[patch 02/20] make the inode i_mmap_lock a reader/writer lock

[Rik van Riel <riel@redhat.com>]

[-- Attachment #1: make-i_mmap_lock-rw.patch --]
[-- Type: text/plain, Size: 19390 bytes --]

I have seen soft cpu lockups in page_referenced_file() due to 
contention on i_mmap_lock() for different pages.  Making the
i_mmap_lock a reader/writer lock should increase parallelism
in vmscan for file back pages mapped into many address spaces.

Read lock the i_mmap_lock for all usage except:

1) mmap/munmap:  linking vma into i_mmap prio_tree or removing
2) unmap_mapping_range:   protecting vm_truncate_count

rmap:  try_to_unmap_file() required new cond_resched_rwlock().
To reduce code duplication, I recast cond_resched_lock() as a
[static inline] wrapper around reworked cond_sched_lock() =>
__cond_resched_lock(void *lock, int type). 
New cond_resched_rwlock() implemented as another wrapper.  


Signed-off-by:  Lee Schermerhorn <lee.schermerhorn@hp.com>
Signed-off-by:  Rik van Riel <riel@redhat.com>

Index: linux-2.6.24-rc5-mm1/include/linux/fs.h
===================================================================
--- linux-2.6.24-rc5-mm1.orig/include/linux/fs.h
+++ linux-2.6.24-rc5-mm1/include/linux/fs.h
@@ -501,7 +501,7 @@ struct address_space {
 	unsigned int		i_mmap_writable;/* count VM_SHARED mappings */
 	struct prio_tree_root	i_mmap;		/* tree of private and shared mappings */
 	struct list_head	i_mmap_nonlinear;/*list VM_NONLINEAR mappings */
-	spinlock_t		i_mmap_lock;	/* protect tree, count, list */
+	rwlock_t		i_mmap_lock;	/* protect tree, count, list */
 	unsigned int		truncate_count;	/* Cover race condition with truncate */
 	unsigned long		nrpages;	/* number of total pages */
 	pgoff_t			writeback_index;/* writeback starts here */
Index: linux-2.6.24-rc5-mm1/include/linux/mm.h
===================================================================
--- linux-2.6.24-rc5-mm1.orig/include/linux/mm.h
+++ linux-2.6.24-rc5-mm1/include/linux/mm.h
@@ -707,7 +707,7 @@ struct zap_details {
 	struct address_space *check_mapping;	/* Check page->mapping if set */
 	pgoff_t	first_index;			/* Lowest page->index to unmap */
 	pgoff_t last_index;			/* Highest page->index to unmap */
-	spinlock_t *i_mmap_lock;		/* For unmap_mapping_range: */
+	rwlock_t *i_mmap_lock;			/* For unmap_mapping_range: */
 	unsigned long truncate_count;		/* Compare vm_truncate_count */
 };
 
Index: linux-2.6.24-rc5-mm1/fs/inode.c
===================================================================
--- linux-2.6.24-rc5-mm1.orig/fs/inode.c
+++ linux-2.6.24-rc5-mm1/fs/inode.c
@@ -210,7 +210,7 @@ void inode_init_once(struct inode *inode
 	INIT_LIST_HEAD(&inode->i_devices);
 	INIT_RADIX_TREE(&inode->i_data.page_tree, GFP_ATOMIC);
 	rwlock_init(&inode->i_data.tree_lock);
-	spin_lock_init(&inode->i_data.i_mmap_lock);
+ 	rwlock_init(&inode->i_data.i_mmap_lock);
 	INIT_LIST_HEAD(&inode->i_data.private_list);
 	spin_lock_init(&inode->i_data.private_lock);
 	INIT_RAW_PRIO_TREE_ROOT(&inode->i_data.i_mmap);
Index: linux-2.6.24-rc5-mm1/fs/hugetlbfs/inode.c
===================================================================
--- linux-2.6.24-rc5-mm1.orig/fs/hugetlbfs/inode.c
+++ linux-2.6.24-rc5-mm1/fs/hugetlbfs/inode.c
@@ -420,6 +420,9 @@ static void hugetlbfs_drop_inode(struct 
 		hugetlbfs_forget_inode(inode);
 }
 
+/*
+ * LOCKING:  __unmap_hugepage_range() requires write lock on i_mmap_lock
+ */
 static inline void
 hugetlb_vmtruncate_list(struct prio_tree_root *root, pgoff_t pgoff)
 {
@@ -454,10 +457,10 @@ static int hugetlb_vmtruncate(struct ino
 	pgoff = offset >> PAGE_SHIFT;
 
 	i_size_write(inode, offset);
-	spin_lock(&mapping->i_mmap_lock);
+	write_lock(&mapping->i_mmap_lock);
 	if (!prio_tree_empty(&mapping->i_mmap))
 		hugetlb_vmtruncate_list(&mapping->i_mmap, pgoff);
-	spin_unlock(&mapping->i_mmap_lock);
+	write_unlock(&mapping->i_mmap_lock);
 	truncate_hugepages(inode, offset);
 	return 0;
 }
Index: linux-2.6.24-rc5-mm1/kernel/fork.c
===================================================================
--- linux-2.6.24-rc5-mm1.orig/kernel/fork.c
+++ linux-2.6.24-rc5-mm1/kernel/fork.c
@@ -272,12 +272,12 @@ static int dup_mmap(struct mm_struct *mm
 				atomic_dec(&inode->i_writecount);
 
 			/* insert tmp into the share list, just after mpnt */
-			spin_lock(&file->f_mapping->i_mmap_lock);
+			write_lock(&file->f_mapping->i_mmap_lock);
 			tmp->vm_truncate_count = mpnt->vm_truncate_count;
 			flush_dcache_mmap_lock(file->f_mapping);
 			vma_prio_tree_add(tmp, mpnt);
 			flush_dcache_mmap_unlock(file->f_mapping);
-			spin_unlock(&file->f_mapping->i_mmap_lock);
+			write_unlock(&file->f_mapping->i_mmap_lock);
 		}
 
 		/*
Index: linux-2.6.24-rc5-mm1/mm/filemap_xip.c
===================================================================
--- linux-2.6.24-rc5-mm1.orig/mm/filemap_xip.c
+++ linux-2.6.24-rc5-mm1/mm/filemap_xip.c
@@ -182,7 +182,7 @@ __xip_unmap (struct address_space * mapp
 	if (!page)
 		return;
 
-	spin_lock(&mapping->i_mmap_lock);
+	read_lock(&mapping->i_mmap_lock);
 	vma_prio_tree_foreach(vma, &iter, &mapping->i_mmap, pgoff, pgoff) {
 		mm = vma->vm_mm;
 		address = vma->vm_start +
@@ -200,7 +200,7 @@ __xip_unmap (struct address_space * mapp
 			page_cache_release(page);
 		}
 	}
-	spin_unlock(&mapping->i_mmap_lock);
+	read_unlock(&mapping->i_mmap_lock);
 }
 
 /*
Index: linux-2.6.24-rc5-mm1/mm/fremap.c
===================================================================
--- linux-2.6.24-rc5-mm1.orig/mm/fremap.c
+++ linux-2.6.24-rc5-mm1/mm/fremap.c
@@ -202,13 +202,13 @@ asmlinkage long sys_remap_file_pages(uns
 			}
 			goto out;
 		}
-		spin_lock(&mapping->i_mmap_lock);
+		write_lock(&mapping->i_mmap_lock);
 		flush_dcache_mmap_lock(mapping);
 		vma->vm_flags |= VM_NONLINEAR;
 		vma_prio_tree_remove(vma, &mapping->i_mmap);
 		vma_nonlinear_insert(vma, &mapping->i_mmap_nonlinear);
 		flush_dcache_mmap_unlock(mapping);
-		spin_unlock(&mapping->i_mmap_lock);
+		write_unlock(&mapping->i_mmap_lock);
 	}
 
 	err = populate_range(mm, vma, start, size, pgoff);
Index: linux-2.6.24-rc5-mm1/mm/hugetlb.c
===================================================================
--- linux-2.6.24-rc5-mm1.orig/mm/hugetlb.c
+++ linux-2.6.24-rc5-mm1/mm/hugetlb.c
@@ -721,9 +721,9 @@ void unmap_hugepage_range(struct vm_area
 	 * do nothing in this case.
 	 */
 	if (vma->vm_file) {
-		spin_lock(&vma->vm_file->f_mapping->i_mmap_lock);
+		write_lock(&vma->vm_file->f_mapping->i_mmap_lock);
 		__unmap_hugepage_range(vma, start, end);
-		spin_unlock(&vma->vm_file->f_mapping->i_mmap_lock);
+		write_unlock(&vma->vm_file->f_mapping->i_mmap_lock);
 	}
 }
 
@@ -964,7 +964,7 @@ void hugetlb_change_protection(struct vm
 	BUG_ON(address >= end);
 	flush_cache_range(vma, address, end);
 
-	spin_lock(&vma->vm_file->f_mapping->i_mmap_lock);
+	read_lock(&vma->vm_file->f_mapping->i_mmap_lock);
 	spin_lock(&mm->page_table_lock);
 	for (; address < end; address += HPAGE_SIZE) {
 		ptep = huge_pte_offset(mm, address);
@@ -979,7 +979,7 @@ void hugetlb_change_protection(struct vm
 		}
 	}
 	spin_unlock(&mm->page_table_lock);
-	spin_unlock(&vma->vm_file->f_mapping->i_mmap_lock);
+	read_unlock(&vma->vm_file->f_mapping->i_mmap_lock);
 
 	flush_tlb_range(vma, start, end);
 }
Index: linux-2.6.24-rc5-mm1/mm/memory.c
===================================================================
--- linux-2.6.24-rc5-mm1.orig/mm/memory.c
+++ linux-2.6.24-rc5-mm1/mm/memory.c
@@ -811,7 +811,7 @@ unsigned long unmap_vmas(struct mmu_gath
 	unsigned long tlb_start = 0;	/* For tlb_finish_mmu */
 	int tlb_start_valid = 0;
 	unsigned long start = start_addr;
-	spinlock_t *i_mmap_lock = details? details->i_mmap_lock: NULL;
+	rwlock_t *i_mmap_lock = details? details->i_mmap_lock: NULL;
 	int fullmm = (*tlbp)->fullmm;
 
 	for ( ; vma && vma->vm_start < end_addr; vma = vma->vm_next) {
@@ -1727,7 +1727,7 @@ unwritable_page:
  * can't efficiently keep all vmas in step with mapping->truncate_count:
  * so instead reset them all whenever it wraps back to 0 (then go to 1).
  * mapping->truncate_count and vma->vm_truncate_count are protected by
- * i_mmap_lock.
+ * write locked i_mmap_lock.
  *
  * In order to make forward progress despite repeatedly restarting some
  * large vma, note the restart_addr from unmap_vmas when it breaks out:
@@ -1792,9 +1792,10 @@ again:
 			goto again;
 	}
 
-	spin_unlock(details->i_mmap_lock);
+//TODO:  why not cond_resched_lock() here [rwlock version]?
+	write_unlock(details->i_mmap_lock);
 	cond_resched();
-	spin_lock(details->i_mmap_lock);
+	write_lock(details->i_mmap_lock);
 	return -EINTR;
 }
 
@@ -1890,7 +1891,7 @@ void unmap_mapping_range(struct address_
 		details.last_index = ULONG_MAX;
 	details.i_mmap_lock = &mapping->i_mmap_lock;
 
-	spin_lock(&mapping->i_mmap_lock);
+	write_lock(&mapping->i_mmap_lock);
 
 	/* Protect against endless unmapping loops */
 	mapping->truncate_count++;
@@ -1905,7 +1906,7 @@ void unmap_mapping_range(struct address_
 		unmap_mapping_range_tree(&mapping->i_mmap, &details);
 	if (unlikely(!list_empty(&mapping->i_mmap_nonlinear)))
 		unmap_mapping_range_list(&mapping->i_mmap_nonlinear, &details);
-	spin_unlock(&mapping->i_mmap_lock);
+	write_unlock(&mapping->i_mmap_lock);
 }
 EXPORT_SYMBOL(unmap_mapping_range);
 
Index: linux-2.6.24-rc5-mm1/mm/migrate.c
===================================================================
--- linux-2.6.24-rc5-mm1.orig/mm/migrate.c
+++ linux-2.6.24-rc5-mm1/mm/migrate.c
@@ -202,12 +202,12 @@ static void remove_file_migration_ptes(s
 	if (!mapping)
 		return;
 
-	spin_lock(&mapping->i_mmap_lock);
+	read_lock(&mapping->i_mmap_lock);
 
 	vma_prio_tree_foreach(vma, &iter, &mapping->i_mmap, pgoff, pgoff)
 		remove_migration_pte(vma, old, new);
 
-	spin_unlock(&mapping->i_mmap_lock);
+	read_unlock(&mapping->i_mmap_lock);
 }
 
 /*
Index: linux-2.6.24-rc5-mm1/mm/mmap.c
===================================================================
--- linux-2.6.24-rc5-mm1.orig/mm/mmap.c
+++ linux-2.6.24-rc5-mm1/mm/mmap.c
@@ -186,7 +186,7 @@ error:
 }
 
 /*
- * Requires inode->i_mapping->i_mmap_lock
+ * Requires write locked inode->i_mapping->i_mmap_lock
  */
 static void __remove_shared_vm_struct(struct vm_area_struct *vma,
 		struct file *file, struct address_space *mapping)
@@ -214,9 +214,9 @@ void unlink_file_vma(struct vm_area_stru
 
 	if (file) {
 		struct address_space *mapping = file->f_mapping;
-		spin_lock(&mapping->i_mmap_lock);
+		write_lock(&mapping->i_mmap_lock);
 		__remove_shared_vm_struct(vma, file, mapping);
-		spin_unlock(&mapping->i_mmap_lock);
+		write_unlock(&mapping->i_mmap_lock);
 	}
 }
 
@@ -439,7 +439,7 @@ static void vma_link(struct mm_struct *m
 		mapping = vma->vm_file->f_mapping;
 
 	if (mapping) {
-		spin_lock(&mapping->i_mmap_lock);
+		write_lock(&mapping->i_mmap_lock);
 		vma->vm_truncate_count = mapping->truncate_count;
 	}
 	anon_vma_lock(vma);
@@ -449,7 +449,7 @@ static void vma_link(struct mm_struct *m
 
 	anon_vma_unlock(vma);
 	if (mapping)
-		spin_unlock(&mapping->i_mmap_lock);
+		write_unlock(&mapping->i_mmap_lock);
 
 	mm->map_count++;
 	validate_mm(mm);
@@ -536,7 +536,7 @@ again:			remove_next = 1 + (end > next->
 		mapping = file->f_mapping;
 		if (!(vma->vm_flags & VM_NONLINEAR))
 			root = &mapping->i_mmap;
-		spin_lock(&mapping->i_mmap_lock);
+		write_lock(&mapping->i_mmap_lock);
 		if (importer &&
 		    vma->vm_truncate_count != next->vm_truncate_count) {
 			/*
@@ -620,7 +620,7 @@ again:			remove_next = 1 + (end > next->
 	if (anon_vma)
 		write_unlock(&anon_vma->rwlock);
 	if (mapping)
-		spin_unlock(&mapping->i_mmap_lock);
+		write_unlock(&mapping->i_mmap_lock);
 
 	if (remove_next) {
 		if (file)
@@ -2061,7 +2061,7 @@ void exit_mmap(struct mm_struct *mm)
 
 /* Insert vm structure into process list sorted by address
  * and into the inode's i_mmap tree.  If vm_file is non-NULL
- * then i_mmap_lock is taken here.
+ * then i_mmap_lock is write locked here.
  */
 int insert_vm_struct(struct mm_struct * mm, struct vm_area_struct * vma)
 {
Index: linux-2.6.24-rc5-mm1/mm/mremap.c
===================================================================
--- linux-2.6.24-rc5-mm1.orig/mm/mremap.c
+++ linux-2.6.24-rc5-mm1/mm/mremap.c
@@ -83,7 +83,7 @@ static void move_ptes(struct vm_area_str
 		 * and we propagate stale pages into the dst afterward.
 		 */
 		mapping = vma->vm_file->f_mapping;
-		spin_lock(&mapping->i_mmap_lock);
+		read_lock(&mapping->i_mmap_lock);
 		if (new_vma->vm_truncate_count &&
 		    new_vma->vm_truncate_count != vma->vm_truncate_count)
 			new_vma->vm_truncate_count = 0;
@@ -115,7 +115,7 @@ static void move_ptes(struct vm_area_str
 	pte_unmap_nested(new_pte - 1);
 	pte_unmap_unlock(old_pte - 1, old_ptl);
 	if (mapping)
-		spin_unlock(&mapping->i_mmap_lock);
+		read_unlock(&mapping->i_mmap_lock);
 }
 
 #define LATENCY_LIMIT	(64 * PAGE_SIZE)
Index: linux-2.6.24-rc5-mm1/mm/rmap.c
===================================================================
--- linux-2.6.24-rc5-mm1.orig/mm/rmap.c
+++ linux-2.6.24-rc5-mm1/mm/rmap.c
@@ -368,7 +368,7 @@ static int page_referenced_file(struct p
 	 */
 	BUG_ON(!PageLocked(page));
 
-	spin_lock(&mapping->i_mmap_lock);
+	read_lock(&mapping->i_mmap_lock);
 
 	/*
 	 * i_mmap_lock does not stabilize mapcount at all, but mapcount
@@ -394,7 +394,7 @@ static int page_referenced_file(struct p
 			break;
 	}
 
-	spin_unlock(&mapping->i_mmap_lock);
+	read_unlock(&mapping->i_mmap_lock);
 	return referenced;
 }
 
@@ -474,12 +474,12 @@ static int page_mkclean_file(struct addr
 
 	BUG_ON(PageAnon(page));
 
-	spin_lock(&mapping->i_mmap_lock);
+	read_lock(&mapping->i_mmap_lock);
 	vma_prio_tree_foreach(vma, &iter, &mapping->i_mmap, pgoff, pgoff) {
 		if (vma->vm_flags & VM_SHARED)
 			ret += page_mkclean_one(page, vma);
 	}
-	spin_unlock(&mapping->i_mmap_lock);
+	read_unlock(&mapping->i_mmap_lock);
 	return ret;
 }
 
@@ -909,7 +909,7 @@ static int try_to_unmap_file(struct page
 	unsigned long max_nl_size = 0;
 	unsigned int mapcount;
 
-	spin_lock(&mapping->i_mmap_lock);
+	read_lock(&mapping->i_mmap_lock);
 	vma_prio_tree_foreach(vma, &iter, &mapping->i_mmap, pgoff, pgoff) {
 		ret = try_to_unmap_one(page, vma, migration);
 		if (ret == SWAP_FAIL || !page_mapped(page))
@@ -946,7 +946,7 @@ static int try_to_unmap_file(struct page
 	mapcount = page_mapcount(page);
 	if (!mapcount)
 		goto out;
-	cond_resched_lock(&mapping->i_mmap_lock);
+	cond_resched_rwlock(&mapping->i_mmap_lock, 0);
 
 	max_nl_size = (max_nl_size + CLUSTER_SIZE - 1) & CLUSTER_MASK;
 	if (max_nl_cursor == 0)
@@ -968,7 +968,7 @@ static int try_to_unmap_file(struct page
 			}
 			vma->vm_private_data = (void *) max_nl_cursor;
 		}
-		cond_resched_lock(&mapping->i_mmap_lock);
+		cond_resched_rwlock(&mapping->i_mmap_lock, 0);
 		max_nl_cursor += CLUSTER_SIZE;
 	} while (max_nl_cursor <= max_nl_size);
 
@@ -980,7 +980,7 @@ static int try_to_unmap_file(struct page
 	list_for_each_entry(vma, &mapping->i_mmap_nonlinear, shared.vm_set.list)
 		vma->vm_private_data = NULL;
 out:
-	spin_unlock(&mapping->i_mmap_lock);
+	read_unlock(&mapping->i_mmap_lock);
 	return ret;
 }
 
Index: linux-2.6.24-rc5-mm1/include/linux/sched.h
===================================================================
--- linux-2.6.24-rc5-mm1.orig/include/linux/sched.h
+++ linux-2.6.24-rc5-mm1/include/linux/sched.h
@@ -1889,12 +1889,23 @@ static inline int need_resched(void)
  * cond_resched() and cond_resched_lock(): latency reduction via
  * explicit rescheduling in places that are safe. The return
  * value indicates whether a reschedule was done in fact.
- * cond_resched_lock() will drop the spinlock before scheduling,
- * cond_resched_softirq() will enable bhs before scheduling.
+ * cond_resched_softirq() will enable bhs before scheduling,
+ * cond_resched_*lock() will drop the *lock before scheduling.
  */
 extern int cond_resched(void);
-extern int cond_resched_lock(spinlock_t * lock);
 extern int cond_resched_softirq(void);
+extern int __cond_resched_lock(void * lock, int lock_type);
+
+#define COND_RESCHED_SPIN  2
+static inline int cond_resched_lock(spinlock_t * lock)
+{
+	return __cond_resched_lock(lock, COND_RESCHED_SPIN);
+}
+
+static inline int cond_resched_rwlock(rwlock_t * lock, int write_lock)
+{
+	return __cond_resched_lock(lock, !!write_lock);
+}
 
 /*
  * Does a critical section need to be broken due to another
Index: linux-2.6.24-rc5-mm1/kernel/sched.c
===================================================================
--- linux-2.6.24-rc5-mm1.orig/kernel/sched.c
+++ linux-2.6.24-rc5-mm1/kernel/sched.c
@@ -4686,34 +4686,78 @@ int __sched cond_resched(void)
 EXPORT_SYMBOL(cond_resched);
 
 /*
- * cond_resched_lock() - if a reschedule is pending, drop the given lock,
+ * helper functions for __cond_resched_lock()
+ */
+static int __need_lockbreak(void *lock, int type)
+{
+	if (likely(type == COND_RESCHED_SPIN))
+		return need_lockbreak((spinlock_t *)lock);
+	else
+		return need_lockbreak((rwlock_t *)lock);
+}
+
+static void __reacquire_lock(void *lock, int type)
+{
+	if (likely(type == COND_RESCHED_SPIN))
+		spin_lock((spinlock_t *)lock);
+	else if (type)
+		write_unlock((rwlock_t *)lock);
+	else
+		read_unlock((rwlock_t *)lock);
+}
+
+static void __drop_lock(void *lock, int type)
+{
+	if (likely(type == COND_RESCHED_SPIN))
+		spin_unlock((spinlock_t *)lock);
+	else if (type)
+		write_unlock((rwlock_t *)lock);
+	else
+		read_unlock((rwlock_t *)lock);
+}
+
+static void __release_lock(void *lock, int type)
+{
+	if (likely(type == COND_RESCHED_SPIN))
+		spin_release(&((spinlock_t *)lock)->dep_map, 1, _RET_IP_);
+	else
+		rwlock_release(&((rwlock_t *)lock)->dep_map, 1, _RET_IP_);
+}
+
+/*
+ * __cond_resched_lock() - if a reschedule is pending, drop the given lock,
  * call schedule, and on return reacquire the lock.
  *
+ * Lock type:
+ *  0 = rwlock held for read
+ *  1 = rwlock held for write
+ *  2 = COND_RESCHED_SPIN = spinlock
+ *
  * This works OK both with and without CONFIG_PREEMPT. We do strange low-level
  * operations here to prevent schedule() from being called twice (once via
- * spin_unlock(), once by hand).
+ * *_unlock(), once by hand).
  */
-int cond_resched_lock(spinlock_t *lock)
+int __cond_resched_lock(void *lock, int type)
 {
 	int ret = 0;
 
-	if (need_lockbreak(lock)) {
-		spin_unlock(lock);
+	if (__need_lockbreak(lock, type)) {
+		__drop_lock(lock, type);
 		cpu_relax();
 		ret = 1;
-		spin_lock(lock);
+		__reacquire_lock(lock, type);
 	}
 	if (need_resched() && system_state == SYSTEM_RUNNING) {
-		spin_release(&lock->dep_map, 1, _THIS_IP_);
-		_raw_spin_unlock(lock);
+		__release_lock(lock, type);
+		__drop_lock(lock, type);
 		preempt_enable_no_resched();
 		__cond_resched();
 		ret = 1;
-		spin_lock(lock);
+		__reacquire_lock(lock, type);
 	}
 	return ret;
 }
-EXPORT_SYMBOL(cond_resched_lock);
+EXPORT_SYMBOL(__cond_resched_lock);
 
 int __sched cond_resched_softirq(void)
 {
Index: linux-2.6.24-rc5-mm1/arch/x86/mm/hugetlbpage.c
===================================================================
--- linux-2.6.24-rc5-mm1.orig/arch/x86/mm/hugetlbpage.c
+++ linux-2.6.24-rc5-mm1/arch/x86/mm/hugetlbpage.c
@@ -68,7 +68,7 @@ static void huge_pmd_share(struct mm_str
 	if (!vma_shareable(vma, addr))
 		return;
 
-	spin_lock(&mapping->i_mmap_lock);
+	read_lock(&mapping->i_mmap_lock);
 	vma_prio_tree_foreach(svma, &iter, &mapping->i_mmap, idx, idx) {
 		if (svma == vma)
 			continue;
@@ -93,7 +93,7 @@ static void huge_pmd_share(struct mm_str
 		put_page(virt_to_page(spte));
 	spin_unlock(&mm->page_table_lock);
 out:
-	spin_unlock(&mapping->i_mmap_lock);
+	read_unlock(&mapping->i_mmap_lock);
 }
 
 /*

-- 
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