On Tue, 2 Oct 2012 14:09:23 -0700 Kent Overstreet <koverstr...@google.com> wrote:
> On Tue, Oct 02, 2012 at 04:22:01PM +1000, NeilBrown wrote: > > On Fri, 28 Sep 2012 09:23:43 -0700 Kent Overstreet <koverstr...@google.com> > > wrote: > > > > > On Mon, Sep 24, 2012 at 02:56:39PM +1000, NeilBrown wrote: > > > > > > > > Hi Jens, > > > > this patch has been sitting in my -next tree for a little while and I > > > > was > > > > hoping for it to go in for the next merge window. > > > > It simply allows bio_split() to be used on bios without a payload, > > > > such as > > > > 'discard'. > > > > > > Thing is, at some point in the stack a discard bio is going to have data > > > - see blk_add_rquest_payload(), and it used to be the single page was > > > added to discard bios above generic_make_request(), in > > > blkdev_issue_discard() or whatever it's called. > > > > > > So while I'm sure your code works, it's just a fragile way of doing it. > > > > > > There's also other types of bios where bi_size has nothing to do with > > > the amount of data in the bi_io_vec - actually I think this is a new > > > thing, since Martin Petersen just added REQ_WRITE_SAME and I don't think > > > there were any other instances besides REQ_DISCARD before. > > > > > > So my preference would be defining a mask (REQ_DISCARD|REQ_WRITE_SAME), > > > and if bio->bi_rw & that mask is true, just duplicate the bvec or > > > whatever. > > > > Hi Kent, > > I'm afraid I don't see the relevance of your comments to the patch. > > > > The current bio_split code can successfully split a bio with zero or one > > bi_vec entry. If there are more than that, we cannot split. > > > > How does it matter whether the bio is a DISCARD or a WRITE_SAME or a DATA or > > whatever? > > Hrm, I think I didn't explain very well. > > After your change, if bio->bi_vcnt != 0, then it splits the bvec. > > The trouble is that discard bios do under certain circumstances have > bio->bi_vcnt != 0, in which case splitting the bvec is the wrong thing > to do - first_sectors will quite likely be bigger than the bvec. > > In practice this isn't currently a problem for discard bios, because > since Christoph added blk_add_request_payload(), discard bios won't have > that bvec added until they hit the scsi layer which will be after any > splitting. But this is a fairly recent and unrelated change, and IMO not > the kind of behaviour I'd want to rely on. > > WRITE_SAME is a problem for the same reason - bio_sectors(bio) may be > large, but the bio will always have a single bvec and splitting the bvec > is always the wrong thing to do for WRITE_SAME. > > So, I think it makes more sense to make the splitting conditional on > !(bio->bi_rw & (REQ_DISCARD|REQ_WRITE_SAME)), in addition to > bio->bi_vcnt == 1. > > ..That make more sense? Yes, that does make some more sense, thanks. However it doesn't convince me that we need to change the patch. I guess my position is that once we get to this code, we absolutely have to split the bio - it maps to two separate devices in a RAID0 or similar so not-splitting is not an option. Maybe various md devices need to detect and reject REQ_DISCARD requests that have a payload and REQ_WRITE_SAME requests? Or would they need to explicitly set a flag to say they accept them? So maybe there is something to fix, but I don't think it is in bit_split, except maybe to add WARN_ON ?? Thanks, NeilBrown
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