linux-ext4 - Re: [PATCH 15/15][e2fsprogs] 64-bit mke2fs cleanup

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Date:	Wed, 16 Jul 2008 17:21:58 +0200
From:	Goswin von Brederlow <goswin-v-b@....de>
To:	"Jose R. Santos" <jrs@...ibm.com>
Cc:	Goswin von Brederlow <goswin-v-b@....de>,
	"Theodore Ts'o" <tytso@....edu>, linux-ext4@...r.kernel.org
Subject: Re: [PATCH 15/15][e2fsprogs] 64-bit mke2fs cleanup

"Jose R. Santos" <jrs@...ibm.com> writes:

> On Wed, 16 Jul 2008 14:50:14 +0200
> Goswin von Brederlow <goswin-v-b@....de> wrote:
>
>> "Jose R. Santos" <jrs@...ibm.com> writes:
>> 
>> > diff --git a/misc/mke2fs.c b/misc/mke2fs.c
>> > index 7171990..817be56 100644
>> > --- a/misc/mke2fs.c
>> > +++ b/misc/mke2fs.c
>> > @@ -1402,12 +1402,12 @@ static void PRS(int argc, char *argv[])
>> >  
>> >  	fs_param.s_log_frag_size = fs_param.s_log_block_size;
>> >  
>> > -	if (noaction && fs_param.s_blocks_count) {
>> > -		dev_size = fs_param.s_blocks_count;
>> > +	if (noaction && ext2fs_blocks_count(&fs_param)) {
>> > +		dev_size = ext2fs_blocks_count(&fs_param);
>> >  		retval = 0;
>> >  	} else {
>> >  	retry:
>> > -		retval = ext2fs_get_device_size(device_name,
>> > +		retval = ext2fs_get_device_size2(device_name,
>> >  						EXT2_BLOCK_SIZE(&fs_param),
>> >  						&dev_size);
>> >  		if ((retval == EFBIG) &&
>> 
>> You should not assume that ext2fs_get_device_size2() will return EFBIG
>> if the size exceed 2^32 blocks. The point of ext2fs_get_device_size2()
>> was to be able to support more than 2^32 blocks and the library
>> function will certainly change to return larger numbers in the future.
>> 
>> So you need something like
>> 
>> if ((retval == EFBIG || dev_size >= (1ULL << 32)) &&
>
> No, ext2fs_get_device_size2() should return EFBIG size if the device is
> larger than 48-bit.  I need to fix that on ext2fs_get_device_size2() but 
> this check here is ok.
>
>> MfG
>>         Goswin
>
> -JRS

Why? The decision what device size if too big depends on the features
for this filesystem. By default 32bit is the limit, with
EXT4_FEATURE_INCOMPAT_64BIT it is 48bit. (shouldn't that be named
EXT4_FEATURE_INCOMPAT_48BIT?). Maybe later there will be a true 64bit
feature.

If you make ext2fs_get_device_size2() return EFBIG for >48bit then you
have to check for EFBIG and conditionally for 2^32 which makes the
code more complex. Better to just check the actual size the device has
in mke2fs.c.

Also the user might give a size that is actually small enough while
the device would be too big. Say, with current mke2fs, you have a disk
that is 16.000001TiB large. You could tell mke2fs to only use 2^32-1
blocks. The EFBIG check prevents that.

See the patch I just send.

MfG
        Goswin
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