Re: [PATCH 0/5] Add process_memwatch syscall

Gabriel Krisman Bertazi
2022-08-10T16:53:16Z

"Peter.Enderborg@sony.com" <Peter.Enderborg@sony.com> writes:
>>
>> This syscall can be used by the CRIU project and other applications which
>> require soft-dirty PTE bit information. The following operations are
>> supported in this syscall:
>> - Get the pages that are soft-dirty.
>> - Clear the pages which are soft-dirty.
>> - The optional flag to ignore the VM_SOFTDIRTY and only track per page
>> soft-dirty PTE bit
>>

Hi Peter,

(For context, I wrote a previous version of this patch and have been
working with Usama on the current patch).

> Why can it not be done as a IOCTL?

Considering an ioctl is basically a namespaced syscall with extra-steps,
surely we can do it :) There are a few reasons we haven't, though:

1) ioctl auditing/controling is much harder than syscall

2) There is a concern for performance, since this might be executed
frequently by windows applications running over wine. There is an extra
cost with unnecessary copy_[from/to]_user that we wanted to avoid, even
though we haven't measured.

3) I originally wrote this at the time process_memadvise was merged. I
felt it fits the same kind of interface exposed by
process_memadvise/process_mrelease, recently merged.

4) Not obvious whether the ioctl would be against pagemap/clear_refs.
Neither file name describes both input and output semantics.

Obviously, all of those reasons can be worked around, and we can turn
this into an ioctl.

Thanks,

--
Gabriel Krisman Bertazi