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

[Dave Hansen]

On 9/7/20 11:00 AM, Gerald Schaefer wrote:
> Commit 1a42010cdc26 ("s390/mm: convert to the generic get_user_pages_fast
> code") introduced a subtle but severe bug on s390 with gup_fast, due to
> dynamic page table folding.

Would it be fair to say that the "fake" page table entries s390
allocates on the stack are what's causing the trouble here?  That might
be a nice thing to open up with here.  "Dynamic page table folding"
really means nothing to me.

> @@ -2521,7 +2521,7 @@ static int gup_pmd_range(pud_t pud, unsigned long addr, unsigned long end,
>  	do {
>  		pmd_t pmd = READ_ONCE(*pmdp);
>  
> -		next = pmd_addr_end(addr, end);
> +		next = pmd_addr_end_folded(pmd, addr, end);
>  		if (!pmd_present(pmd))
>  			return 0;

It looks like you fix this up later, but this would be a problem if left
this way.  There's no documentation for whether I use
pmd_addr_end_folded() or pmd_addr_end() when writing a page table walker.