Subject: Re: [PATCH v6 21/23] powerpc/64s: Use contiguous PMD/PUD instead of HUGEPD

From: "Nicholas Piggin" <[email-masked]>
Date: Wed, 26 Jun 2024 11:23:11 +1000

On Tue Jun 25, 2024 at 3:20 PM AEST, LEROY Christophe wrote:
>
>
> Le 25/06/2024 à 06:49, Nicholas Piggin a écrit :
> > On Tue Jun 25, 2024 at 12:45 AM AEST, Christophe Leroy wrote:
> >> On book3s/64, the only user of hugepd is hash in 4k mode.
> >>
> >> All other setups (hash-64, radix-4, radix-64) use leaf PMD/PUD.
> >>
> >> Rework hash-4k to use contiguous PMD and PUD instead.
> >>
> >> In that setup there are only two huge page sizes: 16M and 16G.
> >>
> >> 16M sits at PMD level and 16G at PUD level.
> >>
> >> pte_update doesn't know page size, lets use the same trick as
> >> hpte_need_flush() to get page size from segment properties. That's
> >> not the most efficient way but let's do that until callers of
> >> pte_update() provide page size instead of just a huge flag.
> >>
> >> Signed-off-by: Christophe Leroy <[email-masked]>
> > 
> > [snip]
> > 
> >> +static inline unsigned long hash__pte_update(struct mm_struct *mm,
> >> +					 unsigned long addr,
> >> +					 pte_t *ptep, unsigned long clr,
> >> +					 unsigned long set,
> >> +					 int huge)
> >> +{
> >> +	unsigned long old;
> >> +
> >> +	old = hash__pte_update_one(ptep, clr, set);
> >> +
> >> +	if (IS_ENABLED(CONFIG_PPC_4K_PAGES) && huge) {
> >> +		unsigned int psize = get_slice_psize(mm, addr);
> >> +		int nb, i;
> >> +
> >> +		if (psize == MMU_PAGE_16M)
> >> +			nb = SZ_16M / PMD_SIZE;
> >> +		else if (psize == MMU_PAGE_16G)
> >> +			nb = SZ_16G / PUD_SIZE;
> >> +		else
> >> +			nb = 1;
> >> +
> >> +		WARN_ON_ONCE(nb == 1);	/* Should never happen */
> >> +
> >> +		for (i = 1; i < nb; i++)
> >> +			hash__pte_update_one(ptep + i, clr, set);
> >> +	}
> >>   	/* huge pages use the old page table lock */
> >>   	if (!huge)
> >>   		assert_pte_locked(mm, addr);
> >>   
> >> -	old = be64_to_cpu(old_be);
> >>   	if (old & H_PAGE_HASHPTE)
> >>   		hpte_need_flush(mm, addr, ptep, old, huge);
> >>   
> > 
> > We definitely need a bit more comment and changelog about the atomicity
> > issues here. I think the plan should be all hash-side access just
> > operates on PTE[0], which should avoid that whole race. There could be
> > some cases that don't follow that. Adding some warnings to catch such
> > things could be good too.
>
> That seems to be the case indeed, as we have the following in 
> hash_page_mm():
>
> #ifndef CONFIG_PPC_64K_PAGES
> 	/*
> 	 * If we use 4K pages and our psize is not 4K, then we might
> 	 * be hitting a special driver mapping, and need to align the
> 	 * address before we fetch the PTE.
> 	 *
> 	 * It could also be a hugepage mapping, in which case this is
> 	 * not necessary, but it's not harmful, either.
> 	 */
> 	if (psize != MMU_PAGE_4K)
> 		ea &= ~((1ul << mmu_psize_defs[psize].shift) - 1);
> #endif /* CONFIG_PPC_64K_PAGES */

Yeah, for that one it works (comment needs updating to say that it
*is* necessary). I think that's the main thing but there's other
possible places where it might not hold -- KVM too, not just the
hash refill.

> > 
> > I'd been meaning to do more on this sooner, sorry. I've started
> > tinkering with adding a bit of debug code. I'll see if I can help with
> > adding a bit of comments.
>
> Yes would we very welcome, I guess you'll send it as followup/fixup 
> patch to the series ?

Yeah, the basic approach I think is good, so it wouldn't be a
big rework.

>
> > 
> > [snip]
> > 
> >> diff --git a/arch/powerpc/mm/book3s64/hugetlbpage.c b/arch/powerpc/mm/book3s64/hugetlbpage.c
> >> index 5a2e512e96db..83c3361b358b 100644
> >> --- a/arch/powerpc/mm/book3s64/hugetlbpage.c
> >> +++ b/arch/powerpc/mm/book3s64/hugetlbpage.c
> >> @@ -53,6 +53,16 @@ int __hash_page_huge(unsigned long ea, unsigned long access, unsigned long vsid,
> >>   		/* If PTE permissions don't match, take page fault */
> >>   		if (unlikely(!check_pte_access(access, old_pte)))
> >>   			return 1;
> >> +		/*
> >> +		 * If hash-4k, hugepages use seeral contiguous PxD entries
> >> +		 * so bail out and let mm make the page young or dirty
> >> +		 */
> >> +		if (IS_ENABLED(CONFIG_PPC_4K_PAGES)) {
> >> +			if (!(old_pte & _PAGE_ACCESSED))
> >> +				return 1;
> >> +			if ((access & _PAGE_WRITE) && !(old_pte & _PAGE_DIRTY))
> >> +				return 1;
> >> +		}
> >>   
> >>   		/*
> >>   		 * Try to lock the PTE, add ACCESSED and DIRTY if it was
> > 
> > I'm hoping we wouldn't have to do this, if we follow the PTE[0] rule.
>
> But we still need all entries to be updated so that page walker which 
> don't know they must use PTE[0] get the right information ?

Ah yeah. Maybe for ACCESSED|DIRTY we can slightly adjust that rule
and apply it to all PTEs. If we can do that then it takes care of
a few other cases too.

Bug what is the consequence of two pte_update racing? Let's say
page_vma_mkclean_one vs setting dirty. Can you end up with some
PTEs dirty and some not?

Thanks,
Nick