On Mon, Jun 30 2008, Andrew Morton wrote:
> On Sun, 08 Jun 2008 19:45:08 +0400
> Dmitri Monakhov <dmonakhov@xxxxxxxxxx> wrote:
>
> > We have to properly decrease all related bio's counters, especially bi_size
> > in order to merge_bvec_fn return right result. Usually this result in
> > false merge rejects for two absolutely valid bio_vecs. This may cause
> > significant performance penalty for example Itanium: page_size == 16k,
> > fs_block_size == 1k and block device is raid with small chunk_size.
> >
>
> Please cc Jens on BIO changes.
>
> > ---
> > fs/bio.c | 16 ++++++++++++----
> > 1 files changed, 12 insertions(+), 4 deletions(-)
> >
> > diff --git a/fs/bio.c b/fs/bio.c
> > index 7856257..d713074 100644
> > --- a/fs/bio.c
> > +++ b/fs/bio.c
> > @@ -332,14 +332,21 @@ static int __bio_add_page(struct request_queue *q, struct bio *bio, struct page
> >
> > if (page == prev->bv_page &&
> > offset == prev->bv_offset + prev->bv_len) {
> > + /* Temprory detacth last bio_vec. */
>
> whoa, drunken speling.
>
> > + bio->bi_size -= prev->bv_len;
> > + bio->bi_vcnt--;
> > + bio->bi_phys_segments--;
> > + bio->bi_hw_segments--;
> > +
This logic isn't quite right, the rules for what constitutes a new hw or
phys segment is not a 1:1 mapping with number of pages in the bio. How
about just dropping the segment decrement? The merge_bvec fn should not
care, and we'll retry and coalesce segment count if we get to the limit
anyway.
--
Jens Axboe
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