Re: [PATCH v11 0/7] Implement IOCTL to get and optionally clear info about PTEs

From: Mike Rapoport <rppt@kernel.org>
To: Andrei Vagin <avagin@gmail.com>
Cc: "Andrew Morton" <akpm@linux-foundation.org>,
	"Muhammad Usama Anjum" <usama.anjum@collabora.com>,
	"Peter Xu" <peterx@redhat.com>,
	"David Hildenbrand" <david@redhat.com>,
	"Michał Mirosław" <emmir@google.com>,
	"Danylo Mocherniuk" <mdanylo@google.com>,
	"Paul Gofman" <pgofman@codeweavers.com>,
	"Cyrill Gorcunov" <gorcunov@gmail.com>,
	"Nadav Amit" <namit@vmware.com>,
	"Alexander Viro" <viro@zeniv.linux.org.uk>,
	"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>,
	linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	linux-mm@kvack.org, linux-kselftest@vger.kernel.org,
	"Greg KH" <gregkh@linuxfoundation.org>,
	kernel@collabora.com
Subject: Re: [PATCH v11 0/7] Implement IOCTL to get and optionally clear info about PTEs
Date: Tue, 21 Mar 2023 14:41:53 +0200	[thread overview]
Message-ID: <ZBmmEfgq8QBAKPFN@kernel.org> (raw)
In-Reply-To: <CANaxB-wGLbM9U_dK=kzs+r5Xy358aKZ0=J_zODiLOcng+dbXog@mail.gmail.com>

On Mon, Mar 20, 2023 at 11:30:00AM -0700, Andrei Vagin wrote:
> On Thu, Mar 9, 2023 at 11:58 AM Andrew Morton <akpm@linux-foundation.org> wrote:
> >
> > On Thu,  9 Mar 2023 18:57:11 +0500 Muhammad Usama Anjum <usama.anjum@collabora.com> wrote:
> >
> > > The information related to pages if the page is file mapped, present and
> > > swapped is required for the CRIU project [5][6]. The addition of the
> > > required mask, any mask, excluded mask and return masks are also required
> > > for the CRIU project [5].
> >
> > It's a ton of new code and what I'm not seeing in here (might have
> > missed it?) is a clear statement of the value of this feature to our
> > users.
> >
> > I see hints that CRIU would like it, but no description of how valuable
> > this is to CRIU's users.
> 
> Hi Andrew,
> 
> The current interface works for CRIU, and I can't say we have anything
> critical with it right now.
> 
> On the other hand, the new interface has a number of significant improvements:
> 
> * it is more granular and allows us to track changed pages more
>   effectively. The current interface can clear dirty bits for the entire
>   process only. In addition, reading info about pages is a separate
>   operation. It means we must freeze the process to read information
>   about all its pages, reset dirty bits, only then we can start dumping
>   pages. The information about pages becomes more and more outdated,
>   while we are processing pages. The new interface solves both these
>   downsides. First, it allows us to read pte bits and clear the
>   soft-dirty bit atomically. It means that CRIU will not need to freeze
>   processes to pre-dump their memory. Second, it clears soft-dirty bits
>   for a specified region of memory. It means CRIU will have actual info
>   about pages to the moment of dumping them.
> 
> * The new interface has to be much faster because basic page filtering
>   is happening in the kernel. With the old interface, we have to read
>   pagemap for each page.

There is still a caveat in using userfaultfd for tracking dirty pages in
CRIU because we still don't support C/R of processes that use uffd. 

> Thanks,
> Andrei
> 
> >
> > So please spend some time preparing this info.
> >
> > Also, are any other applications of this feature anticipated?  If so,
> > what are they?
> >
> > IOW, please sell this stuff to us!

-- 
Sincerely yours,
Mike.