On Fri, 19 Apr 2019 10:27:17 -0700
Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Fri, Apr 19, 2019 at 6:33 AM Martin Schwidefsky
> <schwidefsky@xxxxxxxxxx> wrote:
> >
> > That problem got stuck in my head and I thought more about it. Why not
> > emulate the static folding sequence in the s390 page table code?
>
> So this model seems much closer to what x86 does in its folding, where
> the pattern is basically
>
> > static inline pX-1d_t *pXd_offset(pXd_t *pXd, unsigned long address)
> > {
> > if (pXd_folded(pXd)
> > return (pX-1d_t *) pXd;
> > return (pX-1d_t *) pXd_deref(*pXd) + pXd_index(address);
> > }
>
> which is really how the code is designed to work (ie the folded entry
> doesn't actually do anything to the page directory pointer, it just
> says "ok, we'll use this exact page directory pointer for the next
> lower level instead".
>
> And that's very much what allows the generic gup code to load the
> entry once, and use a temporary, and as you walk down the chain, if it
> is folded it just then uses that (previous) temporary value for the
> next level instead. IOW, the lower level page table is hidden inside
> the upper level one, and folding just means "don't do any offsets,
> don't change any values, just use the entry as-is for the next lower
> level".
>
> So I think that's the right thing to do.
Ok, I added two patches for my s390/linux:features branch
Martin Schwidefsky (2):
s390/mm: make the pxd_offset functions more robust
s390/mm: convert to the generic get_user_pages_fast code
All code changes are inside arch/s390, I plan to include these patches with
the next merge window. That gives us a little bit of time to run our tests.
--
blue skies,
Martin.
"Reality continues to ruin my life." - Calvin.
