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Date:   Mon, 10 Oct 2022 21:52:39 -0700
From:   Andrei Vagin <avagin@gmail.com>
To:     Muhammad Usama Anjum <usama.anjum@collabora.com>
Cc:     Jonathan Corbet <corbet@lwn.net>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Andrew Morton <akpm@linux-foundation.org>,
        Shuah Khan <shuah@kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
        open list <linux-kernel@vger.kernel.org>,
        "open list:PROC FILESYSTEM" <linux-fsdevel@vger.kernel.org>,
        "open list:MEMORY MANAGEMENT" <linux-mm@kvack.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@vger.kernel.org>, kernel@collabora.com,
        Gabriel Krisman Bertazi <krisman@collabora.com>,
        David Hildenbrand <david@redhat.com>,
        Peter Enderborg <peter.enderborg@sony.com>,
        Greg KH <gregkh@linuxfoundation.org>,
        Suren Baghdasaryan <surenb@google.com>,
        Matthew Wilcox <willy@infradead.org>,
        Danylo Mocherniuk <mdanylo@google.com>
Subject: Re: [PATCH v3 0/4] Implement IOCTL to get and clear soft dirty PTE
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On Mon, Oct 03, 2022 at 04:21:22PM +0500, Muhammad Usama Anjum wrote:
> On 9/28/22 10:24 PM, Andrei Vagin wrote:
> > On Wed, Sep 21, 2022 at 11:26 AM Muhammad Usama Anjum
> > <usama.anjum@collabora.com> wrote:
> >>
> >> Hi,
> >>
> >> Thank you for reviewing.
> >>
> >> On 9/19/22 7:58 PM, Andrei Vagin wrote:
> >>>> This ioctl can be used by the CRIU project and other applications which
> >>>> require soft-dirty PTE bit information. The following operations are
> >>>> supported in this ioctl:
> >>>> - Get the pages that are soft-dirty.
> >>>
> >>> I think this interface doesn't have to be limited by the soft-dirty
> >>> bits only. For example, CRIU needs to know whether file, present and swap bits
> >>> are set or not.
> >> These operations can be performed by pagemap procfs file. Definitely
> >> performing them through IOCTL will be faster. But I'm trying to add a
> >> simple IOCTL by which some specific PTE bit can be read and cleared
> >> atomically. This IOCTL can be extended to include other bits like file,
> >> present and swap bits by keeping the interface simple. The following
> >> mask advice is nice. But if we add that kind of masking, it'll start to
> >> look like a filter on top of pagemap. My intention is to not duplicate
> >> the functionality already provided by the pagemap. One may ask, then why
> >> am I adding "get the soft-dirty pages" functionality? I'm adding it to
> >> complement the get and clear operation. The "get" and "get and clear"
> >> operations with special flag (PAGEMAP_SD_NO_REUSED_REGIONS) can give
> >> results quicker by not splitting the VMAs.
> > 
> > This simple interface is good only for a limited number of use-cases.
> > The interface
> > that I suggest doesn't duplicate more code than this one, but it is much more
> > universal. It will be a big mess if you add a separate API for each
> > specific use-case.
> >
> >
> >>> I mean we should be able to specify for what pages we need to get info
> >>> for. An ioctl argument can have these four fields:
> >>> * required bits (rmask & mask == mask) - all bits from this mask have to be set.
> >>> * any of these bits (amask & mask != 0) - any of these bits is set.
> >>> * exclude masks (emask & mask == 0) = none of these bits are set.
> >>> * return mask - bits that have to be reported to user.
> The required mask (rmask) makes sense to me. At the moment, I only know
> about the practical use case for the required mask. Can you share how
> can any and exclude masks help for the CRIU?
> 

I looked at should_dump_page in the CRIU code:
https://github.com/checkpoint-restore/criu/blob/45641ab26d7bb78706a6215fdef8f9133abf8d10/criu/mem.c#L102

When CRIU dumps file private mappings, it needs to get pages that have
PME_PRESENT or PME_SWAP but don't have PME_FILE.

> >>>> - Clear the pages which are soft-dirty.
> >>>> - The optional flag to ignore the VM_SOFTDIRTY and only track per page
> >>>> soft-dirty PTE bit
> >>>>
> >>>> There are two decisions which have been taken about how to get the output
> >>>> from the syscall.
> >>>> - Return offsets of the pages from the start in the vec
> >>>
> >>> We can conside to return regions that contains pages with the same set
> >>> of bits.
> >>>
> >>> struct page_region {
> >>>       void *start;
> >>>       long size;
> >>>       u64 bitmap;
> >>> }
> >>>
> >>> And ioctl returns arrays of page_region-s. I believe it will be more
> >>> compact form for many cases.
> >> Thank you for mentioning this. I'd considered this while development.
> >> But I gave up and used the simple array to return the offsets of the
> >> pages as in the problem I'm trying to solve, the dirty pages may be
> >> present amid non-dirty pages. The range may not be useful in that case.
> > 
> > This is a good example. If we expect more than two consequent pages
> > on average, the "region" interface looks more prefered. I don't know your
> > use-case, but in the case of CRIU, this assumption looks reasonable.
> > 
> >> Also we want to return only a specific number of pages of interest. The
> >> following paragraph explains it.
> >>
> >>>
> >>>> - Stop execution when vec is filled with dirty pages
> >>>> These two arguments doesn't follow the mincore() philosophy where the
> >>>> output array corresponds to the address range in one to one fashion, hence
> >>>> the output buffer length isn't passed and only a flag is set if the page
> >>>> is present. This makes mincore() easy to use with less control. We are
> >>>> passing the size of the output array and putting return data consecutively
> >>>> which is offset of dirty pages from the start. The user can convert these
> >>>> offsets back into the dirty page addresses easily. Suppose, the user want
> >>>> to get first 10 dirty pages from a total memory of 100 pages. He'll
> >>>> allocate output buffer of size 10 and the ioctl will abort after finding the
> >>>> 10 pages. This behaviour is needed to support Windows' getWriteWatch(). The
> >>>> behaviour like mincore() can be achieved by passing output buffer of 100
> >>>> size. This interface can be used for any desired behaviour.
> > 
> > Now, it is more clear where this interface came from. It repeats the
> > interface of Windows' getWriteWatch. I think we have to look wider.
> > The interface that reports regions will be more efficient for many
> > use-cases. As for getWriteWatch, it will require a bit more code in
> > user-space, but this code is trivial.

I added Danylo to CC. I think he has a good use-case for the new
interface. Danylo, could you describe it here.

> > 
> > Thanks,
> > Andrei
> 
> -- 
> Muhammad Usama Anjum