Date:   Tue, 4 Aug 2020 14:50:35 +0200
From:   Jan Kara <jack@...e.cz>
To:     姜迎 <jiangying8582@....com>
Cc:     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

On Wed 01-07-20 23:25:26, 姜迎 wrote:
> Does anyone else have any comments on the PATCH v3？Suggestions are welcome.
> 
> Thanks！

The patch looks good to me FWIW. But as Jiang properly notes current
upstream doesn't need this at all so it's only for -stable kernel releases.
Since there was no report of this problem so far I'm not convinced this is
serious enough to warrant non-upstream patch in -stable but if this bug
indeed breaks some application, please add that info to the changelog and
send the patch to stable@...r.kernel.org for inclusion. In that case also
feel free to add:

Reviewed-by: Jan Kara <jack@...e.cz>

								Honza
> 
> 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
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR

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