Re: [PATCH v2 4/5] mm: Add new ptep_deref() helper to fully encapsulate pte_t

From: Ryan Roberts
Date: Thu May 25 2023 - 05:09:19 EST

On 19/05/2023 10:12, Ryan Roberts wrote:
> On 18/05/2023 20:28, Yu Zhao wrote:
>> On Thu, May 18, 2023 at 5:07 AM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
>>> There are many call sites that directly dereference a pte_t pointer.
>>> This makes it very difficult to properly encapsulate a page table in the
>>> arch code without having to allocate shadow page tables. ptep_deref()
>>> aims to solve this by replacing all direct dereferences with a call to
>>> this function.
>>> The default implementation continues to just dereference the pointer
>>> (*ptep), so generated code should be exactly the same. However, it is
>>> possible for the architecture to override the default with their own
>>> implementation, that can (e.g.) hide certain bits from the core code, or
>>> determine young/dirty status by mixing in state from another source.
>>> While ptep_get() and ptep_get_lockless() already exist, these are
>>> implemented as atomic accesses (e.g. READ_ONCE() in the default case).
>>> So rather than using ptep_get() and risking performance regressions,
>>> introduce an new variant.
>> We should reuse ptep_get():
>> 1. I don't think READ_ONCE() can cause measurable regressions in this case.
>> 2. It's technically wrong without it.
> Can you clarify what you mean by technically wrong? Are you saying that the
> current code that does direct dereferencing is buggy?
> I previously convinced myself that the potential for the compiler generating
> multiple loads was safe because the code in question is under the PTL so there
> are no concurrent stores. And we shouldn't see any tearing for the same reason.
> That said, if there is concensus that we can just use ptep_get() (==
> READ_ONCE()) everywhere, then I agree that would be cleaner. Does anyone object?

Hi all,

A politie bump: It would be great to hear opinions on this before I go ahead and
make the change.