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

[Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>]

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.

>> 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".


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