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

[Balbir Singh <bsingharora@gmail.com>]

On Thu, Dec 1, 2016 at 7:34 PM, Laurent Dufour
<ldufour@linux.vnet.ibm.com> wrote:
> 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.

Is this possible? All sorts of arch's fault
handling/virtualization/file system and drivers (IO/DRM/) hold
mmap_sem.

Balbir Singh.

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