Re: VMA merging updateds?

["Huang, Kai" <kai.huang@intel.com>]

On 23/09/2024 7:48 pm, Jarkko Sakkinen wrote:
> On Sun Sep 22, 2024 at 7:57 PM EEST, Jarkko Sakkinen wrote:
>>> On Sun Sep 22, 2024 at 7:27 PM EEST, Jarkko Sakkinen wrote:
>>>> Hi
>>>>
>>>> I started to look into this old issue with mm subsystem and SGX, i.e.
>>>> can we make SGX VMA's to merge together?
>>>>
>>>> This demonstrates the problem pretty well:
>>>>
>>>> https://lore.kernel.org/linux-sgx/884c7ea454cf2eb0ba2e95f7c25bd42018824f97.camel@kernel.org/
>>>>
>>>> It was result of brk() syscall being applied a few times.
>>
>> Briging some context here. This can be fixed in the run-time by book
>> keeping the ranges and doing unmapping/mapping. I guess this goes
>> beyond what mm should support?
>>
>> I thought to plain check this as it has been two years since my last
>> query on topic (if we could improve either the driver or mm somehow).
> 
> In the past I've substituted kernel's mm merge code with user space
> replacement:
> 
> https://github.com/enarx/mmledger/blob/main/src/lib.rs
> 
> It's essentially a reimplementation of al stuff that goes into
> mm/mmap.c's vma_merge(). I cannot recall anymore whether merges
> which map over existing ranges were working correctly, i.e. was
> the issue only concerning adjacent VMA's.
> 
> What I'm looking here is that can we make some cosntraints that
> if satisfied by the pfnmap code, it could leverage the code from
> vma_merge(). Perhaps by making explicit call to vma_merge()?
> I get that implicit use moves too much responsibility to the mm
> subsystem.
> 

Hi Jarkko,

Just want to understand more on the background:

Are you seeing any real problem due to needing a lot of mmap()s to the 
same enclave, or it is just a problem that doesn't look nice and you 
want to resolve?

I mean, this problem doesn't seem to be SGX-specific but a common one 
for VMAs with VM_PFNMAP (any bit in VM_SPECIAL), e.g., from random 
device drivers with mmap() support.  We will need a good justification 
if we want to make any core-mm change, if any, for this.