[PATCH] block: add filemap_invalidate_lock_killable()

[Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>]

syzbot is reporting hung task at blkdev_fallocate() [1], for it can take
minutes with mapping->invalidate_lock held. Since fallocate() has to accept
size > MAX_RW_COUNT bytes, we can't predict how long it will take. Thus,
mitigate this problem by using killable wait where possible.

  ----------
  #define _GNU_SOURCE
  #include <sys/types.h>
  #include <sys/stat.h>
  #include <fcntl.h>
  #include <unistd.h>

  int main(int argc, char *argv[])
  {
    fork();
    fallocate(open("/dev/nullb0", O_RDWR), 0x11ul, 0ul, 0x7ffffffffffffffful);
    return 0;
  }
  ----------

Link: https://syzkaller.appspot.com/bug?extid=39b75c02b8be0a061bfc [1]
Reported-by: syzbot <syzbot+39b75c02b8be0a061bfc@syzkaller.appspotmail.com>
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
---
 block/fops.c       | 4 +++-
 include/linux/fs.h | 5 +++++
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/block/fops.c b/block/fops.c
index 0da147edbd18..a87050db4670 100644
--- a/block/fops.c
+++ b/block/fops.c
@@ -622,7 +622,9 @@ static long blkdev_fallocate(struct file *file, int mode, loff_t start,
 	if ((start | len) & (bdev_logical_block_size(bdev) - 1))
 		return -EINVAL;
 
-	filemap_invalidate_lock(inode->i_mapping);
+	/* fallocate() might take minutes with lock held. */
+	if (filemap_invalidate_lock_killable(inode->i_mapping))
+		return -EINTR;
 
 	/* Invalidate the page cache, including dirty pages. */
 	error = truncate_bdev_range(bdev, file->f_mode, start, end);
diff --git a/include/linux/fs.h b/include/linux/fs.h
index bbf812ce89a8..27b3d36bb73c 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -828,6 +828,11 @@ static inline void filemap_invalidate_lock(struct address_space *mapping)
 	down_write(&mapping->invalidate_lock);
 }
 
+static inline int filemap_invalidate_lock_killable(struct address_space *mapping)
+{
+	return down_write_killable(&mapping->invalidate_lock);
+}
+
 static inline void filemap_invalidate_unlock(struct address_space *mapping)
 {
 	up_write(&mapping->invalidate_lock);
-- 
2.32.0