[PATCH 3/3] mm/hugetlb: page faults check for fallocate hole punch in progress and wait

[Mike Kravetz]

At page fault time, check i_private which indicates a fallocate hole punch
is in progress.  If the fault falls within the hole, wait for the hole
punch operation to complete before proceeding with the fault.

Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
---
 mm/hugetlb.c | 37 +++++++++++++++++++++++++++++++++++++
 1 file changed, 37 insertions(+)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 3c7db92..540d3a79 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -3580,6 +3580,7 @@ int hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
 	struct page *pagecache_page = NULL;
 	struct hstate *h = hstate_vma(vma);
 	struct address_space *mapping;
+	struct inode *inode = file_inode(vma->vm_file);
 	int need_wait_lock = 0;
 
 	address &= huge_page_mask(h);
@@ -3603,6 +3604,42 @@ int hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
 	idx = vma_hugecache_offset(h, vma, address);
 
 	/*
+	 * page faults could race with fallocate hole punch.  If a page
+	 * is faulted between unmap and deallocation, it will still remain
+	 * in the punched hole.  During hole punch operations, a hugetlb_falloc
+	 * structure will be pointed to by i_private.  If this fault is for
+	 * a page in a hole being punched, wait for the operation to finish
+	 * before proceeding.
+	 *
+	 * Even with this strategy, it is still possible for a page fault to
+	 * race with hole punch.  However, the race window is considerably
+	 * smaller.
+	 */
+	if (unlikely(inode->i_private)) {
+		struct hugetlb_falloc *hugetlb_falloc;
+
+		spin_lock(&inode->i_lock);
+		hugetlb_falloc = inode->i_private;
+		if (hugetlb_falloc && hugetlb_falloc->waitq &&
+		    idx >= hugetlb_falloc->start &&
+		    idx <= hugetlb_falloc->end) {
+			wait_queue_head_t *hugetlb_falloc_waitq;
+			DEFINE_WAIT(hugetlb_fault_wait);
+
+			hugetlb_falloc_waitq = hugetlb_falloc->waitq;
+			prepare_to_wait(hugetlb_falloc_waitq,
+					&hugetlb_fault_wait,
+					TASK_UNINTERRUPTIBLE);
+			spin_unlock(&inode->i_lock);
+			schedule();
+
+			spin_lock(&inode->i_lock);
+			finish_wait(hugetlb_falloc_waitq, &hugetlb_fault_wait);
+		}
+		spin_unlock(&inode->i_lock);
+	}
+
+	/*
 	 * Serialize hugepage allocation and instantiation, so that we don't
 	 * get spurious allocation failures if two CPUs race to instantiate
 	 * the same page in the page cache.
-- 
2.4.3

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