Re: [PATCH] btrfs: zstd: add `zstd-fast` alias mount option for fast modes

[Qu Wenruo]

在 2025/3/31 20:36, Daniel Vacek 写道:
[...]
> > >                        btrfs_set_opt(ctx->mount_opt, COMPRESS);
> > >                        btrfs_clear_opt(ctx->mount_opt, NODATACOW);
> > >                        btrfs_clear_opt(ctx->mount_opt, NODATASUM);
> > > +             } else if (strncmp(param->string, "zstd-fast", 9) == 0) {
> > > +                     ctx->compress_type = BTRFS_COMPRESS_ZSTD;
> > > +                     ctx->compress_level =
> > > +                             -btrfs_compress_str2level(BTRFS_COMPRESS_ZSTD,
> > > +                                                       param->string + 9
> >
> > Can we just use some temporary variable to save the return value of
> > btrfs_compress_str2level()?
>
> I don't see any added value. Isn't `ctx->compress_level` temporary
> enough at this point? Note that the FS is not mounted yet so there's
> no other consumer of `ctx`.
>
> Or did you mean just for readability?

Doing all the same checks and flipping the sign of ctx->compress_level
is already cursed to me.

Isn't something like this easier to understand?

	level = btrfs_compress_str2level();
	if (level > 0)
		ctx->compress_level = -level;
	else
		ctx->compress_level = level;

> > );
> > > +                     if (ctx->compress_level > 0)
> > > +                             ctx->compress_level = -ctx->compress_level;
> >
> > This also means, if we pass something like "compress=zstd-fast:-9", it
> > will still set the level to the correct -9.
> >
> > Not some weird double negative, which is good.
> >
> > But I'm also wondering, should we even allow minus value for "zstd-fast".
>
> It was meant as a safety in a sense that `s/zstd:-/zstd-fast:-/` still
> works the same. Hence it defines that "fast level -3 <===> fast level
> 3" (which is still level -3 in internal zstd representation).
> So if you mounted `compress=zstd-fast:-9` it's understood you actually
> meant `compress=zstd-fast:9` in the same way as if you did
> `compress=zstd:-9`.
>
> I thought this was solid. Or would you rather bail out with -EINVAL?

I prefer to bail out with -EINVAL, but it's only my personal choice.

You'd better wait for feedbacks from other people.

Thanks,
Qu>
> > > +                     btrfs_set_opt(ctx->mount_opt, COMPRESS);
> > > +                     btrfs_clear_opt(ctx->mount_opt, NODATACOW);
> > > +                     btrfs_clear_opt(ctx->mount_opt, NODATASUM);
> > >                } else if (strncmp(param->string, "zstd", 4) == 0) {
> > >                        ctx->compress_type = BTRFS_COMPRESS_ZSTD;
> > >                        ctx->compress_level =
> >
> > Another thing is, if we want to prefer using zstd-fast:9 other than
> > zstd:-9, should we also change our compress handling in
> > btrfs_show_options() to show zstd-fast:9 instead?
>
> At this point it's not about preference but about compatibility. I
> actually tested changing this but as a side-effect it also had an
> influence on `btrfs.compression` xattr (our `compression` property)
> which is rather an incompatible on-disk format change. Hence I falled
> back keeping it simple. Showing `zstd:-9` is still a valid output.
>
> --nX
>
> > Thanks,
> > Qu