linux-kernel - Re: [PATCH] ovl: skip overlayfs superblocks at global sync

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Date:   Thu, 9 Apr 2020 14:28:55 +0300
From:   Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
To:     Amir Goldstein <amir73il@...il.com>
Cc:     linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Miklos Szeredi <miklos@...redi.hu>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        overlayfs <linux-unionfs@...r.kernel.org>,
        Dmitry Monakhov <dmtrmonakhov@...dex-team.ru>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        Theodore Tso <tytso@....edu>
Subject: Re: [PATCH] ovl: skip overlayfs superblocks at global sync

On 09/04/2020 13.23, Amir Goldstein wrote:
> On Thu, Apr 9, 2020 at 11:30 AM Konstantin Khlebnikov
> <khlebnikov@...dex-team.ru> wrote:
>>
>> Stacked filesystems like overlayfs has no own writeback, but they have to
>> forward syncfs() requests to backend for keeping data integrity.
>>
>> During global sync() each overlayfs instance calls method ->sync_fs()
>> for backend although it itself is in global list of superblocks too.
>> As a result one syscall sync() could write one superblock several times
>> and send multiple disk barriers.
>>
>> This patch adds flag SB_I_SKIP_SYNC into sb->sb_iflags to avoid that.
>>
>> Reported-by: Dmitry Monakhov <dmtrmonakhov@...dex-team.ru>
>> Signed-off-by: Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
>> ---
> 
> Seems reasonable.
> You may add:
> Reviewed-by: Amir Goldstein <amir73il@...il.com>
> 
> +CC: containers list

Thanks

> 
> This bring up old memories.
> I posted this way back to fix handling of emergency_remount() in the
> presence of loop mounted fs:
> https://lore.kernel.org/linux-ext4/CAA2m6vfatWKS1CQFpaRbii2AXiZFvQUjVvYhGxWTSpz+2rxDyg@mail.gmail.com/
> 
> But seems to me that emergency_sync() and sync(2) are equally broken
> for this use case.
> 
> I wonder if anyone cares enough about resilience of loop mounted fs to try
> and change the iterate_* functions to iterate supers/bdevs in reverse order...

Now I see reason behind "sync; sync; sync; reboot" =)

Order old -> new allows to not miss new items if list modifies.
Might be important for some users.

bdev iteration seems already reversed: inode_sb_list_add adds to the head

> 
> Thanks,
> Amir.
>