[PATCH v7 1/8] mm/hmm: move page fault handling out of walk callbacks

hmm_range_fault() currently triggers page faults from inside the page-table
walk callbacks: hmm_vma_walk_pmd(), hmm_vma_walk_pud(),
hmm_vma_walk_hugetlb_entry() and the pte-level helper all call
hmm_vma_fault(), which in turn calls handle_mm_fault() while the walker
still holds nested locks.  The pte spinlock is dropped explicitly by each
caller, and the hugetlb path manually drops and retakes
hugetlb_vma_lock_read around the fault to dodge a deadlock against the walk
framework's unconditional unlock.

This layering does not extend cleanly to fault handlers that may release
mmap_lock (VM_FAULT_RETRY, VM_FAULT_COMPLETED). If the lock is dropped
while walk_page_range() is mid-traversal, the VMA can be freed before the
walk framework's matching hugetlb_vma_unlock_read(), turning that unlock
into a use-after-free.

Split the responsibilities the way get_user_pages() does. Walk callbacks
become inspect-only: when they detect a range that needs to be faulted in,
they record it in struct hmm_vma_walk and return a private sentinel
(HMM_FAULT_PENDING). The outer loop in hmm_range_fault() then drops out of
walk_page_range(), invokes a new helper hmm_do_fault() that calls
handle_mm_fault() with only mmap_lock held, and restarts the walk so the
now-present entries are collected into hmm_pfns.

No functional change for existing callers. As a side effect the hugetlb
callback no longer needs the hugetlb_vma_{un}lock_read dance, and every
fault-path exit from the callbacks now releases the pte spinlock on a
single, common path. This refactor is also a precursor for adding an
unlockable variant of hmm_range_fault() in a follow-up patch.

Signed-off-by: Stanislav Kinsburskii <[email protected]>
---
 mm/hmm.c |  118 +++++++++++++++++++++++++++++++++++++++-----------------------
 1 file changed, 75 insertions(+), 43 deletions(-)

diff --git a/mm/hmm.c b/mm/hmm.c
index 4f3f627d2b47..2129b1ee4c35 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -33,8 +33,17 @@
 struct hmm_vma_walk {
 	struct hmm_range	*range;
 	unsigned long		last;
+	unsigned long		end;
+	unsigned int		required_fault;
 };
 
+/*
+ * Internal sentinel returned by walk callbacks when they need a page fault.
+ * The callback stores end/required_fault in hmm_vma_walk; the outer loop
+ * consumes the sentinel and never propagates it to the caller.
+ */
+#define HMM_FAULT_PENDING	-EAGAIN
+
 enum {
 	HMM_NEED_FAULT = 1 << 0,
 	HMM_NEED_WRITE_FAULT = 1 << 1,
@@ -60,37 +69,25 @@ static int hmm_pfns_fill(unsigned long addr, unsigned long end,
 }
 
 /*
- * hmm_vma_fault() - fault in a range lacking valid pmd or pte(s)
- * @addr: range virtual start address (inclusive)
- * @end: range virtual end address (exclusive)
- * @required_fault: HMM_NEED_* flags
- * @walk: mm_walk structure
- * Return: -EBUSY after page fault, or page fault error
+ * hmm_record_fault() - record a range that needs to be faulted in
  *
- * This function will be called whenever pmd_none() or pte_none() returns true,
- * or whenever there is no page directory covering the virtual address range.
+ * Called by the walk callbacks when they discover that part of the range
+ * needs a page fault.  The callback records what to fault and returns
+ * HMM_FAULT_PENDING; the outer loop in hmm_range_fault() drops back out of
+ * walk_page_range() and invokes handle_mm_fault() from a context where no
+ * page-table or hugetlb_vma_lock is held.
  */
-static int hmm_vma_fault(unsigned long addr, unsigned long end,
-			 unsigned int required_fault, struct mm_walk *walk)
+static int hmm_record_fault(unsigned long addr, unsigned long end,
+			    unsigned int required_fault,
+			    struct mm_walk *walk)
 {
 	struct hmm_vma_walk *hmm_vma_walk = walk->private;
-	struct vm_area_struct *vma = walk->vma;
-	unsigned int fault_flags = FAULT_FLAG_REMOTE;
 
 	WARN_ON_ONCE(!required_fault);
 	hmm_vma_walk->last = addr;
-
-	if (required_fault & HMM_NEED_WRITE_FAULT) {
-		if (!(vma->vm_flags & VM_WRITE))
-			return -EPERM;
-		fault_flags |= FAULT_FLAG_WRITE;
-	}
-
-	for (; addr < end; addr += PAGE_SIZE)
-		if (handle_mm_fault(vma, addr, fault_flags, NULL) &
-		    VM_FAULT_ERROR)
-			return -EFAULT;
-	return -EBUSY;
+	hmm_vma_walk->end = end;
+	hmm_vma_walk->required_fault = required_fault;
+	return HMM_FAULT_PENDING;
 }
 
 static unsigned int hmm_pte_need_fault(const struct hmm_vma_walk *hmm_vma_walk,
@@ -174,7 +171,7 @@ static int hmm_vma_walk_hole(unsigned long addr, unsigned long end,
 		return hmm_pfns_fill(addr, end, range, HMM_PFN_ERROR);
 	}
 	if (required_fault)
-		return hmm_vma_fault(addr, end, required_fault, walk);
+		return hmm_record_fault(addr, end, required_fault, walk);
 	return hmm_pfns_fill(addr, end, range, 0);
 }
 
@@ -209,7 +206,7 @@ static int hmm_vma_handle_pmd(struct mm_walk *walk, unsigned long addr,
 	required_fault =
 		hmm_range_need_fault(hmm_vma_walk, hmm_pfns, npages, cpu_flags);
 	if (required_fault)
-		return hmm_vma_fault(addr, end, required_fault, walk);
+		return hmm_record_fault(addr, end, required_fault, walk);
 
 	pfn = pmd_pfn(pmd) + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
 	for (i = 0; addr < end; addr += PAGE_SIZE, i++, pfn++) {
@@ -328,7 +325,7 @@ static int hmm_vma_handle_pte(struct mm_walk *walk, unsigned long addr,
 fault:
 	pte_unmap(ptep);
 	/* Fault any virtual address we were asked to fault */
-	return hmm_vma_fault(addr, end, required_fault, walk);
+	return hmm_record_fault(addr, end, required_fault, walk);
 }
 
 #ifdef CONFIG_ARCH_SUPPORTS_PMD_SOFTLEAF
@@ -371,7 +368,7 @@ static int hmm_vma_handle_absent_pmd(struct mm_walk *walk, unsigned long start,
 					      npages, 0);
 	if (required_fault) {
 		if (softleaf_is_device_private(entry))
-			return hmm_vma_fault(addr, end, required_fault, walk);
+			return hmm_record_fault(addr, end, required_fault, walk);
 		else
 			return -EFAULT;
 	}
@@ -517,7 +514,7 @@ static int hmm_vma_walk_pud(pud_t *pudp, unsigned long start, unsigned long end,
 						      npages, cpu_flags);
 		if (required_fault) {
 			spin_unlock(ptl);
-			return hmm_vma_fault(addr, end, required_fault, walk);
+			return hmm_record_fault(addr, end, required_fault, walk);
 		}
 
 		pfn = pud_pfn(pud) + ((addr & ~PUD_MASK) >> PAGE_SHIFT);
@@ -564,21 +561,8 @@ static int hmm_vma_walk_hugetlb_entry(pte_t *pte, unsigned long hmask,
 	required_fault =
 		hmm_pte_need_fault(hmm_vma_walk, pfn_req_flags, cpu_flags);
 	if (required_fault) {
-		int ret;
-
 		spin_unlock(ptl);
-		hugetlb_vma_unlock_read(vma);
-		/*
-		 * Avoid deadlock: drop the vma lock before calling
-		 * hmm_vma_fault(), which will itself potentially take and
-		 * drop the vma lock. This is also correct from a
-		 * protection point of view, because there is no further
-		 * use here of either pte or ptl after dropping the vma
-		 * lock.
-		 */
-		ret = hmm_vma_fault(addr, end, required_fault, walk);
-		hugetlb_vma_lock_read(vma);
-		return ret;
+		return hmm_record_fault(addr, end, required_fault, walk);
 	}
 
 	pfn = pte_pfn(entry) + ((start & ~hmask) >> PAGE_SHIFT);
@@ -637,6 +621,44 @@ static const struct mm_walk_ops hmm_walk_ops = {
 	.walk_lock	= PGWALK_RDLOCK,
 };
 
+/*
+ * hmm_do_fault - fault in a range recorded by a walk callback
+ *
+ * Called from the outer loop in hmm_range_fault() after a callback
+ * returned HMM_FAULT_PENDING.  At this point we hold only mmap_lock;
+ * the page-table spinlock and any hugetlb_vma_lock acquired by the walk
+ * framework have already been released by the unwind.
+ *
+ * Returns -EBUSY on success (all pages faulted, caller should re-walk).
+ * Returns a negative errno on failure.
+ */
+static int hmm_do_fault(struct mm_struct *mm,
+			struct hmm_vma_walk *hmm_vma_walk)
+{
+	unsigned long addr = hmm_vma_walk->last;
+	unsigned long end = hmm_vma_walk->end;
+	unsigned int required_fault = hmm_vma_walk->required_fault;
+	unsigned int fault_flags = FAULT_FLAG_REMOTE;
+	struct vm_area_struct *vma;
+
+	vma = vma_lookup(mm, addr);
+	if (!vma)
+		return -EFAULT;
+
+	if (required_fault & HMM_NEED_WRITE_FAULT) {
+		if (!(vma->vm_flags & VM_WRITE))
+			return -EPERM;
+		fault_flags |= FAULT_FLAG_WRITE;
+	}
+
+	for (; addr < end; addr += PAGE_SIZE)
+		if (handle_mm_fault(vma, addr, fault_flags, NULL) &
+		    VM_FAULT_ERROR)
+			return -EFAULT;
+
+	return -EBUSY;
+}
+
 /**
  * hmm_range_fault - try to fault some address in a virtual address range
  * @range:	argument structure
@@ -674,6 +696,16 @@ int hmm_range_fault(struct hmm_range *range)
 			return -EBUSY;
 		ret = walk_page_range(mm, hmm_vma_walk.last, range->end,
 				      &hmm_walk_ops, &hmm_vma_walk);
+		/*
+		 * When HMM_FAULT_PENDING is returned a walk callback
+		 * recorded a range that needs handle_mm_fault();
+		 * hmm_do_fault() runs the fault outside walk_page_range()
+		 * (so no page-table or hugetlb_vma_lock is held) and
+		 * returns -EBUSY so the loop re-walks and picks up the
+		 * now-present entries.
+		 */
+		if (ret == HMM_FAULT_PENDING)
+			ret = hmm_do_fault(mm, &hmm_vma_walk);
 		/*
 		 * When -EBUSY is returned the loop restarts with
 		 * hmm_vma_walk.last set to an address that has not been stored