linux-kernel - Re: [PATCH v2] docs: filesystems: document the squashfs specific mount options

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Message-ID: <oonydtkv6anpjbugjvqkp4byqlf3aiwkwf4g7y5w5tthivxomh@b4w4jyceaepz>
Date:   Mon, 30 Oct 2023 18:09:45 +0200
From:   Ariel Miculas <amiculas@...co.com>
To:     Phillip Lougher <phillip@...ashfs.org.uk>
Cc:     linux-doc@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>, serge@...lyn.com,
        Jonathan Corbet <corbet@....net>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] docs: filesystems: document the squashfs specific
 mount options

On 23/10/30 03:40PM, Phillip Lougher wrote:
> On 30/10/2023 12:57, Ariel Miculas wrote:
> > On 23/10/29 08:33PM, Phillip Lougher wrote:
> > > On 29/10/2023 16:19, Ariel Miculas wrote:
> > > > When SQUASHFS_CHOICE_DECOMP_BY_MOUNT is set, the "threads" mount option
> > > > can be used to specify the decompression mode: single-threaded,
> > > > multi-threaded, percpu or the number of threads used for decompression.
> > > > When SQUASHFS_CHOICE_DECOMP_BY_MOUNT is not set and
> > > > SQUASHFS_DECOMP_MULTI is set, the "threads" option can also be used to
> > > > specify the number of threads used for decompression. This mount option
> > > > is only mentioned in fs/squashfs/Kconfig, which makes it difficult to
> > > > find.
> > > > 
> > > > Another mount option available is "errors", which can be configured to
> > > > panic the kernel when squashfs errors are encountered.
> > > > 
> > > > Add both these options to the squashfs documentation, making them more
> > > > noticeable.
> > > > 
> > > > Signed-off-by: Ariel Miculas <amiculas@...co.com>
> > > 
> > > Looks good to me.
> > > 
> > > Reviewed-by: Phillip Lougher <phillip@...ashfs.org.uk>
> > 
> > Unfortunately, it seems this is not quite correct either:
> > There is the config option SQUASHFS_MOUNT_DECOMP_THREADS:
> > ```
> > 	bool "Add the mount parameter 'threads=' for squashfs"
> > 	depends on SQUASHFS
> > 	depends on SQUASHFS_DECOMP_MULTI
> > 	default n
> > 	help
> > 	  Use threads= to set the decompression parallel mode and the number of threads.
> > 	  If SQUASHFS_CHOICE_DECOMP_BY_MOUNT=y
> > 	      threads=<single|multi|percpu|1|2|3|...>
> > 	  else
> > 	      threads=<2|3|...>
> > 	  The upper limit is num_online_cpus() * 2.
> > ```
> > that depends on SQUASHFS_DECOMP_MULTI.
> > So I think I should take my v1 patch and specify that the "threads="
> > mount option depends on SQUASHFS_MOUNT_DECOMP_THREADS. There's no need
> > to specify SQUASHFS_DECOMP_MULTI, because SQUASHFS_MOUNT_DECOMP_THREADS
> > already depends on it.
> 
> Sorry, you have to specify SQUASHFS_DECOMP_MULTI to be able to specify
> SQUASHFS_MOUNT_DECOMP_THREADS if SQUASHFS_DECOMP_BY_MOUNT is unselected.
Agree.
> 
> Just try it, do make menuconfig, ensure SQUASHFS_CHOICE_DECOMP_BY_MOUNT
> is unselected, select Single threaded decompression and you won't be
> able to specify SQUASHFS_MOUNT_DECOMP_THREADS.
True.
> 
> That was the point of my review. What bit don't you understand?
But SQUASHFS_DECOMP_MULTI is not enough, you need to specify
SQUASHFS_MOUNT_DECOMP_THREADS in order to use the "threads=" mount
option.
So instead of saying
```
If SQUASHFS_CHOICE_DECOMP_BY_MOUNT is **not** set and
SQUASHFS_DECOMP_MULTI is set:
```
wouldn't it be right to actually say:
```
If SQUASHFS_CHOICE_DECOMP_BY_MOUNT is **not** set and
SQUASHFS_MOUNT_DECOMP_THREADS is set:
```?

As you've mentioned, you could only set SQUASHFS_MOUNT_DECOMP_THREADS
when SQUASHFS_DECOMP_MULTI is selected. That happens in two cases:
* either SQUASHFS_CHOICE_DECOMP_BY_MOUNT is set, in which case it also
  selects SQUASHFS_MOUNT_DECOMP_THREADS
* either SQUASHFS_CHOICE_DECOMP_BY_MOUNT is not set,
  SQUASHFS_DECOMP_MULTI is set and SQUASHFS_MOUNT_DECOMP_THREADS is also
  set

So I wouldn't even mention SQUASHFS_DECOMP_MULTI in the documentation,
only SQUASHFS_MOUNT_DECOMP_THREADS, because the latter always depends on
the former. And the "threads=" mount option is only available when
SQUASHFS_MOUNT_DECOMP_THREADS is set (which is the configuration I've
missed in v1 and v2).

Regards,
Ariel
> 
> Phillip
> 
> > What do you think?
> > 
> > Regards,
> > Ariel
> > 
>