fs: return if direct I/O will trigger writeback

Find out if the I/O will trigger a wait due to writeback. If yes,
return -EAGAIN.

Return -EINVAL for buffered AIO: there are multiple causes of
delay such as page locks, dirty throttling logic, page loading
from disk etc. which cannot be taken care of.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 25 insertions, 7 deletions

diff --git a/mm/filemap.c b/mm/filemap.c
index 9b39a2390b9e..742034e56100 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2070,10 +2070,17 @@ generic_file_read_iter(struct kiocb *iocb, struct iov_iter *iter)
                 loff_t size;
 
                 size = i_size_read(inode);
-                retval = filemap_write_and_wait_range(mapping, iocb->ki_pos,
-                                        iocb->ki_pos + count - 1);
-                if (retval < 0)
-                        goto out;
+                if (iocb->ki_flags & IOCB_NOWAIT) {
+                        if (filemap_range_has_page(mapping, iocb->ki_pos,
+                                                   iocb->ki_pos + count - 1))
+                                return -EAGAIN;
+                } else {
+                        retval = filemap_write_and_wait_range(mapping,
+                                                iocb->ki_pos,
+                                                iocb->ki_pos + count - 1);
+                        if (retval < 0)
+                                goto out;
+                }
 
                 file_accessed(file);
 
@@ -2674,6 +2681,9 @@ inline ssize_t generic_write_checks(struct kiocb *iocb, struct iov_iter *from)
 
         pos = iocb->ki_pos;
 
+        if ((iocb->ki_flags & IOCB_NOWAIT) && !(iocb->ki_flags & IOCB_DIRECT))
+                return -EINVAL;
+
         if (limit != RLIM_INFINITY) {
                 if (iocb->ki_pos >= limit) {
                         send_sig(SIGXFSZ, current, 0);
@@ -2742,9 +2752,17 @@ generic_file_direct_write(struct kiocb *iocb, struct iov_iter *from)
         write_len = iov_iter_count(from);
         end = (pos + write_len - 1) >> PAGE_SHIFT;
 
-        written = filemap_write_and_wait_range(mapping, pos, pos + write_len - 1);
-        if (written)
-                goto out;
+        if (iocb->ki_flags & IOCB_NOWAIT) {
+                /* If there are pages to writeback, return */
+                if (filemap_range_has_page(inode->i_mapping, pos,
+                                           pos + iov_iter_count(from)))
+                        return -EAGAIN;
+        } else {
+                written = filemap_write_and_wait_range(mapping, pos,
+                                                        pos + write_len - 1);
+                if (written)
+                        goto out;
+        }
 
         /*
          * After a write we want buffered reads to be sure to go to disk to get