linux-ext4 - Re: [PATCH] fcntl: Add 32bit filesystem mode

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Date:   Tue, 21 Apr 2020 14:02:39 +0100
From:   Peter Maydell <peter.maydell@...aro.org>
To:     Andreas Dilger <adilger@...ger.ca>
Cc:     Florian Weimer <fw@...eb.enyo.de>,
        Linus Walleij <linus.walleij@...aro.org>,
        "Theodore Ts'o" <tytso@....edu>,
        Ext4 Developers List <linux-ext4@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        QEMU Developers <qemu-devel@...gnu.org>,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH] fcntl: Add 32bit filesystem mode

On Tue, 21 Apr 2020 at 00:51, Andreas Dilger <adilger@...ger.ca> wrote:
> Another question I had here is whether the filesystem needs to provide
> 32-bit values for other syscalls, such as stat() and statfs()?  For
> ext4, stat() is not going to return a 64-bit inode number, but other
> filesystems might (e.g. Lustre has a mode to do this).  Similarly,
> should statfs() scale up f_bsize until it can return a 32-bit f_blocks
> value?  We also had to do this ages ago for Lustre when 32-bit clients
> couldn't handle > 16TB filesystems, but that is a single disk today.
>
> Should that be added into F_SET_FILE_32BIT_FS also?

Interesting question. The directory-offset is the thing that's
got peoples' attention because it's what has actually been hit
in real-world situations, but other syscalls have the same
potential problem too. The closest I can think of to a 'general
rule' (in terms of what QEMU would like) would be "behave the
same way you would for a compat32 syscall if you had one, or
how you would behave on an actual 32-bit host".

thanks
-- PMM