Date:   Sun, 28 Jun 2020 14:47:43 +0800 (CST)
From:   姜迎 <jiangying8582@....com>
To:     tytso@....edu
Cc:     adilger.kernel@...ger.ca, linux-ext4@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH] ext4: fix direct I/O read error

From: jiangying8582 <jiangying8582@....com>
Date: Wed, 24 Jun 2020 19:02:34 +0800
Subject: [PATCH] ext4: fix direct I/O read error

This patch is used to fix ext4 direct I/O read error when
the read size is not alignment with block size. Compare the
size between read offset with file size, if read offset is
greater than file size, then return 0.

Then, I will use a test to explain the error.
(1) Make the file that is not alignment wiht block size:
        $dd if=/dev/zero of=./test.jar bs=1000 count=3

(2) I wrote a test script named "direct_io_read_file.c" s 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) Compiling the script:
        $gcc direct_io_read_file.c -D_GNU_SOURCE

(4) Exec the script:
        $./a.out

    The result is as following:
        ret=1024
        ret=1024
        ret=952
        ret=-1
        write rts-segmenter-0.3.7.2.jar failed: Invalid argument

I have tested this script 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
is 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 my fix patch, 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 problem.

Why this problem can happen? I think
commit <9fe55eea7e4b> ("Fix race when checking i_size on direct i/o read")
caused.

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. Thanks.

Signed-off-by: jiangying8582 <jiangying8582@....com>
---
 fs/ext4/inode.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 516faa2..d514ff5 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -3821,6 +3821,12 @@ 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;
+
+       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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