Re: [PATCH v5] ext4: fix direct I/O read error for kernel stable rc4.4

On Thu, Aug 06, 2020 at 05:38:11PM +0800, Jiang Ying wrote:

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. It will cause the Applications to fail to read. For example, when the search service downloads a new full index file, the search engine is loading the previous index file and is processing the search request, it can not use buffer io that may squeeze the previous index file in use from pagecache, so the serch service must use direct I/O read.

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") Reviewed-by: Jan Kara jack@suse.cz Reviewed-by: Wang Long wanglong19@meituan.com Signed-off-by: Jiang Ying jiangying8582@126.com

Changes since V4: Fix build error on kernel stable rc 4.4. This patch only for kernel 4.4.

What about for the 4.9.y tree, will this work there too?

thanks,

greg k-h