Re: [PATCH 1/2] blkdev: fix merge_bvec_fn return value checks

[Mike Snitzer <snitzer@redhat.com>]

On Wed, Mar 03 2010 at  1:45pm -0500,
Dmitry Monakhov <dmonakhov@openvz.org> wrote:

> Mike Snitzer <snitzer@redhat.com> writes:
> 
> > On Tue, Mar 2, 2010 at 10:49 PM, Dmitry Monakhov <dmonakhov@openvz.org> wrote:
> >> Jens Axboe <jens.axboe@oracle.com> writes:
> >>
> >>> On Sat, Feb 27 2010, Dmitry Monakhov wrote:
> >>>> merge_bvec_fn() returns bvec->bv_len on success. So we have to check
> >>>> against this value. But in case of fs_optimization merge we compare
> >>>> with wrong value. This patch must be included in
> >>>>  b428cd6da7e6559aca69aa2e3a526037d3f20403
> >>>> But accidentally i've forgot to add this in the initial patch.
> >>>> To make things straight let's replace all such checks.
> >>>> In fact this makes code easy to understand.
> >>>
> >>> Agree, applied.
> >> Ohh.. as you already know this patch break dm-layer. Sorry.
> >> This is because dm->merge may return more than requested. So correct
> >> check must test against less what requested. Correct patch attached.
> >
> > Yes, it is quite common for dm_merge_bvec() to return greater than the
> > requested length.
> >
> > But dm_merge_bvec() returning a maximum length, rather than requested,
> > isn't special.  All the other blk_queue_merge_bvec() callers' merge
> > functions appear to return "maximum amount of bytes we can accept at
> > this offset" too.
> What for? Does it allow us to make some optimization?

I wasn't suggesting the behavior of the current merge functions
returning maximum is important or useful.  I was just pointing out what
the interface is and that dm_merge_bvec() is no different than the
others.

> For example like follows:
> bio_add_pageS(bio, **pages) {
>    /* call merge_fn only one untill all space exhausted */                
>    ret = merge_fn()  (this returns huge value (1024*1024))
>    while (ret) { 
>           bio->bi_io_vec[bio->bi_vcnt - 1].bv_page = page;
>           ...
>           ret -= PAGE_SIZE;
>           bio->bi_vcnt++;
>    }
> }
> IMHO the answer is *NO*, this code will unlikely to work.

Conversely, I see no value in imposing that these 'q->merge_bvec_fn'
functions return at most the requested length.  Can't even really see it
making the __bio_add_page() code more readable.

> > __bio_add_page() only needs to care about whether the
> > 'q->merge_bvec_fn' return is _less than_ the requested length.

Linux has all sorts of internal interfaces that are "odd"... the current
'q->merge_bvec_fn' interface included.  But odd is not a problem (nor is
it "broken") unless you make changes that don't consider how the current
interface is defined.

But I digress...

Mike