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.
> 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.
