On 17.03.22 18:58, Catalin Marinas wrote: > On Thu, Mar 17, 2022 at 11:04:18AM +0100, David Hildenbrand wrote: >> On 16.03.22 19:27, Catalin Marinas wrote: >>> On Tue, Mar 15, 2022 at 03:18:34PM +0100, David Hildenbrand wrote: >>>> @@ -909,12 +925,13 @@ static inline pmd_t pmdp_establish(struct vm_area_struct *vma, >>>> /* >>>> * Encode and decode a swap entry: >>>> * bits 0-1: present (must be zero) >>>> - * bits 2-7: swap type >>>> + * bits 2: remember PG_anon_exclusive >>>> + * bits 3-7: swap type >>>> * bits 8-57: swap offset >>>> * bit 58: PTE_PROT_NONE (must be zero) >>> >>> I don't remember exactly why we reserved bits 0 and 1 when, from the >>> hardware perspective, it's sufficient for bit 0 to be 0 and the whole >>> pte becomes invalid. We use bit 1 as the 'table' bit (when 0 at pmd >>> level, it's a huge page) but we shouldn't check for this on a swap >>> entry. >> >> You mean >> >> arch/arm64/include/asm/pgtable-hwdef.h:#define PTE_TABLE_BIT (_AT(pteval_t, 1) << 1) >> >> right? > > Yes. > >> I wonder why it even exists, for arm64 I only spot: >> >> arch/arm64/include/asm/pgtable.h:#define pte_mkhuge(pte) (__pte(pte_val(pte) & ~PTE_TABLE_BIT)) >> >> I don't really see code that sets PTE_TABLE_BIT. >> >> Similarly, I don't see code that sets PMD_TABLE_BIT/PUD_TABLE_BIT/P4D_TABLE_BIT. >> Most probably setting code is not using the defines, that's why I'm not finding it. > > It gets set as part of P*D_TYPE_TABLE via p*d_populate(). We use the > P*D_TABLE_BIT mostly for checking whether it's a huge page or not (the > arm64 hugetlbpage.c code). > Makes sense, after digging into the arm arm, I agree that it should be safe to reuse bit 1. I'll use this (yet untested) patch in v2:
