Re: [PATCH v2 2/5] mm: avoid unnecessary flush on change_huge_pmd()

Dave Hansen
2021-10-26T18:44:51Z

On 10/26/21 10:44 AM, Nadav Amit wrote:
>> "If software on one logical processor writes to a page while software on
>> another logical processor concurrently clears the R/W flag in the
>> paging-structure entry that maps the page, execution on some processors may
>> result in the entry’s dirty flag being set (due to the write on the first
>> logical processor) and the entry’s R/W flag being clear (due to the update
>> to the entry on the second logical processor). This will never occur on a
>> processor that supports control-flow enforcement technology (CET)”
>>
>> So I guess that this optimization can only be enabled when CET is enabled.
>>
>> :(
> I still wonder whether the SDM comment applies to present bit vs dirty
> bit atomicity as well.

I think it's implicit. From "4.8 ACCESSED AND DIRTY FLAGS":

"Whenever there is a write to a linear address, the processor
sets the dirty flag (if it is not already set) in the paging-
structure entry"

There can't be a "write to a linear address" without a Present=1 PTE.
If it were a Dirty=1,Present=1 PTE, there's no race because there might
not be a write to the PTE at all.

There's also this from the "4.10.4.3 Optional Invalidation" section:

"no TLB entry or paging-structure cache entry is created with
information from a paging-structure entry in which the P flag
is 0."

That means that we don't have to worry about the TLB doing something
bonkers like caching a Dirty=1 bit from a Present=0 PTE.

Is that what you were worried about?