Re: [PATCH v6 03/24] erofs: add super block operations

[Gao Xiang]

Hi Christoph,

On Mon, Sep 02, 2019 at 05:51:09AM -0700, Christoph Hellwig wrote:
> On Sun, Sep 01, 2019 at 04:54:55PM +0800, Gao Xiang wrote:
> > No modification at this... (some comments already right here...)
> 
> >  20 /* 128-byte erofs on-disk super block */
> >  21 struct erofs_super_block {
> > ...
> >  24         __le32 features;        /* (aka. feature_compat) */
> > ...
> >  38         __le32 requirements;    /* (aka. feature_incompat) */
> > ...
> >  41 };
> 
> This is only cosmetic, why not stick to feature_compat and
> feature_incompat?

Okay, will fix. (however, in my mind, I'm some confused why
"features" could be incompatible...)

> 
> > > > +	bh = sb_bread(sb, 0);
> > > 
> > > Is there any good reasons to use buffer heads like this in new code
> > > vs directly using bios?
> > 
> > As you said, I want it in the page cache.
> > 
> > The reason "why not use read_mapping_page or similar?" is simply
> > read_mapping_page -> .readpage -> (for bdev inode) block_read_full_page
> >  -> create_page_buffers anyway...
> > 
> > sb_bread haven't obsoleted... It has similar function though...
> 
> With the different that it keeps you isolated from the buffer_head
> internals.  This seems to be your only direct use of buffer heads,
> which while not deprecated are a bit of an ugly step child.  So if
> you can easily avoid creating a buffer_head dependency in a new
> filesystem I think you should avoid it.

OK, let's use read_mapping_page instead.

> 
> > > > +	sbi->build_time = le64_to_cpu(layout->build_time);
> > > > +	sbi->build_time_nsec = le32_to_cpu(layout->build_time_nsec);
> > > > +
> > > > +	memcpy(&sb->s_uuid, layout->uuid, sizeof(layout->uuid));
> > > > +	memcpy(sbi->volume_name, layout->volume_name,
> > > > +	       sizeof(layout->volume_name));
> > > 
> > > s_uuid should preferably be a uuid_t (assuming it is a real BE uuid,
> > > if it is le it should be a guid_t).
> > 
> > For this case, I have no idea how to deal with...
> > I have little knowledge about this uuid stuff, so I just copied
> > from f2fs... (Could be no urgent of this field...)
> 
> Who fills out this field in the on-disk format and how?

mkfs.erofs, but this field leaves 0 for now. Is that reasonable?
(using libuuid can generate it easily...)

> 
> > The background is Al's comments in erofs v2....
> > (which simplify erofs_fill_super logic)
> > https://lore.kernel.org/r/20190720224955.GD17978@xxxxxxxxxxxxxxxxxx/
> > 
> > with a specific notation...
> > https://lore.kernel.org/r/20190721040547.GF17978@xxxxxxxxxxxxxxxxxx/
> > 
> > "
> > > OTOH, for the case of NULL ->s_root ->put_super() won't be called
> > > at all, so in that case you need it directly in ->kill_sb().
> > "
> 
> Yes.  Although none of that is relevant for this initial version,
> just after more features are added.

This patch uses it actually... since no failure path in erofs_fill_super()
and s_root will be filled nearly at the end of the function...

Thanks,
Gao Xiang