Date:   Mon, 27 Feb 2023 16:18:25 -0500
From:   Peter Xu <peterx@redhat.com>
To:     Nadav Amit <namit@vmware.com>
Cc:     Muhammad Usama Anjum <usama.anjum@collabora.com>,
        Mike Rapoport <rppt@kernel.org>,
        =?utf-8?B?TWljaGHFgiBNaXJvc8WCYXc=?= <emmir@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Cyrill Gorcunov <gorcunov@gmail.com>,
        Paul Gofman <pgofman@codeweavers.com>,
        Danylo Mocherniuk <mdanylo@google.com>,
        Shuah Khan <shuah@kernel.org>,
        Christian Brauner <brauner@kernel.org>,
        Yang Shi <shy828301@gmail.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        "Liam R . Howlett" <Liam.Howlett@oracle.com>,
        Yun Zhou <yun.zhou@windriver.com>,
        Suren Baghdasaryan <surenb@google.com>,
        Alex Sierra <alex.sierra@amd.com>,
        Matthew Wilcox <willy@infradead.org>,
        Pasha Tatashin <pasha.tatashin@soleen.com>,
        Axel Rasmussen <axelrasmussen@google.com>,
        "Gustavo A . R . Silva" <gustavoars@kernel.org>,
        Dan Williams <dan.j.williams@intel.com>,
        kernel list <linux-kernel@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
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        linux-kselftest <linux-kselftest@vger.kernel.org>,
        Greg KH <gregkh@linuxfoundation.org>,
        "kernel@collabora.com" <kernel@collabora.com>,
        David Hildenbrand <david@redhat.com>,
        Andrei Vagin <avagin@gmail.com>
Subject: 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 Thu, Feb 23, 2023 at 05:11:11PM +0000, Nadav Amit wrote:
> 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.

Yes this is an interesting question to think about..

Keeping the data in the pgtable has one good thing that it doesn't need any
complexity on maintaining the log, and no possibility of "log full".

If there's possible "log full" then the next question is whether we should
let the worker wait the monitor if the monitor is not fast enough to
collect those data.  It adds some slight dependency on the two threads, I
think it can make the tracking harder or impossible in latency sensitive
workloads.

The other thing is we can also make the log "never gonna full" by making it
a bitmap covering any registered ranges, but I don't either know whether
it'll be worth it for the effort.

Thanks,

-- 
Peter Xu