Re: [PATCH] add support for larger files in minix filesystem

[Joshua Hudson]

No response. I suppose I must be doing something wrong ...

On 11/1/15, Joshua Hudson <joshudson@xxxxxxxxx> wrote:
> From: Joshua Hudson <joshudson@xxxxxxxxx>
>
> The Minix v3 filesystem and kernel driver have no actual dependency on
> files
> being less than 2GB in size; however the kernel does not allow creating a
> file of 2GB or larger on a Minix v3 filesystem. I was able to remove the
> pseudo-
> dependency easily by changing one line of code (filesystems need to tell
> VFS
> how big of files they allow).
>
> This code won't do anything useful unless the filesystem superblock is
> patched at offset 1040 to a larger value. The largest safe value is
> 0,252,255,255.
>
> Signed-off-by Joshua Hudson <joshudson@xxxxxxxxx>
> ---
> I'm not on the list anymore so if you don't CC me I won't see it.
>
> Background: I've been playing around with work on a new embedded device.
> where the hardware simply isn't powerful enough to sustain ext2 filesystem.
> This resulted in me choosing the Minix filesystem as the operational
> filesystem
> for reasons including find-next-free-block is sixteen times faster than
> FAT.
> I discovered the need to collect about 3gb of sensor data in one tape-like
> run.
> Naturally, this leads to creating a 3gb file; but Linux Kernel can't
> handle that.
>
> Please note I'm pretty far down branches and I don't think a stock kernel
> will run on my system (non-free hardware drivers). Thankfully the Minix fs
> code changes so slowly this should apply all the same. I have tested that
> the patch applies and builds a kernel that fixes the problem.
>
> The fact that the superblock has to be edited for the patch to do anything
> interesting (else it replaces s_maxbytes with the same value) makes this
> quite safe to apply even in the presence of a bug I don't know about.
>
> --- linux-4.2.3/fs/minix/inode.c.orig   2015-11-01 17:13:57.227148723 -0800
> +++ linux-4.2.3/fs/minix/inode.c        2015-11-01 17:21:49.785390753 -0800
> @@ -232,6 +232,7 @@
>                 s->s_max_links = MINIX2_LINK_MAX;
>         } else
>                 goto out_no_fs;
> +       s->s_maxbytes = (unsigned)sbi->s_max_size; /* s_max_size
> cannot be >=4GB and MAX_LFS_SIZE must be >= 4GB */
>
>         /*
>          * Allocate the buffer map to keep the superblock small.
>
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