linux-ext4 - Re: [PATCH 2/2] ext4: avoid resizing to a partial cluster size

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Date:   Fri, 15 Jul 2022 07:48:28 -0400
From:   "Theodore Ts'o" <tytso@....edu>
To:     Jan Kara <jack@...e.cz>
Cc:     "Kiselev, Oleg" <okiselev@...zon.com>,
        "linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH 2/2] ext4: avoid resizing to a partial cluster size

On Fri, Jul 15, 2022 at 11:35:18AM +0200, Jan Kara wrote:
> > available for the filesystem having an “odd” size.  Our preference is for
> > the utilities to silently fix the fs size down to the nearest “safe” size
> > rather than get sporadic errors.   I had submitted a patch for resize2fs
> > that rounds the fs target size down to the nearest cluster boundary.  In
> > principle it’s similar to the size-rounding that is done now for 4K
> > blocks.   Using updated e2fsprogs isn’t mandatory for using ext4 in the
> > newer kernels, so making the kernel safe(r) for bigalloc resizes seems
> > like a good idea.
> 
> I see. Honestly, doing automatic "fixups" of passed arguments to syscalls /
> ioctls has bitten us more than once in the past. That's why I'm cautious
> about that. It seems convenient initially but then when contraints change
> (e.g. you'd want to be rounding to a different number) you suddently find
> you have no way to extend the API without breaking some userspace. That's
> why I prefer to put these "rounding convenience" functions into userspace.
> 
> That being said I don't feel too strongly about this particular case so I
> guess I'll defer the final decision about the policy to Ted.

In this particular case, a file system whose size is not a multiple of
cluster size is never going to be valid, so having the resize ioctl
round down the requested size to largest valid size seems to be a safe
(and useful) thing to do.

						- Ted