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Subject[PATCH 1/4] fs: allow short direct-io reads to be completed via buffered IO V2
V1->V2: Check to see if our current ppos is >= i_size after a short DIO read,
just in case it was actually a short read and we need to just return.

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>
---
mm/filemap.c | 36 +++++++++++++++++++++++++++++++-----
1 files changed, 31 insertions(+), 5 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index 140ebda..829ac9c 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;
--
1.6.6.1


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