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

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Date: Tue, 25 Jun 2024 14:49:45 +1000
From: "Nicholas Piggin" <npiggin@...il.com>
To: "Christophe Leroy" <christophe.leroy@...roup.eu>, "Andrew Morton"
 <akpm@...ux-foundation.org>, "Jason Gunthorpe" <jgg@...dia.com>, "Peter Xu"
 <peterx@...hat.com>, "Oscar Salvador" <osalvador@...e.de>, "Michael
 Ellerman" <mpe@...erman.id.au>
Cc: <linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>,
 <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH v6 21/23] powerpc/64s: Use contiguous PMD/PUD instead of
 HUGEPD

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 <christophe.leroy@...roup.eu>

[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.

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.

[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.

I think is minor enough that should not prevent testing in -mm.

Thanks,
Nick