Re: [PATCH v11 1/7] userfaultfd: Add UFFD WP Async support

[Peter Xu]

 Hello, Muhammad,

On Thu, Mar 09, 2023 at 06:57:12PM +0500, Muhammad Usama Anjum wrote:
> Add new WP Async mode (UFFD_FEATURE_WP_ASYNC) which resolves the page
> faults on its own. It can be used to track that which pages have been
> written-to from the time the pages were write-protected. It is very
> efficient way to track the changes as uffd is by nature pte/pmd based.
> 
> UFFD synchronous WP sends the page faults to the userspace where the
> pages which have been written-to can be tracked. But it is not efficient.
> This is why this asynchronous version is being added. After setting the
> WP Async, the pages which have been written to can be found in the pagemap
> file or information can be obtained from the PAGEMAP_IOCTL.
> 
> Suggested-by: Peter Xu <peterx@xxxxxxxxxx>
> Signed-off-by: Muhammad Usama Anjum <usama.anjum@xxxxxxxxxxxxx>

Here's the patch that can enable WP_ASYNC for all kinds of memories (as I
promised..).  Currently I only tested btrfs (besides the common three)
which is the major fs I use locally, but I guess it'll also enable the rest
no matter what's underneath, just like soft-dirty.

As I mentioned, I just feel it very unfortunate to have a lot of suffixes
for the UFFD_FEATURE_* on types of memory, and I hope we get rid of it for
this WP_ASYNC from the start because the workflow should really be similar
to anon/shmem handling for most of the rest, just a few tweaks here and
there.

I had a feeling that some type of special VMA will work weirdly, but let's
see.. so far I don't come up with any.

If the patch looks fine to you, please consider replace this patch with
patch 1 of mine where I attached.  Then patch 1 can be reviewed alongside
with your series.

Logically patch 1 can be reviewed separately too, because it works
perfectly afaiu without the atomic version of pagemap already.  But on my
side I don't think it justifies anything really matters, so unless someone
thinks it a good idea to post / review / merge it separately, you can keep
that with your new pagemap ioctl.

Patch 2 is only for your reference.  It's not for merging quality so please
don't put it into your series.  I do plan to cleanup the userfaultfd
selftests in the near future first (when I wrote this I am more eager to do
so..).  I also think your final pagemap test cases can cover quite a bit.

Thanks,

-- 
Peter Xu