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

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Message-ID: <ZBNr4nohj/Tw4Zhw@x1n>
Date:   Thu, 16 Mar 2023 15:20:02 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Muhammad Usama Anjum <usama.anjum@...labora.com>
Cc:     David Hildenbrand <david@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michał Mirosław <emmir@...gle.com>,
        Andrei Vagin <avagin@...il.com>,
        Danylo Mocherniuk <mdanylo@...gle.com>,
        Paul Gofman <pgofman@...eweavers.com>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Mike Rapoport <rppt@...nel.org>, Nadav Amit <namit@...are.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Shuah Khan <shuah@...nel.org>,
        Christian Brauner <brauner@...nel.org>,
        Yang Shi <shy828301@...il.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Yun Zhou <yun.zhou@...driver.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Alex Sierra <alex.sierra@....com>,
        Matthew Wilcox <willy@...radead.org>,
        Pasha Tatashin <pasha.tatashin@...een.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        "Gustavo A . R . Silva" <gustavoars@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, linux-kselftest@...r.kernel.org,
        Greg KH <gregkh@...uxfoundation.org>, kernel@...labora.com
Subject: Re: [PATCH v11 1/7] userfaultfd: Add UFFD WP Async support

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@...hat.com>
> Signed-off-by: Muhammad Usama Anjum <usama.anjum@...labora.com>

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

View attachment "0001-userfaultfd-UFFD_FEATURE_WP_ASYNC.patch" of type "text/plain" (16603 bytes)

View attachment "0002-selftests-mm-Add-userfaultfd-unit-test.patch" of type "text/plain" (11054 bytes)