linux-kernel - Re: [RFC PATCH v4 14/16] powerpc/64s: Use contiguous PMD/PUD instead of HUGEPD

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Date: Wed, 29 May 2024 11:23:18 +0200
From: Oscar Salvador <osalvador@...e.com>
To: Christophe Leroy <christophe.leroy@...roup.eu>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Jason Gunthorpe <jgg@...dia.com>, Peter Xu <peterx@...hat.com>,
	Michael Ellerman <mpe@...erman.id.au>,
	Nicholas Piggin <npiggin@...il.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [RFC PATCH v4 14/16] powerpc/64s: Use contiguous PMD/PUD instead
 of HUGEPD

On Mon, May 27, 2024 at 03:30:12PM +0200, 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.


On 4k mode, PMD_SIZE is 2MB and PUD_SIZE is 256MB, right?

> +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;

On 4K, hugepages are either 16M or 16G. How can we end up in a situation
whwere the is pte is huge, but is is neither MMU_PAGE_16G nor MMU_PAGE_16M?

> 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
'several'
> +		 * 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;

I have 0 clue about this code. What would happen if we do not bail out?


-- 
Oscar Salvador
SUSE Labs