linux-kernel - Re: Direct IO for fat

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Date:	Fri, 09 Feb 2007 00:44:06 +0900
From:	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
To:	Jan Kara <jack@...e.cz>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Direct IO for fat

Jan Kara <jack@...e.cz> writes:

>   Hello,

Hello,

>   I've noticed that extending a file using direct IO fails for FAT with
> EINVAL. It's basically because of the following code in fat_direct_IO():
>
> if (rw == WRITE) {
>         /*
>          * FIXME: blockdev_direct_IO() doesn't use
>          * ->prepare_write(),
>          * so we need to update the ->mmu_private to block
>          * boundary.
>          *
>          * But we must fill the remaining area or hole by nul for
>          * updating ->mmu_private.
>          */
>         loff_t size = offset + iov_length(iov, nr_segs);
>         if (MSDOS_I(inode)->mmu_private < size)
>                 return -EINVAL;
> }
>
>   But isn't this check bogus? blockdev_direct_IO writes only to space that
> is already allocated and stops as soon as it needs to extend the file
> (further extension is then handled by buffered writes). So it should
> already do what it needed for FAT. Thanks for an answer in advance.

FAT has to fill the hole completely, but DIO doesn't seems to do.

e.g.
        fd = open("file", O_WRONLY | O_CREAT | O_TRUNC);
        write(fd, buf, 512);
        lseek(fd, 10000, SEEK_SET);
        write(fd, buf, 512);

We need to allocate the blocks on 512 ~ 10000, and fill it with zero.
However, I think DIO doesn't fill it. If I'm missing something, please
let me know, I'll kill that check.

Thanks.
-- 
OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
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