On 03/07/2016 01:38 PM, David Miller wrote:

From: Khalid Aziz <redacted>
Date: Mon, 7 Mar 2016 14:33:56 -0700

quoted

On 03/07/2016 12:16 PM, David Miller wrote:

quoted

From: Khalid Aziz <redacted>
Date: Mon, 7 Mar 2016 11:24:54 -0700

quoted

Tags can be cleared by user by setting tag to 0. Tags are
automatically cleared by the hardware when the mapping for a virtual
address is removed from TSB (which is why swappable pages are a
problem), so kernel does not have to do it as part of clean up.

You might be able to crib some bits for the Tag in the swp_entry_t,
it's
64-bit and you can therefore steal bits from the offset field.

That way you'll have the ADI tag in the page tables, ready to
re-install
at swapin time.

That is a possibility but limited in scope. An address range covered
by a single TTE can have large number of tags. Version tags are set on
cacheline. In extreme case, one could set a tag for each set of
64-bytes in a page. Also tags are set completely in userspace and no
transition occurs to kernel space, so kernel has no idea of what tags
have been set. I have not found a way to query the MMU on tags.

I will think some more about it.

That would mean that ADI is impossible to use for swappable memory.

...

If that's true I'm extremely disappointed that they devoted so much
silicon and engineering to this feature yet didn't take that one
critical step to make it generally useful. :(

You can easily read ADI tags with a simple ldxa #ASI_MCD_PRIMARY 
instruction.

Rob


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