Subject: Re: [PATCH v4 03/34] s390: Use pt_frag_refcount for pagetables

From: Mike Rapoport <[email-masked]>
Date: Wed, 14 Jun 2023 16:21:37 +0300

On Mon, Jun 12, 2023 at 02:03:52PM -0700, Vishal Moola (Oracle) wrote:
> s390 currently uses _refcount to identify fragmented page tables.
> The page table struct already has a member pt_frag_refcount used by
> powerpc, so have s390 use that instead of the _refcount field as well.
> This improves the safety for _refcount and the page table tracking.
> 
> This also allows us to simplify the tracking since we can once again use
> the lower byte of pt_frag_refcount instead of the upper byte of _refcount.
> 
> Signed-off-by: Vishal Moola (Oracle) <[email-masked]>

One nit below, otherwise

Acked-by: Mike Rapoport (IBM) <[email-masked]>

> ---
>  arch/s390/mm/pgalloc.c | 38 +++++++++++++++-----------------------
>  1 file changed, 15 insertions(+), 23 deletions(-)
> 
> diff --git a/arch/s390/mm/pgalloc.c b/arch/s390/mm/pgalloc.c
> index 66ab68db9842..6b99932abc66 100644
> --- a/arch/s390/mm/pgalloc.c
> +++ b/arch/s390/mm/pgalloc.c
> @@ -182,20 +182,17 @@ void page_table_free_pgste(struct page *page)
>   * As follows from the above, no unallocated or fully allocated parent
>   * pages are contained in mm_context_t::pgtable_list.
>   *
> - * The upper byte (bits 24-31) of the parent page _refcount is used
> + * The lower byte (bits 0-7) of the parent page pt_frag_refcount is used
>   * for tracking contained 2KB-pgtables and has the following format:
>   *
>   *   PP  AA
> - * 01234567    upper byte (bits 24-31) of struct page::_refcount
> + * 01234567    upper byte (bits 0-7) of struct page::pt_frag_refcount

Nit:              lower

>   *   ||  ||
>   *   ||  |+--- upper 2KB-pgtable is allocated
>   *   ||  +---- lower 2KB-pgtable is allocated
>   *   |+------- upper 2KB-pgtable is pending for removal
>   *   +-------- lower 2KB-pgtable is pending for removal
>   *
> - * (See commit 620b4e903179 ("s390: use _refcount for pgtables") on why
> - * using _refcount is possible).
> - *
>   * When 2KB-pgtable is allocated the corresponding AA bit is set to 1.
>   * The parent page is either:
>   *   - added to mm_context_t::pgtable_list in case the second half of the
> @@ -243,11 +240,12 @@ unsigned long *page_table_alloc(struct mm_struct *mm)
>  		if (!list_empty(&mm->context.pgtable_list)) {
>  			page = list_first_entry(&mm->context.pgtable_list,
>  						struct page, lru);
> -			mask = atomic_read(&page->_refcount) >> 24;
> +			mask = atomic_read(&page->pt_frag_refcount);
>  			/*
>  			 * The pending removal bits must also be checked.
>  			 * Failure to do so might lead to an impossible
> -			 * value of (i.e 0x13 or 0x23) written to _refcount.
> +			 * value of (i.e 0x13 or 0x23) written to
> +			 * pt_frag_refcount.
>  			 * Such values violate the assumption that pending and
>  			 * allocation bits are mutually exclusive, and the rest
>  			 * of the code unrails as result. That could lead to
> @@ -259,8 +257,8 @@ unsigned long *page_table_alloc(struct mm_struct *mm)
>  				bit = mask & 1;		/* =1 -> second 2K */
>  				if (bit)
>  					table += PTRS_PER_PTE;
> -				atomic_xor_bits(&page->_refcount,
> -							0x01U << (bit + 24));
> +				atomic_xor_bits(&page->pt_frag_refcount,
> +							0x01U << bit);
>  				list_del(&page->lru);
>  			}
>  		}
> @@ -281,12 +279,12 @@ unsigned long *page_table_alloc(struct mm_struct *mm)
>  	table = (unsigned long *) page_to_virt(page);
>  	if (mm_alloc_pgste(mm)) {
>  		/* Return 4K page table with PGSTEs */
> -		atomic_xor_bits(&page->_refcount, 0x03U << 24);
> +		atomic_xor_bits(&page->pt_frag_refcount, 0x03U);
>  		memset64((u64 *)table, _PAGE_INVALID, PTRS_PER_PTE);
>  		memset64((u64 *)table + PTRS_PER_PTE, 0, PTRS_PER_PTE);
>  	} else {
>  		/* Return the first 2K fragment of the page */
> -		atomic_xor_bits(&page->_refcount, 0x01U << 24);
> +		atomic_xor_bits(&page->pt_frag_refcount, 0x01U);
>  		memset64((u64 *)table, _PAGE_INVALID, 2 * PTRS_PER_PTE);
>  		spin_lock_bh(&mm->context.lock);
>  		list_add(&page->lru, &mm->context.pgtable_list);
> @@ -323,22 +321,19 @@ void page_table_free(struct mm_struct *mm, unsigned long *table)
>  		 * will happen outside of the critical section from this
>  		 * function or from __tlb_remove_table()
>  		 */
> -		mask = atomic_xor_bits(&page->_refcount, 0x11U << (bit + 24));
> -		mask >>= 24;
> +		mask = atomic_xor_bits(&page->pt_frag_refcount, 0x11U << bit);
>  		if (mask & 0x03U)
>  			list_add(&page->lru, &mm->context.pgtable_list);
>  		else
>  			list_del(&page->lru);
>  		spin_unlock_bh(&mm->context.lock);
> -		mask = atomic_xor_bits(&page->_refcount, 0x10U << (bit + 24));
> -		mask >>= 24;
> +		mask = atomic_xor_bits(&page->pt_frag_refcount, 0x10U << bit);
>  		if (mask != 0x00U)
>  			return;
>  		half = 0x01U << bit;
>  	} else {
>  		half = 0x03U;
> -		mask = atomic_xor_bits(&page->_refcount, 0x03U << 24);
> -		mask >>= 24;
> +		mask = atomic_xor_bits(&page->pt_frag_refcount, 0x03U);
>  	}
>  
>  	page_table_release_check(page, table, half, mask);
> @@ -368,8 +363,7 @@ void page_table_free_rcu(struct mmu_gather *tlb, unsigned long *table,
>  	 * outside of the critical section from __tlb_remove_table() or from
>  	 * page_table_free()
>  	 */
> -	mask = atomic_xor_bits(&page->_refcount, 0x11U << (bit + 24));
> -	mask >>= 24;
> +	mask = atomic_xor_bits(&page->pt_frag_refcount, 0x11U << bit);
>  	if (mask & 0x03U)
>  		list_add_tail(&page->lru, &mm->context.pgtable_list);
>  	else
> @@ -391,14 +385,12 @@ void __tlb_remove_table(void *_table)
>  		return;
>  	case 0x01U:	/* lower 2K of a 4K page table */
>  	case 0x02U:	/* higher 2K of a 4K page table */
> -		mask = atomic_xor_bits(&page->_refcount, mask << (4 + 24));
> -		mask >>= 24;
> +		mask = atomic_xor_bits(&page->pt_frag_refcount, mask << 4);
>  		if (mask != 0x00U)
>  			return;
>  		break;
>  	case 0x03U:	/* 4K page table with pgstes */
> -		mask = atomic_xor_bits(&page->_refcount, 0x03U << 24);
> -		mask >>= 24;
> +		mask = atomic_xor_bits(&page->pt_frag_refcount, 0x03U);
>  		break;
>  	}
>  
> -- 
> 2.40.1
> 
> 

-- 
Sincerely yours,
Mike.