Re: [PATCH] btrfs-progs: add supported attr flags to btrfs(5)

[David Sterba <dsterba@suse.cz>]

On Fri, Jun 27, 2014 at 09:56:10AM -0500, Eric Sandeen wrote:
> > * and 'X' does not mean "no compression" and never has, although I'd
> >   like to see a chattr bit for that because we have the corresponding
> >   inode bit
> 
> Ok, then I'm not sure what it does mean.  Supposedly these flags are supported;
> via check_flags(), called by setflags(), which I was basing these on:
> 
>         if (flags & ~(FS_IMMUTABLE_FL | FS_APPEND_FL | \
>                       FS_NOATIME_FL | FS_NODUMP_FL | \
>                       FS_SYNC_FL | FS_DIRSYNC_FL | \
>                       FS_NOCOMP_FL | FS_COMPR_FL |
>                       FS_NOCOW_FL))
> 
> and the kernel header says that's:
> 
> #define FS_NOCOMP_FL                    0x00000400 /* Don't compress */

Passing this bit directly via ioctl works as expected, but to my
knowledge there is no chattr letter allocated for it.

> chattr(1) says: "compression raw access (X)," and also "The ’X’ attribute
> is used by the experimental compression patches to indicate that a raw
> contents of a compressed file  can  be  accessed  directly.  It currently 
> may not be set or reset using chattr(1), although it can be displayed by lsattr(1)."
> 
> Hum, ok, so we are starting to go off the rails here, aren't we ;)

Yeah. And there's no support for accessing raw compressed data.

> e2fsprogs has this flag translation:
>          { EXT2_NOCOMPR_FL, "X", "Compression_Raw_Access" },
> for:
> #define EXT2_NOCOMPR_FL                 0x00000400 /* Access raw compressed data */
> 
> and btrfs_ioctl_setflags claims to handle it:
> 
>         if (flags & FS_NOCOMP_FL) {
>                 ip->flags &= ~BTRFS_INODE_COMPRESS;
>                 ip->flags |= BTRFS_INODE_NOCOMPRESS;
> 		...
> 
> so hopefully you can understand my confusion? ;)

Oh I do now, but it's ext2 fault :)

> The comment says:
> 
>          * The COMPRESS flag can only be changed by users, while the NOCOMPRESS
>          * flag may be changed automatically if compression code won't make
>          * things smaller.
> 
> (but doesn't says "may *only* be...")

And thats IMO right (at least I expect it work like that), the user may
set or drop the NOCOMPRESS flag. The comment says that it may appear
without user interaction.

> But OTOH, chattr won't ever even *pass* "X" to the fs, will it.
> 
> So I guess I'm lost.  It looks like there's code to handle an incoming
> "X" but I don't think chattr will send it.
> 
> Do we ever get an outbound "X" for an opportunistically not-compressed file?
> If so, maybe that still needs to be specified.

AFAICS 'X' is not listed among the standard chattr options and chattr.c
in e2fsprogs has no support for that.

There is

lib/e2p/pf.c:   { EXT2_NOCOMPR_FL, "X", "Compression_Raw_Access" },

but this is used only locally by print_flags.

I hope this answers your questions, 'X' has no meaning for btrfs now.