linux-kernel - Re: [PATCH RFC v2 0/4] Add support for sharing page tables across processes (Previously mshare)

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Date:   Mon, 31 Jul 2023 14:50:59 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>,
        Rongwei Wang <rongwei.wang@...ux.alibaba.com>
Cc:     linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org,
        "xuyu@...ux.alibaba.com" <xuyu@...ux.alibaba.com>
Subject: Re: [PATCH RFC v2 0/4] Add support for sharing page tables across
 processes (Previously mshare)

On 31.07.23 14:25, Matthew Wilcox wrote:
> On Mon, Jul 31, 2023 at 12:35:00PM +0800, Rongwei Wang wrote:
>> Hi Matthew
>>
>> May I ask you another question about mshare under this RFC? I remember you
>> said you will redesign the mshare to per-vma not per-mapping (apologize if
>> remember wrongly) in last time MM alignment session. And I also refer to you
>> to re-code this part in our internal version (based on this RFC). It seems
>> that per VMA will can simplify the structure of pgtable sharing, even
>> doesn't care the different permission of file mapping. these are advantages
>> (maybe) that I can imagine. But IMHO, It seems not a strongly reason to
>> switch per-mapping to per-vma.
>>
>> And I can't imagine other considerations of upstream. Can you share the
>> reason why redesigning in a per-vma way, due to integation with hugetlbfs
>> pgtable sharing or anonymous page sharing?
> 
> It was David who wants to make page table sharing be per-VMA.  I think
> he is advocating for the wrong approach.  In any case, I don't have time
> to work on mshare and Khalid is on leave until September, so I don't
> think anybody is actively working on mshare.

Not that I also don't have any time to look into this, but my comment 
essentially was that we should try decoupling page table sharing (reduce 
memory consumption, shorter rmap walk) from the mprotect(PROT_READ) use 
case.

For page table sharing I was wondering whether there could be ways to 
just have that done semi-automatically. Similar to how it's done for 
hugetlb. There are some clear limitations: mappings < PMD_SIZE won't be 
able to benefit.

It's still unclear whether that is a real limitation. Some use cases 
were raised (put all user space library mappings into a shared area), 
but I realized that these conflict with MAP_PRIVATE requirements of such 
areas. Maybe I'm wrong and this is easily resolved.

At least it's not the primary use case that was raised. For the primary 
use cases (VMs, databases) that map huge areas, it might not be a 
limitation.

Regarding mprotect(PROT_READ), my point was that mprotect() is most 
probably the wrong tool to use (especially, due to signal handling). 
Instead, I was suggesting having a way to essentially protect pages in a 
shmem file -- and get notified whenever wants to write to such a page 
either via the page tables or via write() and friends. We do have the 
write-notify infrastructure for filesystems in place that we might 
extend/reuse. That mechanism could benefit from shared page tables by 
having to do less rmap walks.

Again, I don't have time to look into that (just like everybody else as 
it appears) and might miss something important. Just sharing my thoughts 
that I raised in the call.

-- 
Cheers,

David / dhildenb