Re: [RFC PATCH v2 1/3] mm/gup: fix gup_fast with dynamic page table folding

From: Jason Gunthorpe
Date: Thu Sep 10 2020 - 17:59:47 EST

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.