On Tue, Dec 18, 2012 at 3:42 AM, Jens Axboe wrote:
> Bah. Does the below fix it up for you?
Grr. This is still bullshit.
alignment = sector << 9;
is fundamentally crap, because 'sector_t' may well be 32-bit
(non-large-block device case). And we're supposed (surprise surprise)
to be able to handle devices larger than 4GB in size.
So doing *any* of these calculations in bytes is pure and utter crap.
You need to do them in sectors. That's what "sector_t" means, and
that's damn well how everything should work. Anything that works in
bytes is simply pure crap. And don't talk to me about 64-bit math and
doing it in "u64" or "loff_t", that's just utterly moronic too.
Besides, "sector_div()" is only sensible when you're looking for the
remainder of a sector number. That's true in the first case (sector
really is a sector number - it's the starting sector of the
partition), but the source of alignment and granularity are actually
just "unsigned int" (and that's in bytes, not sectors), so using
sector_t afterwards is crazy too. You should have used just '%'.
Looking around, there are other places where this idiocy happens too
(blkdev_issue_discard() seems to think the granularity/alignments are
sector_t's too, for example).
Anyway, here's a patch to fix the crazy types and the bogus second
"sector_div()". It's whitespace-damaged, because not only have I not
tested it, I also think somebody needs to look at things in general.
The whole "discard_alignment" handling is extremely odd. I don't think
it should be called "alignment" at all - because it isn't. It's an
alignment *offset*. Look at the normal (non-discard) case, where it's
called "alignment_offset" like it should be.
So the math is confused, the types are confused, and the naming is
confused. Please, somebody check this out, because now *I* am
And btw, that whole commit happened too f*cking late too. When I get a
pull request, it should damn well have been tested already, and it
should have been developed *before* the merge window started. Not the
day before the pull request.
I'm grumpy, because all of this code is UTTER SH*T, and it was sent to me.
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index acb4f7bbbd32..c23cae25a0c0 100644
@@ -1188,14 +1188,25 @@ static inline int
queue_discard_alignment(struct request_queue *q)
static inline int queue_limit_discard_alignment(struct queue_limits
*lim, sector_t sector)
- sector_t alignment = sector << 9;
- alignment = sector_div(alignment, lim->discard_granularity);
+ /* Why are these in bytes, not sectors? */
+ unsigned int alignment, granularity, offset;
- alignment = lim->discard_granularity + lim->discard_alignment
- return sector_div(alignment, lim->discard_granularity);
+ alignment = lim->discard_alignment >> 9;
+ granularity = lim->discard_granularity >> 9;
+ if (!alignment || !granularity)
+ return 0;
+ /* Offset of the partition start in 'granularity' sectors */
+ offset = sector_div(sector, granularity);
+ /* And why do we do this modulus *again* in blkdev_issue_discard()?
+ offset = (granularity + alignment - offset) % granularity;
+ /* Turn it back into bytes, gaah */
+ return offset << 9;
static inline int bdev_discard_alignment(struct block_device *bdev)