Re: [PATCH v3 0/4] Implement IOCTL to get and clear soft dirty PTE

[Andrei Vagin <avagin@gmail.com>]

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.
> >
> >> - 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.

Thanks,
Andrei