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Date: Thu, 10 Sep 2020 18:59:21 -0300
From: Jason Gunthorpe <jgg@ziepe.ca>
To: Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [RFC PATCH v2 1/3] mm/gup: fix gup_fast with dynamic page table
 folding
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Cc: Peter Zijlstra <peterz@infradead.org>,
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 <linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org>

On Thu, Sep 10, 2020 at 12:32:05PM -0700, Linus Torvalds wrote:
> Yeah, I get hung up on naming sometimes. I don't tend to care much
> about private local variables ("i" is a perfectly fine variable name),
> but these kinds of somewhat subtle cross-architecture definitions I
> feel matter.

One of the first replys to this patch was to ask "when would I use
_orig vs normal", so you are not alone. The name should convey it..

So, I suggest pXX_offset_unlocked()

Since it is safe to call without the page table lock, while pXX_offset()
requires the page table lock to be held as the internal *pXX is a data
race otherwise.

Patch 1 might be OK for a stable backport, but to get to a clear
pXX_offset_unlocked() all the arches would want to be changed to
implement that API and the generic code would provide the wrapper:

#define pXX_offset(pXXp, address) pXX_offset_unlocked(pXXp, *(pXXp), address)

Arches would not have a *pXX inside their code.

Then we can talk about auditing call sites of pXX_offset and think
about using the _unlocked version in places where the page table lock
is not held.

For instance mm/pagewalk.c should be changed. So should
huge_pte_offset() and probably other places. These places might
already be exsting data-race bugs.

It is code-as-documentation indicating an unlocked page table walk.

Now it is not just a S390 story but a change that makes the data
concurrency much clearer, so I think I prefer this version to the
addr_end one too.

Regards,
Jason