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Date:   Wed, 1 Jul 2020 23:25:26 +0800
From:   姜迎 <jiangying8582@....com>
To:     Markus.Elfring@....de, tytso@....edu, adilger.kernel@...ger.ca,
        linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org,
        wanglong19@...tuan.com, heguanjun@...tuan.com
Subject: Re: [PATCH v3] ext4: fix direct I/O read error

Does anyone else have any comments on the PATCH v3?Suggestions are welcome.

Thanks!

发自我的iPhone

> 在 2020年6月29日,下午5:45,Jiang Ying <jiangying8582@....com> 写道:
> 
> 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@...tuan.com>
> Signed-off-by: Wang Long <wanglong19@...tuan.com>
> Signed-off-by: Jiang Ying <jiangying8582@....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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