[PATCH v2] mm/filemap: fix a data race in filemap_fault()

[Qian Cai <cai@lca.pw>]

struct file_ra_state ra.mmap_miss could be accessed concurrently during
page faults as noticed by KCSAN,

 BUG: KCSAN: data-race in filemap_fault / filemap_map_pages

 write to 0xffff9b1700a2c1b4 of 4 bytes by task 3292 on cpu 30:
  filemap_fault+0x920/0xfc0
  do_sync_mmap_readahead at mm/filemap.c:2384
  (inlined by) filemap_fault at mm/filemap.c:2486
  __xfs_filemap_fault+0x112/0x3e0 [xfs]
  xfs_filemap_fault+0x74/0x90 [xfs]
  __do_fault+0x9e/0x220
  do_fault+0x4a0/0x920
  __handle_mm_fault+0xc69/0xd00
  handle_mm_fault+0xfc/0x2f0
  do_page_fault+0x263/0x6f9
  page_fault+0x34/0x40

 read to 0xffff9b1700a2c1b4 of 4 bytes by task 3313 on cpu 32:
  filemap_map_pages+0xc2e/0xd80
  filemap_map_pages at mm/filemap.c:2625
  do_fault+0x3da/0x920
  __handle_mm_fault+0xc69/0xd00
  handle_mm_fault+0xfc/0x2f0
  do_page_fault+0x263/0x6f9
  page_fault+0x34/0x40

 Reported by Kernel Concurrency Sanitizer on:
 CPU: 32 PID: 3313 Comm: systemd-udevd Tainted: G        W    L 5.5.0-next-20200210+ #1
 Hardware name: HPE ProLiant DL385 Gen10/ProLiant DL385 Gen10, BIOS A40 07/10/2019

ra.mmap_miss is used to contribute the readahead decisions, a data race
could be undesirable. Both the read and write is only under
non-exclusive mmap_sem, two concurrent writers could even overflow the
counter. Fixing the underflow by writing to a local variable before
committing a final store to ra.mmap_miss given a small inaccuracy of the
counter should be acceptable.

Suggested-by: Kirill A. Shutemov <kirill@shutemov.name>
Signed-off-by: Qian Cai <cai@lca.pw>
---

v2: fix the underflow issue pointed out by Matthew.

 mm/filemap.c | 20 +++++++++++++-------
 1 file changed, 13 insertions(+), 7 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index 1784478270e1..2e298db2e80f 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2365,6 +2365,7 @@ static struct file *do_sync_mmap_readahead(struct vm_fault *vmf)
 	struct address_space *mapping = file->f_mapping;
 	struct file *fpin = NULL;
 	pgoff_t offset = vmf->pgoff;
+	unsigned int mmap_miss;
 
 	/* If we don't want any read-ahead, don't bother */
 	if (vmf->vma->vm_flags & VM_RAND_READ)
@@ -2380,14 +2381,15 @@ static struct file *do_sync_mmap_readahead(struct vm_fault *vmf)
 	}
 
 	/* Avoid banging the cache line if not needed */
-	if (ra->mmap_miss < MMAP_LOTSAMISS * 10)
-		ra->mmap_miss++;
+	mmap_miss = READ_ONCE(ra->mmap_miss);
+	if (mmap_miss < MMAP_LOTSAMISS * 10)
+		WRITE_ONCE(ra->mmap_miss, ++mmap_miss);
 
 	/*
 	 * Do we miss much more than hit in this file? If so,
 	 * stop bothering with read-ahead. It will only hurt.
 	 */
-	if (ra->mmap_miss > MMAP_LOTSAMISS)
+	if (mmap_miss > MMAP_LOTSAMISS)
 		return fpin;
 
 	/*
@@ -2413,13 +2415,15 @@ static struct file *do_async_mmap_readahead(struct vm_fault *vmf,
 	struct file_ra_state *ra = &file->f_ra;
 	struct address_space *mapping = file->f_mapping;
 	struct file *fpin = NULL;
+	unsigned int mmap_miss;
 	pgoff_t offset = vmf->pgoff;
 
 	/* If we don't want any read-ahead, don't bother */
 	if (vmf->vma->vm_flags & VM_RAND_READ)
 		return fpin;
-	if (ra->mmap_miss > 0)
-		ra->mmap_miss--;
+	mmap_miss = READ_ONCE(ra->mmap_miss);
+	if (mmap_miss)
+		WRITE_ONCE(ra->mmap_miss, --mmap_miss);
 	if (PageReadahead(page)) {
 		fpin = maybe_unlock_mmap_for_io(vmf, fpin);
 		page_cache_async_readahead(mapping, ra, file,
@@ -2586,6 +2590,7 @@ void filemap_map_pages(struct vm_fault *vmf,
 	unsigned long max_idx;
 	XA_STATE(xas, &mapping->i_pages, start_pgoff);
 	struct page *page;
+	unsigned int mmap_miss = READ_ONCE(file->f_ra.mmap_miss);
 
 	rcu_read_lock();
 	xas_for_each(&xas, page, end_pgoff) {
@@ -2622,8 +2627,8 @@ void filemap_map_pages(struct vm_fault *vmf,
 		if (page->index >= max_idx)
 			goto unlock;
 
-		if (file->f_ra.mmap_miss > 0)
-			file->f_ra.mmap_miss--;
+		if (mmap_miss > 0)
+			mmap_miss--;
 
 		vmf->address += (xas.xa_index - last_pgoff) << PAGE_SHIFT;
 		if (vmf->pte)
@@ -2643,6 +2648,7 @@ void filemap_map_pages(struct vm_fault *vmf,
 			break;
 	}
 	rcu_read_unlock();
+	WRITE_ONCE(file->f_ra.mmap_miss, mmap_miss);
 }
 EXPORT_SYMBOL(filemap_map_pages);
 
-- 
2.21.0 (Apple Git-122.2)