mm/filemap.c: fix a data race in filemap_fault()

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 underflow the counter.  Fix
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.

Signed-off-by: Kirill A. Shutemov <kirill@shutemov.name>
Signed-off-by: Qian Cai <cai@lca.pw>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Tested-by: Qian Cai <cai@lca.pw>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Marco Elver <elver@google.com>
Link: http://lkml.kernel.org/r/20200211030134.1847-1-cai@lca.pw
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/filemap.c b/mm/filemap.c
index 653190943aa72cf27480153424bac5077cd3eaa6..1aaea26556cc7e4ab702af1e96262d67a0607cf0 100644 (file)
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2468,6 +2468,7 @@ 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)
@@ -2483,14 +2484,15 @@ 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;
 
        /*
@@ -2516,13 +2518,15 @@ 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 || !ra->ra_pages)
                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,
@@ -2688,6 +2692,7 @@ 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) {
@@ -2724,8 +2729,8 @@ 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)
@@ -2745,6 +2750,7 @@ next:
                        break;
        }
        rcu_read_unlock();
+       WRITE_ONCE(file->f_ra.mmap_miss, mmap_miss);
 }
 EXPORT_SYMBOL(filemap_map_pages);