[PATCH 0/3] RFC: Implement MemoryWorkingSetExInformation.

[Ken Thomases]

On Dec 17, 2019, at 10:21 PM, Andrew Wesie <awesie at gmail.com> wrote:
> 
> On Tue, Dec 17, 2019 at 8:31 PM Andrew Wesie <awesie at gmail.com> wrote:
>> 
>> On Tue, Dec 17, 2019 at 8:07 PM Ken Thomases <ken at codeweavers.com> wrote:
>>> 
>>> Are the semantics of the fields of MEMORY_WORKING_SET_EX_BLOCK documented or explained somewhere?  It's hard to comment intelligently without that.
>>> 
>> 
>> Not really. My knowledge is limited to what my test has shown:
>> - Demand-zero page is "Invalid"
>> - Once you read from a demand-zero page (or write to it), it is now "Valid"
>> - If you change a page protection to NOACCESS or GUARD, it is "Invalid"
>> - If you change a page protection from NOACCESS or GUARD to
>> READ_ONLY, READWRITE, ... without GUARD, it is still "Invalid"
>> - If you then read from that page (or write to it), it is now "Valid"
>> 
> 
> One additional hint that I just found in VirtualQuery documentation
> is: "To detect whether copy-on-write has occurred for a specific page,
> either access the page or lock it using the VirtualLock function to
> make sure the page is resident in memory, then use the
> QueryWorkingSetEx function ...".
> 
> This seems to imply that when pages are mapped into a process, they
> are not resident (e.g. "Valid") until they are accessed or locked.
> Additionally, from my previous email, pages become non-resident when
> their protection changes to PAGE_NOACCESS or PAGE_GUARD. The page will
> then only become resident once it is accessed (read or write) again.
> 
> The Linux kernel supports the concept of a "soft dirty" bit in the PTE
> that is accessible to user space with pagemaps. This is almost what we
> need, except it only handles writes and we need it to mark pages on
> read as well.

I wonder if it wouldn't be good enough to just use the current page protection.  If a page allows access, you have to assume another thread could access it at any time.  So, we could just pretend one has.

NOACCESS or GUARD -> invalid
anything else -> valid

Do you happen to know if that would satisfy League of Legends?

-Ken