Re: [PATCH v6 05/11] block: remove split code in blkdev_issue_{discard,write_same}

[Ming Lin]

On Wed, 2015-10-21 at 11:33 -0400, Mike Snitzer wrote:
> On Wed, Oct 21 2015 at 11:01am -0400,
> Ming Lin <mlin@xxxxxxxxxx> wrote:
> 
> > On Wed, 2015-10-21 at 09:39 -0400, Jeff Moyer wrote:
> > > Christoph Hellwig <hch@xxxxxxxxxxxxx> writes:
> > > 
> > > > Jens, Ming:
> > > >
> > > > are you fine with the one liner change to get back to the old I/O
> > > > pattern?  While it looks like the cards fault I'd like to avoid this
> > > > annoying regression.
> > > 
> > > I'm not Jens or Ming, but your patch looks fine to me, though you'll
> > > want to remove the MAX_BIO_SECTORS definition since it's now unused.
> > > It's not clear to me why the limit was lowered in the first place.
> > 
> > UINT_MAX >> 9 is not power of 2 and it causes dm-thinp discard fails.
> > 
> > At the lengthy discussion:
> > [PATCH v5 01/11] block: make generic_make_request handle arbitrarily sized bios
> > We agreed to cap discard to 2G as an interim solution for 4.3 until the
> > dm-thinp discard code is rewritten.
> 
> But did Jens ever commit that change to cap at 2G?  I don't recall
> seeing it.

Yes, commit b49a0871

> 
> > Hi Mike,
> > 
> > Will the dm-thinp discard rewritten ready for 4.4?
> 
> No.  I'm not clear what needs changing in dm-thinp.  I'll have to
> revisit the thread to refresh my memory.
> 
> BTW, DM thinp can easily handle discards that aren't a power-of-2 so
> long as the requested discard is a factor of the thinp blocksize.

You are right. It's not about power-of-2.

Copy my old post here about why dm-thinp discard may fail with "UINT_MAX
>> 9".

      4G: 8388608 sectors
UINT_MAX: 8388607 sectors

dm-thinp block size = default discard granularity = 128 sectors

blkdev_issue_discard(sector=0, nr_sectors=8388608)

[start_sector, end_sector]
[0, 8388607]
    [0, 8388606], then dm-thinp splits it to 2 bios
        [0, 8388479]
        [8388480, 8388606] ---> this has problem in process_discard_bio(),
                                because the discard size(7 sectors) covers less than a block(128 sectors)
    [8388607, 8388607] ---> same problem 


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