On Tue, Jul 31, 2012 at 09:12:08PM +0200, Peter Zijlstra wrote:
> If we marked a THP with our special PROT_NONE protections, ensure we
> don't loose them over a split.
>
> Collapse seems to always allocate a new (huge) page which should
> already end up on the new target node so loosing protections there
> isn't a problem.
This looks an optimization too, as it reduces a few branches.
If you didn't introduce an unnecessary goto it would have made the
actual change more readable and the patch much smaller. (you could
have cleaned it up with a later patch if you disliked the codying
style that tried to avoid using unnecessary gotos)
The s/barrier/ACCESS_ONCE/ I'll merge it in my tree as a separate
commit, as it's not related to sched-numa.
>
> Cc: Rik van Riel <r...@redhat.com>
> Cc: Paul Turner <p...@google.com>
> Signed-off-by: Peter Zijlstra <a.p.zijls...@chello.nl>
> ---
> arch/x86/include/asm/pgtable.h | 1
> mm/huge_memory.c | 104
> +++++++++++++++++++----------------------
> 2 files changed, 50 insertions(+), 55 deletions(-)
> --- a/arch/x86/include/asm/pgtable.h
> +++ b/arch/x86/include/asm/pgtable.h
> @@ -350,6 +350,7 @@ static inline pgprot_t pgprot_modify(pgp
> }
>
> #define pte_pgprot(x) __pgprot(pte_flags(x) & PTE_FLAGS_MASK)
> +#define pmd_pgprot(x) __pgprot(pmd_val(x) & ~_HPAGE_CHG_MASK)
>
> #define canon_pgprot(p) __pgprot(massage_pgprot(p))
>
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1353,64 +1353,60 @@ static int __split_huge_page_map(struct
> int ret = 0, i;
> pgtable_t pgtable;
> unsigned long haddr;
> + pgprot_t prot;
>
> spin_lock(&mm->page_table_lock);
> pmd = page_check_address_pmd(page, mm, address,
> PAGE_CHECK_ADDRESS_PMD_SPLITTING_FLAG);
> - if (pmd) {
> - pgtable = get_pmd_huge_pte(mm);
> - pmd_populate(mm, &_pmd, pgtable);
> -
> - for (i = 0, haddr = address; i < HPAGE_PMD_NR;
> - i++, haddr += PAGE_SIZE) {
> - pte_t *pte, entry;
> - BUG_ON(PageCompound(page+i));
> - entry = mk_pte(page + i, vma->vm_page_prot);
> - entry = maybe_mkwrite(pte_mkdirty(entry), vma);
> - if (!pmd_write(*pmd))
> - entry = pte_wrprotect(entry);
> - else
> - BUG_ON(page_mapcount(page) != 1);
> - if (!pmd_young(*pmd))
> - entry = pte_mkold(entry);
> - pte = pte_offset_map(&_pmd, haddr);
> - BUG_ON(!pte_none(*pte));
> - set_pte_at(mm, haddr, pte, entry);
> - pte_unmap(pte);
> - }
> + if (!pmd)
> + goto unlock;
>
> - smp_wmb(); /* make pte visible before pmd */
> - /*
> - * Up to this point the pmd is present and huge and
> - * userland has the whole access to the hugepage
> - * during the split (which happens in place). If we
> - * overwrite the pmd with the not-huge version
> - * pointing to the pte here (which of course we could
> - * if all CPUs were bug free), userland could trigger
> - * a small page size TLB miss on the small sized TLB
> - * while the hugepage TLB entry is still established
> - * in the huge TLB. Some CPU doesn't like that. See
> - * http://support.amd.com/us/Processor_TechDocs/41322.pdf,
> - * Erratum 383 on page 93. Intel should be safe but is
> - * also warns that it's only safe if the permission
> - * and cache attributes of the two entries loaded in
> - * the two TLB is identical (which should be the case
> - * here). But it is generally safer to never allow
> - * small and huge TLB entries for the same virtual
> - * address to be loaded simultaneously. So instead of
> - * doing "pmd_populate(); flush_tlb_range();" we first
> - * mark the current pmd notpresent (atomically because
> - * here the pmd_trans_huge and pmd_trans_splitting
> - * must remain set at all times on the pmd until the
> - * split is complete for this pmd), then we flush the
> - * SMP TLB and finally we write the non-huge version
> - * of the pmd entry with pmd_populate.
> - */
> - set_pmd_at(mm, address, pmd, pmd_mknotpresent(*pmd));
> - flush_tlb_range(vma, address, address + HPAGE_PMD_SIZE);
> - pmd_populate(mm, pmd, pgtable);
> - ret = 1;
> + prot = pmd_pgprot(*pmd);
> + pgtable = get_pmd_huge_pte(mm);
> + pmd_populate(mm, &_pmd, pgtable);
> +
> + for (i = 0, haddr = address; i < HPAGE_PMD_NR; i++, haddr += PAGE_SIZE)
> {
> + pte_t *pte, entry;
> +
> + BUG_ON(PageCompound(page+i));
> + entry = mk_pte(page + i, prot);
> + entry = pte_mkdirty(entry);
> + if (!pmd_young(*pmd))
> + entry = pte_mkold(entry);
> + pte = pte_offset_map(&_pmd, haddr);
> + BUG_ON(!pte_none(*pte));
> + set_pte_at(mm, haddr, pte, entry);
> + pte_unmap(pte);
> }
> +
> + smp_wmb(); /* make ptes visible before pmd, see __pte_alloc */
> + /*
> + * Up to this point the pmd is present and huge.
> + *
> + * If we overwrite the pmd with the not-huge version, we could trigger
> + * a small page size TLB miss on the small sized TLB while the hugepage
> + * TLB entry is still established in the huge TLB.
> + *
> + * Some CPUs don't like that. See
> + * http://support.amd.com/us/Processor_TechDocs/41322.pdf, Erratum 383
> + * on page 93.
> + *
> + * Thus it is generally safer to never allow small and huge TLB entries
> + * for overlapping virtual addresses to be loaded. So we first mark the
> + * current pmd not present, then we flush the TLB and finally we write
> + * the non-huge version of the pmd entry with pmd_populate.
> + *
> + * The above needs to be done under the ptl because pmd_trans_huge and
> + * pmd_trans_splitting must remain set on the pmd until the split is
> + * complete. The ptl also protects against concurrent faults due to
> + * making the pmd not-present.
> + */
> + set_pmd_at(mm, address, pmd, pmd_mknotpresent(*pmd));
> + flush_tlb_range(vma, address, address + HPAGE_PMD_SIZE);
> + pmd_populate(mm, pmd, pgtable);
> + ret = 1;
> +
> +unlock:
> spin_unlock(&mm->page_table_lock);
>
> return ret;
> @@ -2241,9 +2237,7 @@ static int khugepaged_wait_event(void)
> static void khugepaged_do_scan(struct page **hpage)
> {
> unsigned int progress = 0, pass_through_head = 0;
> - unsigned int pages = khugepaged_pages_to_scan;
> -
> - barrier(); /* write khugepaged_pages_to_scan to local stack */
> + unsigned int pages = ACCESS_ONCE(khugepaged_pages_to_scan);
>
> while (progress < pages) {
> cond_resched();
>
>
> --
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> the body of a message to majord...@vger.kernel.org
> More majordomo info at http://vger.kernel.org/majordomo-info.html
> Please read the FAQ at http://www.tux.org/lkml/
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
