> On Mon, Aug 15, 2016 at 11:23:28AM -0700, Christoph Hellwig wrote:
> > On Mon, Aug 15, 2016 at 11:11:22PM +0800, Ming Lei wrote:
> > > After arbitrary bio size is supported, the incoming bio may
> > > be very big. We have to split the bio into small bios so that
> > > each holds at most BIO_MAX_PAGES bvecs for safety reason, such
> > > as bio_clone().
> >
> > I still think working around a rough driver submitting too large
> > I/O is a bad thing until we've done a full audit of all consuming
> > bios through ->make_request, and we've enabled it for the common
> > path as well.
>
> bcache originally had workaround code to split too-large bios when it
> first went upstream - that was dropped only after the patches to make
> generic_make_request() handle arbitrary size bios went in. So to do what
> you're suggesting would mean reverting that bcache patch and bringing
> that code back, which from my perspective would be a step in the wrong
> direction. I just want to get this over and done with.
>
> >
> > > bool do_split = true;
> > > struct bio *new = NULL;
> > > const unsigned max_sectors = get_max_io_size(q, bio);
> > > + unsigned bvecs = 0;
> > > +
> > > + *no_merge = true;
> > >
> > > bio_for_each_segment(bv, bio, iter) {
> > > /*
> > > + * With arbitrary bio size, the incoming bio may be very
> > > + * big. We have to split the bio into small bios so that
> > > + * each holds at most BIO_MAX_PAGES bvecs because
> > > + * bio_clone() can fail to allocate big bvecs.
> > > + *
> > > + * It should have been better to apply the limit per
> > > + * request queue in which bio_clone() is involved,
> > > + * instead of globally. The biggest blocker is
> > > + * bio_clone() in bio bounce.
> > > + *
> > > + * If bio is splitted by this reason, we should allow
> > > + * to continue bios merging.
> > > + *
> > > + * TODO: deal with bio bounce's bio_clone() gracefully
> > > + * and convert the global limit into per-queue limit.
> > > + */
> > > + if (bvecs++ >= BIO_MAX_PAGES) {
> > > + *no_merge = false;
> > > + goto split;
> > > + }
> >
> > That being said this simple if check here is simple enough that it's
> > probably fine. But I see no need to uglify the whole code path
> > with that no_merge flag. Please drop if for now, and if we start
> > caring for this path in common code we should just move the
> > REQ_NOMERGE setting into the actual blk_bio_*_split helpers.
>
> Agreed about the no_merge thing.
By removing `no_merge` this patch should cherry-peck into stable v4.3+
without merge issues by avoiding bi_rw refactor interference, too.
Ming, can you send out a V4 without `no_merge` ?
--
Eric Wheeler
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