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

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Message-ID: <ec6e5c54-2ff8-9af5-a25d-d99921a3da06@google.com>
Date:   Wed, 23 Mar 2022 18:54:37 -0700
From:   Junaid Shahid <junaids@...gle.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        pbonzini@...hat.com, jmattson@...gle.com, pjt@...gle.com,
        oweisse@...gle.com, alexandre.chartre@...cle.com,
        rppt@...ux.ibm.com, dave.hansen@...ux.intel.com,
        peterz@...radead.org, tglx@...utronix.de, luto@...nel.org,
        linux-mm@...ck.org
Subject: Re: [RFC PATCH 10/47] mm: asi: Support for global non-sensitive
 direct map allocations

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