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

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Date:   Thu, 19 Nov 2020 12:26:34 +0000
From:   "Lyashkov, Alexey" <alexey.lyashkov@....com>
To:     "Theodore Y. Ts'o" <tytso@....edu>,
        Благодаренко Артём 
        <artem.blagodarenko@...il.com>
CC:     "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

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