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Date:   Thu, 25 May 2023 15:35:01 -0700 (PDT)
From:   Hugh Dickins <hughd@google.com>
To:     Peter Xu <peterx@redhat.com>
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Subject: Re: [PATCH 01/31] mm: use pmdp_get_lockless() without surplus
 barrier()
In-Reply-To: <ZG6PwAvIO4Z7lpkq@x1n>
Message-ID: <427ea01f-345a-6086-d145-fe573894dbe@google.com>
References: <68a97fbe-5c1e-7ac6-72c-7b9c6290b370@google.com> <34467cca-58b6-3e64-1ee7-e3dc43257a@google.com> <ZG6PwAvIO4Z7lpkq@x1n>
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On Wed, 24 May 2023, Peter Xu wrote:
> On Sun, May 21, 2023 at 09:49:45PM -0700, Hugh Dickins wrote:
> > Use pmdp_get_lockless() in preference to READ_ONCE(*pmdp), to get a more
> > reliable result with PAE (or READ_ONCE as before without PAE); and remove
> > the unnecessary extra barrier()s which got left behind in its callers.
> 
> Pure question: does it mean that some of below path (missing barrier()
> ones) could have problem when CONFIG_PAE, hence this can be seen as a
> (potential) bug fix?

I don't think so; or at least, I am not claiming that this fixes any.

It really depends on what use is made of the pmdval afterwards, and
I've not checked through them.  The READ_ONCE()s which were there,
were good enough to make sure that the compiler did not reevaluate
the pmdval later on, with perhaps a confusingly different result.

But, at least in the x86 PAE case, they were not good enough to ensure
that the two halves of the entry match up; and, sad to say, nor is that
absolutely guaranteed by these conversions to pmdp_get_lockless() -
because of the "HOWEVER" below.  PeterZ's comments in linux/pgtable.h
are well worth reading through.

You might question why I made these changes at all: some days
I question them too.  Better though imperfect?  Or deceptive?

Hugh

> > 
> > HOWEVER: Note the small print in linux/pgtable.h, where it was designed
> > specifically for fast GUP, and depends on interrupts being disabled for
> > its full guarantee: most callers which have been added (here and before)
> > do NOT have interrupts disabled, so there is still some need for caution.