Re: [PATCH v2 1/1] block: fix blk_queue_split() resource exhaustion

[Mike Snitzer]

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).