Jens Axboe wrote:
On Thu, Dec 13 2007, Mark Lord wrote:
Jens Axboe wrote:
On Thu, Dec 13 2007, Jens Axboe wrote:
On Thu, Dec 13 2007, Mark Lord wrote:
Jens Axboe wrote:
On Thu, Dec 13 2007, Mark Lord wrote:
Mark Lord wrote:
Jens Axboe wrote:
On Thu, Dec 13 2007, Mark Lord wrote:
Matthew Wilcox wrote:
On Thu, Dec 13, 2007 at 01:48:18PM -0500, Mark Lord wrote:
Problem confirmed. 2.6.23.8 regularly generates segments up to
64KB for libata,
but 2.6.24 uses only 4KB segments and a *few* 8KB segments.
Just a suspicion ... could this be slab vs slub? ie check your
configs
are the same / similar between the two kernels.
..
Mmmm.. a good thought, that one.
But I just rechecked, and both have CONFIG_SLAB=y
My guess is that something got changed around when Jens
reworked the block layer for 2.6.24.
I'm going to dig around in there now.
I didn't rework the block layer for 2.6.24 :-). The core block layer
changes since 2.6.23 are:
- Support for empty barriers. Not a likely candidate.
- Shared tag queue fixes. Totally unlikely.
- sg chaining support. Not likely.
- The bio changes from Neil. Of the bunch, the most likely suspects
in
this area, since it changes some of the code involved with merges and
blk_rq_map_sg().
- Lots of simple stuff, again very unlikely.
Anyway, it sounds odd for this to be a block layer problem if you do
see
occasional segments being merged. So it sounds more like the input
data
having changed.
Why not just bisect it?
..
Because the early 2.6.24 series failed to boot on this machine
due to bugs in the block layer -- so the code that caused this
regression
is probably in the stuff from before the kernels became usable here.
..
That sounds more harsh than intended --> the earlier 2.6.24 kernels
(up to
the first couple of -rc* ones failed here because of incompatibilities
between the block/bio changes and libata.
That's better, I think!
No worries, I didn't pick it up as harsh just as an odd conclusion :-)
If I were you, I'd just start from the first -rc that booted for you. If
THAT has the bug, then we'll think of something else. If you don't get
anywhere, I can run some tests tomorrow and see if I can reproduce it
here.
..
I believe that *anyone* can reproduce it, since it's broken long before
the requests ever get to SCSI or libata. Which also means that *anyone*
who wants to can bisect it, as well.
I don't do "bisects".
It was just a suggestion on how to narrow it down, do as you see fit.
But I will dig a bit more and see if I can find the culprit.
Sure, I'll dig around as well.
Just tried something simple. I only see one 12kb segment so far, so not
a lot by any stretch. I also DONT see any missed merges signs, so it
would appear that the pages in the request are simply not contigious
physically.
diff --git a/block/ll_rw_blk.c b/block/ll_rw_blk.c
index e30b1a4..1e34b6f 100644
--- a/block/ll_rw_blk.c
+++ b/block/ll_rw_blk.c
@@ -1330,6 +1330,8 @@ int blk_rq_map_sg(struct request_queue *q, struct
request *rq,
goto new_segment;
sg->length += nbytes;
+ if (sg->length > 8192)
+ printk("sg_len=%d\n", sg->length);
} else {
new_segment:
if (!sg)
@@ -1349,6 +1351,8 @@ new_segment:
sg = sg_next(sg);
}
+ if (bvprv && (page_address(bvprv->bv_page) +
bvprv->bv_len == page_address(bvec->bv_page)))
+ printk("missed merge\n");
sg_set_page(sg, bvec->bv_page, nbytes,
bvec->bv_offset);
nsegs++;
}
..
Yeah, the first part is similar to my own hack.
For testing, try "dd if=/dev/sda of=/dev/null bs=4096k".
That *really* should end up using contiguous pages on most systems.
I figured out the git thing, and am now building some in-between kernels to
try.
OK, it's a vm issue, I have tens of thousand "backward" pages after a
boot - IOW, bvec->bv_page is the page before bvprv->bv_page, not
reverse. So it looks like that bug got reintroduced.
...
Mmm.. shouldn't one of the front- or back- merge logics work for either order?
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