log_groups_per_flex) * 4 +
>>> +				  fs->super->s_reserved_gdt_blocks;
>>> +			if (credits > journal_blocks) {
>>
>> so maybe just scale this by 8, i.e. "if (credits > journal_blocks/8)"
> 
> Fair enough, I'll change that.
> 
>>
>> And actually, I find myself wondering if this same calculation can be used
>> in calc_reserved_gdt_blocks, rather than using the magic numbers?
> 
> Unfortunately it can not. At the time we're calculating number of gdt
> blocks we have no idea how big the journal will be.
> 
> And at the time we already know how big the journal will be we
> already have blocks allocated for the resize_inode so we can not
> just simply change s_reserved_gdt_blocks here.

Ugh.  That sounds like a mess.

But it feels like we need centralized functions or macros that express the
relationships between these values, rather than sprinkling "* 4" and "if 1024"
around.  It seems to have the potential to get out of sync, but I've only
given it cursory thought...

-Eric
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