Re: [PATCH] ubifs: Add support for zstd compression.

From: Richard Weinberger <richard@nod.at>
To: Emil Lenngren <emil.lenngren@gmail.com>
Cc: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>,
	linux-mtd <linux-mtd@lists.infradead.org>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	Michele Dionisio <michele.dionisio@gmail.com>
Subject: Re: [PATCH] ubifs: Add support for zstd compression.
Date: Fri, 7 Jun 2019 22:09:24 +0200 (CEST)	[thread overview]
Message-ID: <1644731533.84685.1559938164477.JavaMail.zimbra@nod.at> (raw)
In-Reply-To: <CAO1O6sdU=kAYS2sTKwiagxrbg+fMer9nvbwA9C4LoFMgH7e1dQ@mail.gmail.com>

Emil,

----- Ursprüngliche Mail -----
> In fs/ubifs/sb.c we have
> 
> static int get_default_compressor(struct ubifs_info *c)
> {
>    if (ubifs_compr_present(c, UBIFS_COMPR_LZO))
>        return UBIFS_COMPR_LZO;
> 
>    if (ubifs_compr_present(c, UBIFS_COMPR_ZLIB))
>        return UBIFS_COMPR_ZLIB;
> 
>    return UBIFS_COMPR_NONE;
> }
> 
> Maybe add an entry for zstd here as well?

Where would you add it? If we add it after UBIFS_COMPR_ZLIB,
it will effectively never get selected, unless someone builds a kernel
without lzo and zlib but zstd.
If we add it before UBIFS_COMPR_ZLIB, it will be used always and users
end up with unreadable files if they reboot to an older kernel.
Please note that we didn't raise the UBIFS format version for zstd.

So I'm not sure what is the best choice for the default filesystem.

Thanks,
//richard

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