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

[Muhammad Usama Anjum <usama.anjum@collabora.com>]

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?

>>>
>>>> - 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

-- 
Muhammad Usama Anjum