Re: [PATCH v10 3/6] fs/proc/task_mmu: Implement IOCTL to get and/or the clear info about PTEs

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> On Feb 22, 2023, at 11:10 PM, Muhammad Usama Anjum <usama.anjum@xxxxxxxxxxxxx> wrote:
> 
> Hi Nadav, Mike, Michał,
> 
> Can you please share your thoughts at [A] below?

I promised I won't talk about the API, but was persuaded to reconsider. I have a
general question regarding the suitablity of currently proposed high-level API.
To explore some alternatives, I'd like to suggest an alternative that may have
some advantages. If these have already been considered and dismissed, feel free
to ignore.

I believe we have two distinct usage scenarios: (1) vectored reads from pagemap,
and (2) atomic UFFD WP-read/protect. It's possible that these require separate
interfaces

Regarding vectored reads, I believe the simplest solution is to maintain the
current pagemap entry format for output and extend it if necessary. The input
can be a vector of ranges. I'm uncertain about the purpose of fields such
as 'anyof_mask' in 'pagemap_scan_arg', so I can't confirm their necessity and
whether the input need to be made. more complicated. There is a possibility
that fields such as 'anyof_mask' might expose internal APIs, so I hope they’re
not required.

For the atomic operation of 'PAGE_IS_WRITTEN' + 'PAGEMAP_WP_ENGAGE', a different
mechanism might be necessary. This function appears to be UFFD-specific.
Instead of the proposed IOCTL, an alternative option is to
use 'UFFD_FEATURE_WP_ASYNC' to log the pages that were written, similar to
page-modification logging on Intel. Since this feature appears to be specific
to UFFD, I believe it would be more appropriate to include the log as part of
the UFFD mechanism rather than the pagemap.

>From my experience with UFFD, proper ordering of events  is crucial, although it
is not always done well. Therefore, we should aim for improvement, not
regression. I believe that utilizing the pagemap-based mechanism for WP'ing
might be a step in the wrong direction. I think that it would have been better
to emit a 'UFFD_FEATURE_WP_ASYNC' WP-log (and ordered) with UFFD #PF and
events. The 'UFFD_FEATURE_WP_ASYNC'-log may not need to wake waiters on the
file descriptor unless the log is full.

I am sorry that I chime in that late, but I think the complications that the
proposed mechanism might raise are not negligible. And anyhow this patch-set
still requires quite a bit of work before it can be merged.





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