Re: [EXT] Re: COW in userspace

[Ralf Ramsauer <ralf.ramsauer@oth-regensburg.de>]

On 23/08/2021 10:02, David Hildenbrand wrote:
> On 20.08.21 15:13, Ralf Ramsauer wrote:
>> Dear mm folks,
>>
>> I have an issue, where it would be great to have a COW-backed virtual
>> memory area within an userspace process. I know there's the possibility
>> to have a file-backed MAP_SHARED vma, which is later duplicated with
>> MAP_PRIVATE, but that's not exactly what I'm looking for.
>>
>> Say I have an anonymous page-aligned VMA a, with MAP_PRIVATE and
>> PROT_RW. Userspace happily writes to/reads from it. At some point in
>> time, I want to 'snapshot' that single VMA within the context of the
>> process and without the need to fork(). Say there's something like
>>
>>    a = mmap(0, len, PROT_RW, MAP_ANON | MAP_POPULATE, -1, 0);
>>    [... fill a ...]
>>
>>    b = mmdup(a, len, PROT_READ);
>>
>> b shall be the new base pointer of a new VMA that is backed by COW
>> mechanisms. After mmdup, those regular COW mechanisms do the rest: both
>> VMAs (a and b) will fault on subsequent writes and duplicate the
>> previously shared physical mapping, pretty much what cow_fault or
>> shared_fault does.
>>
>> Afaict, this, or at least something like this is currently not supported
>> by the kernel. Is that correct? If so, why? Generally spoken, is it a
>> bad idea?
> 
> Not sure if it helps (most probably not), QEMU uses uffd-wp for
> background snapshots of VM memory. It's different, though, as you'll
> only have a single mapping and will be catching modifications to your
> single mapping, such that you can "safe away" relevant snapshot pages
> before any modifications.

Thanks for the pointer, David. I'll have a look.

> 
> You mention "both VMAs (a and b) will fault on subsequent writes", so
> would you actually be allowing PROT_WRITE access to b ("snapshot")?
> 

In general, yes, both should be allowed to be PROT_WRITE. So no matter
"which side" causes the fault, simply both will lead to duplication.

If it would make things easier, then it would also be absolutely fine to
have the snapshot PROT_READ, which would suffice my requirements as well.

Thanks
  Ralf