Re: [PATCH 15/23] s390: allow pte_offset_map_lock() to fail

[Claudio Imbrenda <imbrenda@linux.ibm.com>]

On Tue, 23 May 2023 18:49:14 -0700 (PDT)
Hugh Dickins <hughd@google.com> wrote:

> On Tue, 23 May 2023, Claudio Imbrenda wrote:
> > 
> > so if I understand the above correctly, pte_offset_map_lock will only
> > fail if the whole page table has disappeared, and in that case, it will
> > never reappear with zero pages, therefore we can safely skip (in that
> > case just break). if we were to do a continue instead of a break, we
> > would most likely fail again anyway.  
> 
> Yes, that's the most likely; and you hold mmap_write_lock() there,
> and VM_NOHUGEPAGE on all vmas, so I think it's the only foreseeable
> possibility.
> 
> > 
> > in that case I would still like a small change in your patch: please
> > write a short (2~3 lines max) comment about why it's ok to do things
> > that way  
> 
> Sure.
> 
> But I now see that I've disobeyed you, and gone to 4 lines (but in the
> comment above the function, so as not to distract from the code itself):
> is this good wording to you?  I needed to research how they were stopped
> from coming in afterwards, so wanted to put something greppable in there.
> 
> And, unless I'm misunderstanding, that "after THP was enabled" was
> always supposed to say "after THP was disabled" (because splitting a
> huge zero page pmd inserts a a page table full of little zero ptes).

indeed, thanks for noticing and fixing it

> 
> Or would you prefer the comment in the commit message instead,
> or down just above the pte_offset_map_lock() line?
> 
> It would much better if I could find one place at the mm end, to
> enforce its end of the contract; but cannot think how to do that.
> 
> Hugh
> 
> --- a/arch/s390/mm/gmap.c
> +++ b/arch/s390/mm/gmap.c
> @@ -2537,7 +2537,12 @@ static inline void thp_split_mm(struct mm_struct *mm)
>   * Remove all empty zero pages from the mapping for lazy refaulting
>   * - This must be called after mm->context.has_pgste is set, to avoid
>   *   future creation of zero pages
> - * - This must be called after THP was enabled
> + * - This must be called after THP was disabled.
> + *
> + * mm contracts with s390, that even if mm were to remove a page table,
> + * racing with the loop below and so causing pte_offset_map_lock() to fail,
> + * it will never insert a page table containing empty zero pages once
> + * mm_forbids_zeropage(mm) i.e. mm->context.has_pgste is set.
>   */
>  static int __zap_zero_pages(pmd_t *pmd, unsigned long start,
>  			   unsigned long end, struct mm_walk *walk)

looks good, thanks