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) <vishal.moola@xxxxxxxxx>
One nit below, otherwise
Acked-by: Mike Rapoport (IBM) <rppt@xxxxxxxxxx>
> ---
> 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.
