Re: [PATCH 0/5] Add process_memwatch syscall

[David Hildenbrand]

On 26.07.22 18:18, Muhammad Usama Anjum wrote:
> Hello,

Hi,

> 
> This patch series implements a new syscall, process_memwatch. Currently,
> only the support to watch soft-dirty PTE bit is added. This syscall is
> generic to watch the memory of the process. There is enough room to add
> more operations like this to watch memory in the future.
> 
> Soft-dirty PTE bit of the memory pages can be viewed by using pagemap
> procfs file. The soft-dirty PTE bit for the memory in a process can be
> cleared by writing to the clear_refs file. This series adds features that
> weren't possible through the Proc FS interface.
> - There is no atomic get soft-dirty PTE bit status and clear operation
>   possible.

Such an interface might be easy to add, no?

> - The soft-dirty PTE bit of only a part of memory cannot be cleared.

Same.

So I'm curious why we need a new syscall for that.

> 
> Historically, soft-dirty PTE bit tracking has been used in the CRIU
> project. The Proc FS interface is enough for that as I think the process
> is frozen. We have the use case where we need to track the soft-dirty
> PTE bit for running processes. We need this tracking and clear mechanism
> of a region of memory while the process is running to emulate the
> getWriteWatch() syscall of Windows. This syscall is used by games to keep
> track of dirty pages and keep processing only the dirty pages. This
> syscall can be used by the CRIU project and other applications which
> require soft-dirty PTE bit information.
> 
> As in the current kernel there is no way to clear a part of memory (instead
> of clearing the Soft-Dirty bits for the entire processi) and get+clear
> operation cannot be performed atomically, there are other methods to mimic
> this information entirely in userspace with poor performance:
> - The mprotect syscall and SIGSEGV handler for bookkeeping
> - The userfaultfd syscall with the handler for bookkeeping

You write "poor performance". Did you actually implement a prototype
using userfaultfd-wp? Can you share numbers for comparison?

Adding an new syscall just for handling a corner case feature
(soft-dirty, which we all love, of course) needs good justification.

> 
>         long process_memwatch(int pidfd, unsigned long start, int len,
>                               unsigned int flags, void *vec, int vec_len);
> 
> 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

Huh, why? VM_SOFTDIRTY is an internal implementation detail and should
remain such.

VM_SOFTDIRTY translates to "all pages in this VMA are soft-dirty".

-- 
Thanks,

David / dhildenb