[PATCH hmm 2/8] mm/hmm: don't free the cached pgmap while scanning

[Jason Gunthorpe]

From: Jason Gunthorpe <jgg at mellanox.com>

The pgmap is held in the hmm_vma_walk variable in hope of speeding up
future get_dev_pagemap() calls by hitting the same pointer. The algorithm
doesn't actually care about how long the pgmap is held for.

Move the put of the cached pgmap to after the walk is completed and delete
all the other now redundant puts.

This solves a possible leak of the reference in hmm_vma_walk_pmd() if a
hmm_vma_handle_pte() fails while looping.

Fixes: 992de9a8b751 ("mm/hmm: allow to mirror vma of a file on a DAX backed filesystem")
Signed-off-by: Jason Gunthorpe <jgg at mellanox.com>
---
 mm/hmm.c | 31 +++++++++----------------------
 1 file changed, 9 insertions(+), 22 deletions(-)

We talked about just deleting this stuff, but I think it makes alot sense for
hmm_range_fault() to trigger fault on devmap pages that are not compatible
with the caller - so lets just fix the leak on error path for now.

diff --git a/mm/hmm.c b/mm/hmm.c
index 35f85424176d14..9e8f68eb83287a 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -239,10 +239,6 @@ static int hmm_vma_handle_pmd(struct mm_walk *walk, unsigned long addr,
 		}
 		pfns[i] = hmm_device_entry_from_pfn(range, pfn) | cpu_flags;
 	}
-	if (hmm_vma_walk->pgmap) {
-		put_dev_pagemap(hmm_vma_walk->pgmap);
-		hmm_vma_walk->pgmap = NULL;
-	}
 	hmm_vma_walk->last = end;
 	return 0;
 }
@@ -360,10 +356,6 @@ static int hmm_vma_handle_pte(struct mm_walk *walk, unsigned long addr,
 	return 0;
 
 fault:
-	if (hmm_vma_walk->pgmap) {
-		put_dev_pagemap(hmm_vma_walk->pgmap);
-		hmm_vma_walk->pgmap = NULL;
-	}
 	pte_unmap(ptep);
 	/* Fault any virtual address we were asked to fault */
 	return hmm_vma_walk_hole_(addr, end, fault, write_fault, walk);
@@ -446,16 +438,6 @@ static int hmm_vma_walk_pmd(pmd_t *pmdp,
 			return r;
 		}
 	}
-	if (hmm_vma_walk->pgmap) {
-		/*
-		 * We do put_dev_pagemap() here and not in hmm_vma_handle_pte()
-		 * so that we can leverage get_dev_pagemap() optimization which
-		 * will not re-take a reference on a pgmap if we already have
-		 * one.
-		 */
-		put_dev_pagemap(hmm_vma_walk->pgmap);
-		hmm_vma_walk->pgmap = NULL;
-	}
 	pte_unmap(ptep - 1);
 
 	hmm_vma_walk->last = addr;
@@ -529,10 +511,6 @@ static int hmm_vma_walk_pud(pud_t *pudp, unsigned long start, unsigned long end,
 			pfns[i] = hmm_device_entry_from_pfn(range, pfn) |
 				  cpu_flags;
 		}
-		if (hmm_vma_walk->pgmap) {
-			put_dev_pagemap(hmm_vma_walk->pgmap);
-			hmm_vma_walk->pgmap = NULL;
-		}
 		hmm_vma_walk->last = end;
 		goto out_unlock;
 	}
@@ -694,6 +672,15 @@ long hmm_range_fault(struct hmm_range *range, unsigned int flags)
 			return -EBUSY;
 		ret = walk_page_range(mm, hmm_vma_walk.last, range->end,
 				      &hmm_walk_ops, &hmm_vma_walk);
+		/*
+		 * A pgmap is kept cached in the hmm_vma_walk to avoid expensive
+		 * searching in the probably common case that the pgmap is the
+		 * same for the entire requested range.
+		 */
+		if (hmm_vma_walk.pgmap) {
+			put_dev_pagemap(hmm_vma_walk.pgmap);
+			hmm_vma_walk.pgmap = NULL;
+		}
 	} while (ret == -EBUSY);
 
 	if (ret)
-- 
2.25.1