Re: [PATCH hmm 7/8] mm/hmm: return -EFAULT when setting HMM_PFN_ERROR on requested valid pages

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On 3/11/20 11:35 AM, Jason Gunthorpe wrote:
From: Jason Gunthorpe <jgg@xxxxxxxxxxxx>

hmm_range_fault() should never return 0 if the caller requested a valid
page, but the pfns output for that page would be HMM_PFN_ERROR.

hmm_pte_need_fault() must always be called before setting HMM_PFN_ERROR to
detect if the page is in faulting mode or not.

Fix two cases in hmm_vma_walk_pmd() and reorganize some of the duplicated
code.

Fixes: d08faca018c4 ("mm/hmm: properly handle migration pmd")
Fixes: da4c3c735ea4 ("mm/hmm/mirror: helper to snapshot CPU page table")
Signed-off-by: Jason Gunthorpe <jgg@xxxxxxxxxxxx>

Reviewed-by: Ralph Campbell <rcampbell@xxxxxxxxxx>

---
  mm/hmm.c | 38 +++++++++++++++++++++-----------------
  1 file changed, 21 insertions(+), 17 deletions(-)

diff --git a/mm/hmm.c b/mm/hmm.c
index bf676cfef3e8ee..f61fddf2ef6505 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -363,8 +363,10 @@ static int hmm_vma_walk_pmd(pmd_t *pmdp,
  {
  	struct hmm_vma_walk *hmm_vma_walk = walk->private;
  	struct hmm_range *range = hmm_vma_walk->range;
-	uint64_t *pfns = range->pfns;
-	unsigned long addr = start, i;
+	uint64_t *pfns = &range->pfns[(start - range->start) >> PAGE_SHIFT];
+	unsigned long npages = (end - start) >> PAGE_SHIFT;
+	unsigned long addr = start;
+	bool fault, write_fault;
  	pte_t *ptep;
  	pmd_t pmd;
@@ -374,14 +376,6 @@ static int hmm_vma_walk_pmd(pmd_t *pmdp,
  		return hmm_vma_walk_hole(start, end, -1, walk);
if (thp_migration_supported() && is_pmd_migration_entry(pmd)) {
-		bool fault, write_fault;
-		unsigned long npages;
-		uint64_t *pfns;
-
-		i = (addr - range->start) >> PAGE_SHIFT;
-		npages = (end - addr) >> PAGE_SHIFT;
-		pfns = &range->pfns[i];
-
  		hmm_range_need_fault(hmm_vma_walk, pfns, npages,
  				     0, &fault, &write_fault);
  		if (fault || write_fault) {
@@ -390,8 +384,15 @@ static int hmm_vma_walk_pmd(pmd_t *pmdp,
  			return -EBUSY;
  		}
  		return hmm_pfns_fill(start, end, range, HMM_PFN_NONE);
-	} else if (!pmd_present(pmd))
+	}
+
+	if (!pmd_present(pmd)) {
+		hmm_range_need_fault(hmm_vma_walk, pfns, npages, 0, &fault,
+				     &write_fault);
+		if (fault || write_fault)
+			return -EFAULT;
  		return hmm_pfns_fill(start, end, range, HMM_PFN_ERROR);

Shouldn't this fill with HMM_PFN_NONE instead of HMM_PFN_ERROR?
Otherwise, when a THP is swapped out, you will get a different
value than if a PTE is swapped out and you are prefetching/snapshotting.

+	}
if (pmd_devmap(pmd) || pmd_trans_huge(pmd)) {
  		/*
@@ -408,8 +409,7 @@ static int hmm_vma_walk_pmd(pmd_t *pmdp,
  		if (!pmd_devmap(pmd) && !pmd_trans_huge(pmd))
  			goto again;
- i = (addr - range->start) >> PAGE_SHIFT;
-		return hmm_vma_handle_pmd(walk, addr, end, &pfns[i], pmd);
+		return hmm_vma_handle_pmd(walk, addr, end, pfns, pmd);
  	}
/*
@@ -418,15 +418,19 @@ static int hmm_vma_walk_pmd(pmd_t *pmdp,
  	 * entry pointing to pte directory or it is a bad pmd that will not
  	 * recover.
  	 */
-	if (pmd_bad(pmd))
+	if (pmd_bad(pmd)) {
+		hmm_range_need_fault(hmm_vma_walk, pfns, npages, 0, &fault,
+				     &write_fault);
+		if (fault || write_fault)
+			return -EFAULT;
  		return hmm_pfns_fill(start, end, range, HMM_PFN_ERROR);
+	}
ptep = pte_offset_map(pmdp, addr);
-	i = (addr - range->start) >> PAGE_SHIFT;
-	for (; addr < end; addr += PAGE_SIZE, ptep++, i++) {
+	for (; addr < end; addr += PAGE_SIZE, ptep++, pfns++) {
  		int r;
- r = hmm_vma_handle_pte(walk, addr, end, pmdp, ptep, &pfns[i]);
+		r = hmm_vma_handle_pte(walk, addr, end, pmdp, ptep, pfns);
  		if (r) {
  			/* hmm_vma_handle_pte() did pte_unmap() */
  			hmm_vma_walk->last = addr;

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