On Mon, Apr 29, 2024 at 07:54:05PM +0000, Chaitanya Kulkarni wrote:
> On 4/29/24 03:23, Keith Busch wrote:
> > static void brd_submit_bio(struct bio *bio)
> > {
> > struct brd_device *brd = bio->bi_bdev->bd_disk->private_data;
> > @@ -247,6 +264,12 @@ static void brd_submit_bio(struct bio *bio)
> > struct bio_vec bvec;
> > struct bvec_iter iter;
> >
> > + if (unlikely(op_is_discard(bio->bi_opf))) {
>
> I've been told that unlikely should not be used with discard as it is
> bad for discard workloads, if that is still true, then can you please
> remove unlikely ?
I don't think discard workloads are likely on a ramdisk.
> Also, if you are doing this can you please also add support for
> write-zeroes for the sake of completeness ? unless that support is
> not desired for brd ...
That is orthoganal to the goals here. I just need to temporarily reclaim
memory for other purposes when the disk is temporarily not being use; I
don't care about the zero'ing out part.
I found that previous attempts at supporting discard on brd didn't make
it, supposedly because of some kind of writeback deadlock. I have no
idea what that comment way back from 2.6.35 is referring to though: we
allocate pages with NOIO or NOWAIT, so either brd can get a new page
when its needed or it fails, and either is fine, but deadlock shouldn't
happen.
