From: Yu Zhao <yuzhao@google.com>
To: Ryan Roberts <ryan.roberts@arm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
SeongJae Park <sj@kernel.org>,
Christoph Hellwig <hch@infradead.org>,
"Matthew Wilcox (Oracle)" <willy@infradead.org>,
"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
Lorenzo Stoakes <lstoakes@gmail.com>,
Uladzislau Rezki <urezki@gmail.com>, Zi Yan <ziy@nvidia.com>,
linux-kernel@vger.kernel.org, linux-mm@kvack.org,
damon@lists.linux.dev
Subject: Re: [PATCH v2 4/5] mm: Add new ptep_deref() helper to fully encapsulate pte_t
Date: Thu, 25 May 2023 20:02:37 -0600 [thread overview]
Message-ID: <CAOUHufZDYB_9QBau+9w-ZQWjFTQpnMf-i-jJn-T7=nFizYGDhw@mail.gmail.com> (raw)
In-Reply-To: <692e9e7e-ee00-368b-6a31-60a895f7011c@arm.com>
On Fri, May 19, 2023 at 3:12 AM Ryan Roberts <ryan.roberts@arm.com> wrote:
>
> On 18/05/2023 20:28, Yu Zhao wrote:
> > On Thu, May 18, 2023 at 5:07 AM Ryan Roberts <ryan.roberts@arm.com> 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?
Sorry for not being clear.
I think we can agree that *ptep is volatile. Not being treated as such
seems a bad idea to me. I don't think it'd cause any real problems --
most warnings KCSAN reported didn't either, but we fixed them anyway.
So should we fix this case as well while we are at it.
> 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?
(No objection to NOT using it either. Just a recommendation, since
it's already there.)
next prev parent reply other threads:[~2023-05-26 2:03 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-05-18 11:07 [PATCH v2 0/5] Encapsulate PTE contents from non-arch code Ryan Roberts
2023-05-18 11:07 ` [PATCH v2 1/5] mm: vmalloc must set pte via arch code Ryan Roberts
2023-05-19 12:09 ` Uladzislau Rezki
2023-05-24 18:47 ` Mike Rapoport
2023-05-18 11:07 ` [PATCH v2 2/5] mm: damon must atomically clear young on ptes and pmds Ryan Roberts
2023-05-18 17:13 ` SeongJae Park
2023-05-19 8:53 ` Ryan Roberts
2023-05-18 23:19 ` Yu Zhao
2023-05-19 9:02 ` Ryan Roberts
2023-05-19 19:54 ` SeongJae Park
2023-05-22 8:53 ` Ryan Roberts
2023-05-24 18:49 ` Mike Rapoport
2023-05-18 11:07 ` [PATCH v2 3/5] mm: Fix failure to unmap pte on highmem systems Ryan Roberts
2023-05-24 18:59 ` Mike Rapoport
2023-05-18 11:07 ` [PATCH v2 4/5] mm: Add new ptep_deref() helper to fully encapsulate pte_t Ryan Roberts
2023-05-18 19:28 ` Yu Zhao
2023-05-19 9:12 ` Ryan Roberts
2023-05-25 9:08 ` Ryan Roberts
2023-05-26 2:02 ` Yu Zhao [this message]
2023-05-24 19:06 ` Mike Rapoport
2023-05-24 19:11 ` Ryan Roberts
2023-05-18 11:07 ` [PATCH v2 5/5] mm: ptep_deref() conversion Ryan Roberts
2023-05-18 12:08 ` Jason Gunthorpe
2023-05-18 17:22 ` SeongJae Park
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