Re: Linux 5.1-rc5

[Martin Schwidefsky]

On Thu, 18 Apr 2019 08:49:32 -0700
Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:

> On Thu, Apr 18, 2019 at 1:02 AM Martin Schwidefsky
> <schwidefsky@xxxxxxxxxx> wrote:
> >
> > The problematic lines in the generic gup code are these three:
> >
> > 1845:   pmdp = pmd_offset(&pud, addr);
> > 1888:   pudp = pud_offset(&p4d, addr);
> > 1916:   p4dp = p4d_offset(&pgd, addr);
> >
> > Passing the pointer of a *copy* of a page table entry to pxd_offset() does
> > not work with the page table folding on s390.  
> 
> Hmm. I wonder why. x86 too does the folding thing for the p4d and pud case.
> 
> The folding works with the local copy just the same way it works with
> the orignal value.

The difference is that with the static page table folding pgd_offset()
does the index calculation of the actual hardware top-level table. With
dynamic page table folding as s390 is doing it, if the task does not use
a 5-level page table pgd_offset() will see a pgd_index() of 0, the indexing
of the actual top-level table is done later with p4d_offset(), pud_offset()
or pmd_offset(). 

As an example, with a three level page table we have three indexes x/y/z.
The common code "thinks" 5 indexing steps, with static folding the index
sequence is x 0 0 y z. With dynamic folding the sequence is 0 0 x y z.
By moving the first indexing operation to pgd_offset the static sequence
does not add an index to a non-dereferenced pointer to a stack variable,
the dynamic sequence does.

> But I see that s390 does some other kind of folding and does that
> addition of the p*d_index() unconditionally.
> 
> I guess that does mean that s390 will just have to have its own walker.
> 
> For the issue of the page refcount overflow it really isn't a huge
> deal. Adding the refcount checking is simple (see the example patch I
> gave for powerpc - you'll just have a couple of extra cases since you
> do it all, rather than just the special hugetlb cases).
> 
> Obviously in general it would have been nicer to share as much code as
> possible, but let's not make things unnecessarily complex if s390 is
> just fundamentally different..

It would have been nice to use the generic code (less bugs) but not at
the price of over-complicating things. And that page table folding thing
always makes my head hurt.

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.