[PATCH 09/16] mm/hmm: add output flag for compound page mapping

[Ralph Campbell]

hmm_range_fault() returns an array of page frame numbers and flags for
how the pages are mapped in the requested process' page tables. The PFN
can be used to get the struct page with hmm_pfn_to_page() and the page size
order can be determined with compound_order(page) but if the page is larger
than order 0 (PAGE_SIZE), there is no indication that the page is mapped
using a larger page size. To be fully general, hmm_range_fault() would need
to return the mapping size to handle cases like a 1GB compound page being
mapped with 2MB PMD entries. However, the most common case is the mapping
size is the same as the underlying compound page size.
Add a new output flag to indicate this so that callers know it is safe to
use a large device page table mapping if one is available.

Signed-off-by: Ralph Campbell <rcampbell@xxxxxxxxxx>
---
 include/linux/hmm.h |  4 +++-
 mm/hmm.c            | 10 +++++++---
 2 files changed, 10 insertions(+), 4 deletions(-)

diff --git a/include/linux/hmm.h b/include/linux/hmm.h
index f4a09ed223ac..d0db78025baa 100644
--- a/include/linux/hmm.h
+++ b/include/linux/hmm.h
@@ -41,12 +41,14 @@ enum hmm_pfn_flags {
 	HMM_PFN_VALID = 1UL << (BITS_PER_LONG - 1),
 	HMM_PFN_WRITE = 1UL << (BITS_PER_LONG - 2),
 	HMM_PFN_ERROR = 1UL << (BITS_PER_LONG - 3),
+	HMM_PFN_COMPOUND = 1UL << (BITS_PER_LONG - 4),
 
 	/* Input flags */
 	HMM_PFN_REQ_FAULT = HMM_PFN_VALID,
 	HMM_PFN_REQ_WRITE = HMM_PFN_WRITE,
 
-	HMM_PFN_FLAGS = HMM_PFN_VALID | HMM_PFN_WRITE | HMM_PFN_ERROR,
+	HMM_PFN_FLAGS = HMM_PFN_VALID | HMM_PFN_WRITE | HMM_PFN_ERROR |
+			HMM_PFN_COMPOUND,
 };
 
 /*
diff --git a/mm/hmm.c b/mm/hmm.c
index e9a545751108..d145d44256df 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -170,7 +170,9 @@ static inline unsigned long pmd_to_hmm_pfn_flags(struct hmm_range *range,
 {
 	if (pmd_protnone(pmd))
 		return 0;
-	return pmd_write(pmd) ? (HMM_PFN_VALID | HMM_PFN_WRITE) : HMM_PFN_VALID;
+	return pmd_write(pmd) ?
+			(HMM_PFN_VALID | HMM_PFN_COMPOUND | HMM_PFN_WRITE) :
+			(HMM_PFN_VALID | HMM_PFN_COMPOUND);
 }
 
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
@@ -389,7 +391,9 @@ static inline unsigned long pud_to_hmm_pfn_flags(struct hmm_range *range,
 {
 	if (!pud_present(pud))
 		return 0;
-	return pud_write(pud) ? (HMM_PFN_VALID | HMM_PFN_WRITE) : HMM_PFN_VALID;
+	return pud_write(pud) ?
+			(HMM_PFN_VALID | HMM_PFN_COMPOUND | HMM_PFN_WRITE) :
+			(HMM_PFN_VALID | HMM_PFN_COMPOUND);
 }
 
 static int hmm_vma_walk_pud(pud_t *pudp, unsigned long start, unsigned long end,
@@ -484,7 +488,7 @@ static int hmm_vma_walk_hugetlb_entry(pte_t *pte, unsigned long hmask,
 
 	pfn = pte_pfn(entry) + ((start & ~hmask) >> PAGE_SHIFT);
 	for (; addr < end; addr += PAGE_SIZE, i++, pfn++)
-		range->hmm_pfns[i] = pfn | cpu_flags;
+		range->hmm_pfns[i] = pfn | cpu_flags | HMM_PFN_COMPOUND;
 
 	spin_unlock(ptl);
 	return 0;
-- 
2.20.1