Re: [RFC PATCH v2 0/7] Speculative page faults

[Laurent Dufour <ldufour@linux.vnet.ibm.com>]

On 18/11/2016 15:08, Andi Kleen wrote:
> Laurent Dufour <ldufour@linux.vnet.ibm.com> writes:
> 
>> This is a port on kernel 4.8 of the work done by Peter Zijlstra to
>> handle page fault without holding the mm semaphore.
> 
> One of the big problems with patches like this today is that it is
> unclear what mmap_sem actually protects. It's a big lock covering lots
> of code. Parts in the core VM, but also do VM callbacks in file systems
> and drivers rely on it too?
> 
> IMHO the first step is a comprehensive audit and then writing clear
> documentation on what it is supposed to protect. Then based on that such
> changes can be properly evaluated.

Hi Andi,

Sorry for the late answer...

I do agree, this semaphore is massively used and it would be nice to
have all its usage documented.

I'm currently tracking all the mmap_sem use in 4.8 kernel (about 380
hits) and I'm trying to identify which it is protecting.

In addition, I think it may be nice to limit its usage to code under mm/
so that in the future it may be easier to find its usage.

Thanks,
Laurent.

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