Re: [RFC PATCH 10/47] mm: asi: Support for global non-sensitive direct map allocations

Junaid Shahid
2022-03-24T01:54:46Z

On 3/23/22 16:48, Junaid Shahid wrote:
> Hi Matthew,
>
> On 3/23/22 14:06, Matthew Wilcox wrote:
>> On Tue, Feb 22, 2022 at 09:21:46PM -0800, Junaid Shahid wrote:
>>> standard ASI instances. A new page flag is also added so that when
>>> these pages are freed, they can also be unmapped from the ASI page
>>> tables.
>>
>> It's cute how you just throw this in as an aside.  Page flags are
>> in high demand and just adding them is not to be done lightly.  Is
>> there any other way of accomplishing what you want?
>>
>
> I suppose we may be able to use page_ext instead. That certainly should be
> feasible for the PG_local_nonsensitive flag introduced in a later patch,
> although I am not completely sure about the PG_global_nonsensitive flag. That
> could get slightly tricky (though likely still possible to do) in case we need
> to allocate any non-sensitive memory before page_ext is initialized. One concern
> with using page_ext could be the extra memory usage on large machines.
>
> BTW is page flag scarcity an issue on 64-bit systems as well, or only 32-bit
> systems? ASI is only supported on 64-bit systems (at least currently).
>

One other thing that we could do to remove the need for the
PG_global_nonsensitive flag altogether (though not the PG_local_nonsensitive
flag) would be to always try to unmap pages from the asi_global_nonsensitive_pgd
in free_pages(). Basically, that would mean adding a page table walk to every
free_pages() rather than just non-sensitive free_pages(). Do you think that may
be a better trade-off in order to avoid the flag?

Thanks,
Junaid