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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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