Re: [Xen-devel] [PATCH] mm, oom: distinguish blockable mode for mmu notifiers

[Michal Hocko <mhocko@xxxxxxxxxx>]

On Fri 24-08-18 23:52:25, Tetsuo Handa wrote:
> On 2018/08/24 22:32, Michal Hocko wrote:
> > On Fri 24-08-18 22:02:23, Tetsuo Handa wrote:
> >> I worry that (currently
> >> out-of-tree) users of this API are involving work / recursion.
> > 
> > I do not give a slightest about out-of-tree modules. They will have to
> > accomodate to the new API. I have no problems to extend the
> > documentation and be explicit about this expectation.
> 
> You don't need to care about out-of-tree modules. But you need to hear from
> mm/hmm.c authors/maintainers when making changes for mmu-notifiers.
> 
> > diff --git a/include/linux/mmu_notifier.h b/include/linux/mmu_notifier.h
> > index 133ba78820ee..698e371aafe3 100644
> > --- a/include/linux/mmu_notifier.h
> > +++ b/include/linux/mmu_notifier.h
> > @@ -153,7 +153,9 @@ struct mmu_notifier_ops {
> >      *
> >      * If blockable argument is set to false then the callback cannot
> >      * sleep and has to return with -EAGAIN. 0 should be returned
> > -    * otherwise.
> > +    * otherwise. Please note that if invalidate_range_start approves
> > +    * a non-blocking behavior then the same applies to
> > +    * invalidate_range_end.
> 
> Prior to 93065ac753e44438 ("mm, oom: distinguish blockable mode for mmu
> notifiers"), whether to utilize MMU_INVALIDATE_DOES_NOT_BLOCK was up to
> mmu-notifiers users.
> 
>       -        * If both of these callbacks cannot block, and invalidate_range
>       -        * cannot block, mmu_notifier_ops.flags should have
>       -        * MMU_INVALIDATE_DOES_NOT_BLOCK set.
>       +        * If blockable argument is set to false then the callback 
> cannot
>       +        * sleep and has to return with -EAGAIN. 0 should be returned
>       +        * otherwise.
> 
> Even out-of-tree mmu-notifiers users had rights not to accommodate (i.e.
> make changes) immediately by not setting MMU_INVALIDATE_DOES_NOT_BLOCK.
> 
> Now we are in a merge window. And we noticed a possibility that out-of-tree
> mmu-notifiers users might have trouble with making changes immediately in 
> order
> to follow 93065ac753e44438 if expectation for mm/hmm.c changes immediately.
> And you are trying to ignore such possibility by just updating expected 
> behavior
> description instead of giving out-of-tree users a grace period to check and 
> update
> their code.

This is just ridiculous. I have no idea what you are trying to achieve
here but please read through Documentation/process/stable-api-nonsense.rst
before you try to make strong statements again.

I have changed an in-kernel interface. I have gone through all users and
fixed them up. It is really appreciated to double check after me and I
am willing to fix up any fallouts. But that is just about it. I do not
get a whit about _any_ out of tree drivers when changing the interface.
I am willing to answer any questions regarding this change so developers
of those drivers know how to do their change properly but doing so is
completely their business.
 
> >> and keeps "all operations protected by hmm->mirrors_sem held for write are
> >> atomic". This suggests that "some operations protected by hmm->mirrors_sem 
> >> held
> >> for read will sleep (and in the worst case involves memory allocation
> >> dependency)".
> > 
> > Yes and so what? The clear expectation is that neither of the range
> > notifiers do not sleep in !blocking mode. I really fail to see what you
> > are trying to say.
> 
> I'm saying "Get ACK from Jérôme about mm/hmm.c changes".

HMM is a library layer for other driver, until those get merged the same
applies for them as well.
-- 
Michal Hocko
SUSE Labs

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