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Subject[PATCH v3] ext4: fix direct I/O read error
Date
This patch is used to fix ext4 direct I/O read error when
the read size is not aligned with block size.

Then, I will use a test to explain the error.

(1) Make a file that is not aligned with block size:
$dd if=/dev/zero of=./test.jar bs=1000 count=3

(2) I wrote a source file named "direct_io_read_file.c" as following:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/file.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <string.h>
#define BUF_SIZE 1024

int main()
{
int fd;
int ret;

unsigned char *buf;
ret = posix_memalign((void **)&buf, 512, BUF_SIZE);
if (ret) {
perror("posix_memalign failed");
exit(1);
}
fd = open("./test.jar", O_RDONLY | O_DIRECT, 0755);
if (fd < 0){
perror("open ./test.jar failed");
exit(1);
}

do {
ret = read(fd, buf, BUF_SIZE);
printf("ret=%d\n",ret);
if (ret < 0) {
perror("write test.jar failed");
}
} while (ret > 0);

free(buf);
close(fd);
}

(3) Compile the source file:
$gcc direct_io_read_file.c -D_GNU_SOURCE

(4) Run the test program:
$./a.out

The result is as following:
ret=1024
ret=1024
ret=952
ret=-1
write test.jar failed: Invalid argument.

I have tested this program on XFS filesystem, XFS does not have
this problem, because XFS use iomap_dio_rw() to do direct I/O
read. And the comparing between read offset and file size is done
in iomap_dio_rw(), the code is as following:

if (pos < size) {
retval = filemap_write_and_wait_range(mapping, pos,
pos + iov_length(iov, nr_segs) - 1);

if (!retval) {
retval = mapping->a_ops->direct_IO(READ, iocb,
iov, pos, nr_segs);
}
...
}

...only when "pos < size", direct I/O can be done, or 0 will be return.

I have tested the fix patch on Ext4, it is up to the mustard of
EINVAL in man2(read) as following:
#include <unistd.h>
ssize_t read(int fd, void *buf, size_t count);

EINVAL
fd is attached to an object which is unsuitable for reading;
or the file was opened with the O_DIRECT flag, and either the
address specified in buf, the value specified in count, or the
current file offset is not suitably aligned.

So I think this patch can be applied to fix ext4 direct I/O error.

However Ext4 introduces direct I/O read using iomap infrastructure
on kernel 5.5, the patch is commit <b1b4705d54ab>
("ext4: introduce direct I/O read using iomap infrastructure"),
then Ext4 will be the same as XFS, they all use iomap_dio_rw() to do direct
I/O read. So this problem does not exist on kernel 5.5 for Ext4.

From above description, we can see this problem exists on all the kernel
versions between kernel 3.14 and kernel 5.4. Please apply this patch
on these kernel versions, or please use the method on kernel 5.5 to fix
this problem.

Fixes: 9fe55eea7e4b ("Fix race when checking i_size on direct i/o read")
Co-developed-by: Wang Long <wanglong19@meituan.com>
Signed-off-by: Wang Long <wanglong19@meituan.com>
Signed-off-by: Jiang Ying <jiangying8582@126.com>

Changes since V2:
Optimize the description of the commit message and make a variation for
the patch, e.g. with:

Before:
loff_t size;
size = i_size_read(inode);
After:
loff_t size = i_size_read(inode);

Changes since V1:
Signed-off use real name and add "Fixes:" flag

---
fs/ext4/inode.c | 5 +++++
1 file changed, 5 insertions(+)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 516faa2..a66b0ac 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -3821,6 +3821,11 @@ static ssize_t ext4_direct_IO_read(struct kiocb *iocb, struct iov_iter *iter)
struct inode *inode = mapping->host;
size_t count = iov_iter_count(iter);
ssize_t ret;
+ loff_t offset = iocb->ki_pos;
+ loff_t size = i_size_read(inode);
+
+ if (offset >= size)
+ return 0;

/*
* Shared inode_lock is enough for us - it protects against concurrent
--
1.8.3.1
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