Re: let md auto-detect 128+ raid members, fix potential racecondition

From: Andrew Morton
Date: Sun Jul 30 2006 - 17:19:43 EST

On Sun, 30 Jul 2006 17:56:31 -0300
Alexandre Oliva <aoliva@xxxxxxxxxx> wrote:

> On Jul 30, 2006, Andrew Morton <akpm@xxxxxxxx> wrote:
> > On Sun, 30 Jul 2006 03:56:21 -0300
> > Alexandre Oliva <aoliva@xxxxxxxxxx> wrote:
> >> -void md_autodetect_dev(dev_t dev);
> >> +int md_register_autodetect_dev(dev_t dev);
> > Put it in a header file, please.
> AFAICT it really isn't supposed to be used elsewhere. I suppose I
> could add it to either blkdev.h, fs.h or raid/md.h, since it's more of
> glue code between two modules than something that belongs to one
> specific module. E.g., if it goes in raid/md.h, where it feels the
> most appropriate, then fs/parititions/check.c has to include it, which
> doesn't sound right. OTOH, if it goes in blkdev.h or fs.h, then a lot
> of code ends up seeing the declaration that shouldn't be available.
> Thoughts?

If the function is exported by md then md.h would be an appropriate place
for the declaration.

> Maybe we could replace this with some register/unregister notifier
> interface, such that add_partitions() could then notify multiple
> watchers when a new partition is configured. This would remove the
> backwards dependency here, but I feel it should be done in a separate
> patch. I don't mind if they're integrated at once, but I don't feel
> that changing two unrelated issues at once is a good approach.

If we went that way then patch #1 would be "add a notifier" and patch #2
would be "use it in md".

Do we anticipate that there would ever be other users of this notifier
callback API?

> >> #ifdef CONFIG_BLK_DEV_MD
> >> - if (state->parts[p].flags)
> >> - md_autodetect_dev(bdev->bd_dev+p);
> >> + if (state->parts[p].flags
> >> + && md_register_autodetect_dev(bdev->bd_dev+p))
> >> + printk(KERN_ERR "md: out of memory registering %s%d\n",
> >> + disk->disk_name, p);
> >> #endif
> > What happens if CONFIG_BLK_DEV_MD=m?
> AFAIK then you'd get a link failure. One more reason to go with the
> notifier approach, I guess. It wouldn't quite enable md to
> auto-detect from partitions set up before the module was loaded, but
> it would at least remove this presumed link error.
> Another approach would be to split the autodetect stuff out of md.c
> into a separate file that goes in the main kernel image (if
> CONFIG_MD=y, it's never m) even if CONFIG_BLK_DEV_MD=m. Would this be
> a desirable arrangement?

I guess it sounds logical, but I'm not sure what the end result would look
like. Which amounts to noncommittal Sunday afternoon waffling ;)

The notifier certainly makes sense if we anticipate other users of it. If
we think it'll always be an md-special thing then yeah, I guess we can code
it that way.
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