fs: allow short direct-io reads to be completed via buffered IO

This is similar to what already happens in the write case.  If we have a short
read while doing O_DIRECT, instead of just returning, fallthrough and try to
read the rest via buffered IO.  BTRFS needs this because if we encounter a
compressed or inline extent during DIO, we need to fallback on buffered.  If the
extent is compressed we need to read the entire thing into memory and
de-compress it into the users pages.  I have tested this with fsx and everything
works great.  Thanks,

Signed-off-by: Josef Bacik <josef@redhat.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>

1 files changed, 31 insertions, 5 deletions

diff --git a/mm/filemap.c b/mm/filemap.c
index 140ebda9640f..829ac9cdbd70 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1263,7 +1263,7 @@ generic_file_aio_read(struct kiocb *iocb, const struct iovec *iov,
 {
         struct file *filp = iocb->ki_filp;
         ssize_t retval;
-        unsigned long seg;
+        unsigned long seg = 0;
         size_t count;
         loff_t *ppos = &iocb->ki_pos;
 
@@ -1290,21 +1290,47 @@ generic_file_aio_read(struct kiocb *iocb, const struct iovec *iov,
                                 retval = mapping->a_ops->direct_IO(READ, iocb,
                                                         iov, pos, nr_segs);
                         }
-                        if (retval > 0)
+                        if (retval > 0) {
                                 *ppos = pos + retval;
-                        if (retval) {
+                                count -= retval;
+                        }
+
+                        /*
+                         * Btrfs can have a short DIO read if we encounter
+                         * compressed extents, so if there was an error, or if
+                         * we've already read everything we wanted to, or if
+                         * there was a short read because we hit EOF, go ahead
+                         * and return.  Otherwise fallthrough to buffered io for
+                         * the rest of the read.
+                         */
+                        if (retval < 0 || !count || *ppos >= size) {
                                 file_accessed(filp);
                                 goto out;
                         }
                 }
         }
 
+        count = retval;
         for (seg = 0; seg < nr_segs; seg++) {
                 read_descriptor_t desc;
+                loff_t offset = 0;
+
+                /*
+                 * If we did a short DIO read we need to skip the section of the
+                 * iov that we've already read data into.
+                 */
+                if (count) {
+                        if (count > iov[seg].iov_len) {
+                                count -= iov[seg].iov_len;
+                                continue;
+                        }
+                        offset = count;
+                        count = 0;
+                }
 
                 desc.written = 0;
-                desc.arg.buf = iov[seg].iov_base;
-                desc.count = iov[seg].iov_len;
+                desc.arg.buf = iov[seg].iov_base + offset;
+                desc.count = iov[seg].iov_len - offset;
                 if (desc.count == 0)
                         continue;
                 desc.error = 0;