On Tue, Jul 12 2016 at 10:18pm -0400,
Eric Wheeler <bcache@xxxxxxxxxxxxxxxxxx> wrote:
> On Tue, 12 Jul 2016, NeilBrown wrote:
>
> > On Tue, Jul 12 2016, Lars Ellenberg wrote:
> > ....
> > >
> > > Instead, I suggest to distinguish between recursive calls to
> > > generic_make_request(), and pushing back the remainder part in
> > > blk_queue_split(), by pointing current->bio_lists to a
> > > struct recursion_to_iteration_bio_lists {
> > > struct bio_list recursion;
> > > struct bio_list queue;
> > > }
> > >
> > > By providing each q->make_request_fn() with an empty "recursion"
> > > bio_list, then merging any recursively submitted bios to the
> > > head of the "queue" list, we can make the recursion-to-iteration
> > > logic in generic_make_request() process deepest level bios first,
> > > and "sibling" bios of the same level in "natural" order.
> > >
> > > Signed-off-by: Lars Ellenberg <lars.ellenberg@xxxxxxxxxx>
> > > Signed-off-by: Roland Kammerer <roland.kammerer@xxxxxxxxxx>
> >
> > Reviewed-by: NeilBrown <neilb@xxxxxxxx>
> >
> > Thanks again for doing this - I think this is a very significant
> > improvement and could allow other simplifications.
>
> Thank you Lars for all of this work!
>
> It seems like there have been many 4.3+ blockdev stacking issues and this
> will certainly address some of those (maybe all of them?). (I think we
> hit this while trying drbd in 4.4 so we dropped back to 4.1 without
> issue.) It would be great to hear 4.4.y stable pick this up if
> compatible.
>
>
> Do you believe that this patch would solve any of the proposals by others
> since 4.3 related to bio splitting/large bios? I've been collecting a
> list, none of which appear have landed yet as of 4.7-rc7 (but correct me
> if I'm wrong):
>
> A. [PATCH v2] block: make sure big bio is splitted into at most 256 bvecs
> by Ming Lei: https://patchwork.kernel.org/patch/9169483/
>
> B. block: don't make BLK_DEF_MAX_SECTORS too big
> by Shaohua Li: http://www.spinics.net/lists/linux-bcache/msg03525.html
>
> C. [1/3] block: flush queued bios when process blocks to avoid deadlock
> by Mikulas Patocka: https://patchwork.kernel.org/patch/9204125/
> (was https://patchwork.kernel.org/patch/7398411/)
>
> D. dm-crypt: Fix error with too large bios
> by Mikulas Patocka: https://patchwork.kernel.org/patch/9138595/
>
> The A,B,D are known to fix large bio issues when stacking dm+bcache
> (though the B,D are trivial and probably necessary even with your patch).
>
> Patch C was mentioned earlier in this thread by Mike Snitzer and you
> commented briefly that his patch might solve the issue; given that, and in
> the interest of minimizing duplicate effort, which of the following best
> describes the situation?
>
> 1. Your patch could supersede Mikulas's patch; they address the same
> issue.
>
> 2. Mikulas's patch addresses different issues such and both patches
> should be applied.
>
> 3. There is overlap between both your patch and Mikulas's such that both
> #1,#2 are true and effort to solve this has been duplicated.
>
>
> If #3, then what might be done to resolve the overlap?
Mikulas confirmed to me that he believes Lars' v2 patch will fix the
dm-snapshot problem, which is being tracked with this BZ:
https://bugzilla.kernel.org/show_bug.cgi?id=119841
We'll see how testing goes (currently underway).
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