linux-ext4 - Re: [PATCH] libfs: Fix DIO mode aligment

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Date:   Tue, 8 Dec 2020 11:33:27 +0300
From:   Благодаренко Артём 
        <artem.blagodarenko@...il.com>
To:     "Lyashkov, Alexey" <alexey.lyashkov@....com>
Cc:     "Theodore Y. Ts'o" <tytso@....edu>,
        "linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
        "adilger.kernel@...ger.ca" <adilger.kernel@...ger.ca>,
        Theodore Tso <tytso@...gle.com>
Subject: Re: [PATCH] libfs: Fix DIO mode aligment

Hello Theodore,

Is this case important enough to fix the code?

Thanks.
Best regards,
Artem Blagodarenko.


> On 19 Nov 2020, at 15:26, Lyashkov, Alexey <alexey.lyashkov@....com> wrote:
> 
> Tso,
> 
> This situation hit with modern hdd with 4k block size and e2image changed to use DIRECT IO instead of buffered.
> e2fsprogs tries to read a super lock on offset 1k and it caused to set FS block size to 1k and second block reading.
> (many other places exist, but it simplest).
>>> 
>        if (superblock) {
>                if (!block_size) {
>                        retval = EXT2_ET_INVALID_ARGUMENT;
>                        goto cleanup;
>                }
>                io_channel_set_blksize(fs->io, block_size);
>                group_block = superblock;
>                fs->orig_super = 0;
>        } else {
>                io_channel_set_blksize(fs->io, SUPERBLOCK_OFFSET); <<<<< this is problem
>                superblock = 1;
>                group_block = 0;
>                retval = ext2fs_get_mem(SUPERBLOCK_SIZE, &fs->orig_super);
>                if (retval)
>                        goto cleanup;
>        }
>        retval = io_channel_read_blk(fs->io, superblock, -SUPERBLOCK_SIZE,
>                                     fs->super);
>>> 
> It caused errors like
> # e2image -Q /dev/md65 /tmp/node05_image_out
> e2image 1.45.6.cr1 (14-Aug-2020)
> e2image: Attempt to read block from filesystem resulted in short read while trying to open /dev/md65
> Couldn’t find valid filesystem superblock.
> 
> It looks like I don't first person to found a bug, as someone was add 
> 
> Alex
> 
> On 17/11/2020, 22:19, "Theodore Y. Ts'o" <tytso@....edu> wrote:
> 
>    On Tue, Nov 17, 2020 at 06:30:11PM +0300, Благодаренко Артём wrote:
>> Hello,
>> 
>> Any thoughts about this change? Thanks.
> 
>    I'm trying to think of situations where this could actually trigger in
>    real life.  The only one I can think of is if a file system with a 1k
>    block file system is located on an an Advanced FormatDrive with a 4k
>    sector size.
> 
>    What was the use case where this was actually an issue?
> 
>         	     	      	    	     - Ted
>