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SubjectRe: [PATCH] block: fix accounting bug on cross partition merges
On Thu, Dec 23, 2010 at 06:04:11PM +0100, Jerome Marchand wrote:
> On 12/23/2010 04:39 PM, Vivek Goyal wrote:
> >> diff --git a/block/blk-core.c b/block/blk-core.c
> >> index 4ce953f..72d12d2 100644
> >> --- a/block/blk-core.c
> >> +++ b/block/blk-core.c
> >> @@ -64,13 +64,21 @@ static void drive_stat_acct(struct request *rq, int new_io)
> >> return;
> >>
> >> cpu = part_stat_lock();
> >> - part = disk_map_sector_rcu(rq->rq_disk, blk_rq_pos(rq));
> >>
> >> - if (!new_io)
> >> + if (!new_io) {
> >> + part = rq->part;
> >> part_stat_inc(cpu, part, merges[rw]);
> >> - else {
> >> + } else {
> >> + part = disk_map_sector_rcu(rq->rq_disk, blk_rq_pos(rq));
> >> + if(part->partno && !kref_test_and_get(&part->ref))
> >> + /*
> >
> > Do we have to check this part->partno both while taking and releasing
> > reference. Can't we take one extra reference for disk->part0, at
> > alloc_disk_node() time so that it is never freed and only freed when
> > disk is going away and gendisk is being freed.
> >
> > That way, you don't have to differentiate between disk->part0 and rest
> > of the partitions while taking or dropping references.
>
> We could do it in any way, as long as we don't end up trying to free
> disk->part0. I choose not to touch part0->ref at all, but we could also
> drop all the part->partno test, and get a reference on part0 when we use
> it as a backup. I have no strong opinion about a way or an other.

Ok, I personally like taking an extra reference to disk->part0 and put
a proper comment there so that at rest of the places we don't try to
differentiate between part0 and other partitions.
Having said that I am not too concerned about it. It is just one of the
minor details. So post the new version of patch after testing.

Thanks
Vivek


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