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Subject[PATCH 03/31] mm/thp: Preserve pgprot across huge page split
We're going to play games with page-protections, ensure we don't lose
them over a THP 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.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Reviewed-by: Rik van Riel <riel@redhat.com>
Cc: Paul Turner <pjt@google.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
arch/x86/include/asm/pgtable.h | 1
mm/huge_memory.c | 105 +++++++++++++++++++----------------------
2 files changed, 51 insertions(+), 55 deletions(-)

Index: tip/arch/x86/include/asm/pgtable.h
===================================================================
--- tip.orig/arch/x86/include/asm/pgtable.h
+++ tip/arch/x86/include/asm/pgtable.h
@@ -349,6 +349,7 @@ 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))

Index: tip/mm/huge_memory.c
===================================================================
--- tip.orig/mm/huge_memory.c
+++ tip/mm/huge_memory.c
@@ -1343,63 +1343,60 @@ 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 = pgtable_trans_huge_withdraw(mm);
- pmd_populate(mm, &_pmd, pgtable);
-
- haddr = address;
- for (i = 0; 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);
- }
-
- 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.
- */
- pmdp_invalidate(vma, address, pmd);
- pmd_populate(mm, pmd, pgtable);
- ret = 1;
+ if (!pmd)
+ goto unlock;
+
+ prot = pmd_pgprot(*pmd);
+ pgtable = pgtable_trans_huge_withdraw(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;
@@ -2287,10 +2284,8 @@ static void khugepaged_do_scan(void)
{
struct page *hpage = NULL;
unsigned int progress = 0, pass_through_head = 0;
- unsigned int pages = khugepaged_pages_to_scan;
bool wait = true;
-
- barrier(); /* write khugepaged_pages_to_scan to local stack */
+ unsigned int pages = ACCESS_ONCE(khugepaged_pages_to_scan);

while (progress < pages) {
if (!khugepaged_prealloc_page(&hpage, &wait))



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