Re: [Ext2-devel] [PATCH 2/2] ext2/3: Support2^32-1blocks(e2fsprogs)

On Tue, 2006-03-28 at 09:15 +0200, Laurent Vivier wrote:
Le mar 28/03/2006 à 00:58, Ravikiran G Thirumalai a écrit :
On Mon, Mar 27, 2006 at 01:10:49PM -0800, Andrew Morton wrote:
Mingming Cao <cmm@xxxxxxxxxx> wrote:

I am wondering if we have (or plan to have) "long long " type of percpu
counters? Andrew, Kiran, do you know?

It seems right now the percpu counters are used mostly by ext2/3 for
filesystem free blocks accounting. Right now the counter is "long" type,
which is not enough if we want to extend the filesystem limit from 2**31
to 2**32 on 32 bit machine.

The patch from Takashi copies the whole percpu_count.h and create a new
percpu_llcounter.h to support longlong type percpu counters. I am
wondering is there any better way for this?


I can't immediately think of anything smarter.

One could of course implement a 64-bit percpu counter by simply
concatenating two 32-bit counters. That would be a little less efficient,
but would introduce less source code and would mean that we don't need to
keep two different implemetations in sync. But one would need to do a bit
of implementation, see how bad it looks.

Since long long is 64 bits on both 32bit and 64 bit arches, we can just
change percpu_counter type to long long (or s64) and just have one
implementation of percpu_counter?
But reads and writes on 64 bit counters may not be atomic on all 32 bit arches.
So the implementation might have to be reviewed for that.

As 64bit per cpu counter is used only by ext3 and needed only on 64bit
architecture and when CONFIG_LBD is set, perhaps we can have only one
implementation, 32bit in the case of 32bit arch and 64bit in the case of
64bit arch + LBD, as I did in my 64bit patches for ext3 ?


The current percpu counter on 32 bit machine is "long", a signed value.
It's a problem for ext3 on 32 bit arch also, as the total number of free
blocks in ext3 is a type of u32. Isn't it? Did I miss something?


Mingming
Cheers,
Laurent

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