Re: [PATCH 09/10 V2] Use FIEMAP for FIBMAP calls

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Thu, Feb 07, 2019 at 12:59:54PM +0100, Carlos Maiolino wrote:
> On Wed, Feb 06, 2019 at 12:44:31PM -0800, Darrick J. Wong wrote:
> > On Wed, Feb 06, 2019 at 02:37:53PM +0100, Carlos Maiolino wrote:
> > > > > > In any case, I think a better solution to the multi-device problem is to
> > > > > > start returning device information via struct fiemap_extent, at least
> > > > > > inside the kernel.  Use one of the reserved fields to declare a new
> > > > > > '__u32 fe_device' field in struct fiemap_extent which can be the dev_t
> > > > > > device number, and then you can check that against inode->i_sb->s_bdev
> > > > > > to avoid returning results for the non-primary device of a multi-device
> > > > > > filesystem.
> > > > > 
> > > > > I agree we should address it here, but I don't think fiemap_extent is the right
> > > > > place for it, it is linked to the UAPI, and changing it is usually not a good
> > > > > idea.
> > > > 
> > > > Adding a FIEMAP_EXTENT flag or two to turn one of the fe_reserved fields
> > > > into some sort of dev_t/per-device cookie should be fine.  Userspace
> > > > shouldn't be expecting any meaning in reserved areas.
> > > > 
> > > > > I think I got your idea anyway, but, what if, instead returning the bdev in
> > > > > fiemap_extent, we instead, send a flag (via fi_flags) to the filesystem, to
> > > > > idenfify a FIBMAP or a FIEMAP call, and let the filesystem decide what to do
> > > > > with such information?
> > > > 
> > > > I don't like the idea of adding a FIEMAP_FLAG to distinguish callers.
> > > 
> > > Ok, may I ask why not?
> > 
> > I think it's a bad idea to add a flag to FIEMAP to change its behavior
> > to suit an older and even crappier legacy interface (i.e. FIBMAP).
> > 
> > FIBMAP is architecturally broken in that we can't /ever/ provide the
> > context of "which device does this map to?"
> > 
> > FIEMAP is architecturally deficient as well, but its ioctl structure
> > definition is flexible enough that we can report "which device does this
> > map to".
> > 
> > I want to enhance FIEMAP to deal with multi-device filesystems
> > correctly, and as much as I want to kill FIBMAP, I can't because of zipl
> > and *lilo.
> > 
> > > My apologies if I am wrong, but, per my understanding, there is
> > > nothing today, which tells userspace which device belongs the extent
> > > map reported by FIEMAP.
> > 
> > Right...
> > 
> > > If it belongs to the RT device in XFS, or whatever disk in a raid in
> > > BTRFS, we simply do not provide such information.
> > 
> > Right...
> > 
> > > So, the goal is to provide a way to tell the filesystem if a FIEMAP or
> > > a FIBMAP has been requested, so the current behavior of both ioctls
> > > won't change.
> > 
> > ...but from my point of view, the FIEMAP behavior *ought* to change to
> > be more expressive.  Once that's done, we can use the more expressive
> > FIEMAP output to solve the problem of FIBMAP vs. multi-disk filesystems.
> > 
> > The whole point of having fe_reserved* fields in struct fiemap_extent is
> > so that we can add a new FIEMAP_EXTENT_ flag so that the filesystem can
> > start returning data in a reserved field.  New userspace programs that
> > know about the flag can start reading information from the new field if
> > they see the flag, and old userspace programs don't know about the flag
> > and won't be any worse off.
> > 
> > > Enabling filesystems to return device information into fiemap_extent
> > > requires modification of all filesystems to provide such information,
> > > which will not have any use other than matching the mounted device to
> > > the device where the extent is.
> > 
> > Perhaps it would help for me to present a more concrete proposal:
> > 
> > --- a/include/uapi/linux/fiemap.h	2019-01-18 10:53:44.000000000 -0800
> > +++ b/include/uapi/linux/fiemap.h	2019-02-06 12:25:52.813935941 -0800
> > @@ -22,7 +22,19 @@ struct fiemap_extent {
> >  	__u64 fe_length;   /* length in bytes for this extent */
> >  	__u64 fe_reserved64[2];
> >  	__u32 fe_flags;    /* FIEMAP_EXTENT_* flags for this extent */
> > -	__u32 fe_reserved[3];
> > +
> > +	/*
> > +	 * Underlying device that this extent is stored on.
> > +	 *
> > +	 * If FIEMAP_EXTENT_DEV_T is set, this field is a dev_t containing the
> > +	 * major and minor numbers of a device.  If FIEMAP_EXTENT_DEV_COOKIE is
> > +	 * set, this field is a 32-bit cookie that can be used to distinguish
> > +	 * between backing devices but has no intrinsic meaning.  If neither
> > +	 * EXTENT_DEV flag is set, this field is meaningless.  Only one of the
> > +	 * EXTENT_DEV flags may be set at any time.
> > +	 */
> > +	__u32 fe_device;
> > +	__u32 fe_reserved[2];
> >  };
> >  
> >  struct fiemap {
> > @@ -66,5 +78,14 @@ struct fiemap {
> >  						    * merged for efficiency. */
> >  #define FIEMAP_EXTENT_SHARED		0x00002000 /* Space shared with other
> >  						    * files. */
> > +#define FIEMAP_EXTENT_DEV_T		0x00004000 /* fe_device is a dev_t
> > +						    * structure containing the
> > +						    * major and minor numbers
> > +						    * of a block device. */
> > +#define FIEMAP_EXTENT_DEV_COOKIE	0x00008000 /* fe_device is a 32-bit
> > +						    * cookie that can be used
> > +						    * to distinguish physical
> > +						    * devices but otherwise
> > +						    * has no meaning. */
> >  
> >  #endif /* _LINUX_FIEMAP_H */
> > 
> > Under this scheme, XFS can set FIEMAP_EXTENT_DEV_T in fe_flags and start
> > encoding fe_device = new_encode_dev(xfs_get_device_for_file()).
> > 
> > Some clustered filesystem or whatever could set FIEMAP_EXTENT_DEV_COOKIE
> > and encode the replica number in fe_device.
> > 
> 
> All of this makes sense, but I'm struggling to understand what you mean by
> replica number here, and why it justify a second flag.

I left in the "device cookie" thing in the proposal to accomodate a
request from the Lustre folks to be able to report which replica is
storing a particular extent map.  Apparently the replica id is simply a
32-bit number that isn't inherently useful, hence the vagueness around
what "cookie" really means...

...oh, right, lustre fell out of drivers/staging/.  You could probably
leave it out then.

> > Existing filesystems can be left unchanged, in which case neither
> > EXTENT_DEV flag is set in fe_flags and the bits in fe_device are
> > meaningless, the same as they are today.  Reporting fe_device is entirely
> > optional.
> > 
> > Userspace programs will now be able to tell which device the file data
> > lives on, which has been sort-of requested for years, if the filesystem
> > chooses to start exporting that information.
> > 
> > Your FIBMAP-via-FIEMAP backend can do something like:
> > 
> > /* FIBMAP only returns results for the same block device backing the fs. */
> > if ((fe->fe_flags & EXTENT_DEV_T) && fe->fe_device != inode->i_sb->sb_device)
> > 	return 0;
> > 
> > /* Can't tell what is the backing device, bail out. */
> > if (fe->fe_flags & EXTENT_DEV_COOKIE)
> > 	return 0;
> > 
> 
> Ok, the first conditional, is ok, the second one is not making sense to me.
> Looks like you are basically using it to flag the filesystem can't tell
> exactly which device the current extent is, let's say for example, distributed
> filesystems, where the physical extent can actually be on a different machine.
> But I can't say for sure, can you give me more details about what you are trying
> to achieve here?

You've understood me correctly. :)

> > /*
> >  * Either fe_device matches the backing device or the implementation
> >  * doesn't tell us about the backing device, so assume it's ok.
> >  */
> > <return FIBMAP results>
> >
> 
> This actually looks to contradict what you have been complaining, about some
> filesystems which doesn't support FIBMAP currently, will now suddenly start to
> support. Assuming it's ok if the implementation doesn't tell us about the
> backing device, will simply make FIBMAP work. Let's say BTRFS doesn't report the
> backing device, assuming it's ok will just fall into your first complain.

Sorry, this thread has been going on so long that I forgot your goal for
this series. :/

Specifically, I had forgotten that you're removing the ->bmap pointer,
which means that filesystems don't have any particular way to signal
"Yes on FIEMAP, no on FIBMAP".  Somehow I had thought that you were
merely creating a generic_file_bmap() that would call FIEMAP and ripping
out all the adhoc bmap implementations.

Hmm, how many filesystems support FIEMAP and not FIBMAP?

btrfs, nilfs2, and overlayfs.  Also bad_inode.c...?

Hmm, how many filesystems support FIBMAP and not FIEMAP?

adfs, affs, befs, bfs, efs, exofs, fat, freevxfs, fuse(?), nfs, hfsplus,
isofs, jfs, minixfs, ntfs, qnx[46], reiserfs, sysv, udf, and ufs.

> Anyway, I think I need to understand more your usage idea for EXTENT_DEV_COOKIE
> you mentioned.

I think you've understood it about as well as I can explain it.  Maybe
Andreas will have more to say about the lustre replica id, but OTOH it's
gone and so there's no user of it, so we could just drop it until lustre
comes back.

> > So that's how I'd solve a longstanding design problem of FIEMAP and then
> > take advantage of that solution to remedy my objections to the proposed
> > "Use FIEMAP for FIBMAP" series.  It doesn't require a FIEMAP_FLAG
> > behavior flag that userspace knows about but isn't allowed to pass in.
> >
> 
> > > A FIEMAP_FLAG will also require FS changes, but IMHO, less intrusive
> > > than the device id in fiemap_extent. I don't see much advantage in
> > > adding the device id instead of using the flag.
> > > 
> > > A problem I see using a new FIEMAP_FLAG, is it 'could' be also passed via
> > > userspace, so, it would require a check to make sure it didn't come from
> > > userspace if ioctl_fiemap() was used.
> > > 
> > > I think there are 2 other possibilities which can be used to fix this.
> > > 
> > > - Use a boolean inside fiemap_extent_info to identify a fibmap call, or,
> > > - If the device id is a must for you, maybe add the device id into
> > >   fiemap_extent_info instead of fiemap_extent.
> > 
> > That won't work with btrfs, which can store file extents on multiple
> > different physical devices.
> > 
> > >   So we don't mess with a UAPI exported data structure and still
> > >   provides a way to the filesystems to provide which device the mapped
> > >   extent is in.
> > > 
> > > What you think?
> > > 
> > > Cheers
> > > 
> > > 
> > > > 
> > > > --D
> > > > 
> > > > > > 
> > > > > > > +
> > > > > > > +	return error;
> > > > > > > +}
> > > > > > > +
> > > > > > >  /**
> > > > > > >   *	bmap	- find a block number in a file
> > > > > > >   *	@inode:  inode owning the block number being requested
> > > > > > > @@ -1594,10 +1628,14 @@ EXPORT_SYMBOL(iput);
> > > > > > >   */
> > > > > > >  int bmap(struct inode *inode, sector_t *block)
> > > > > > >  {
> > > > > > > -	if (!inode->i_mapping->a_ops->bmap)
> > > > > > > +	if (inode->i_op->fiemap)
> > > > > > > +		return bmap_fiemap(inode, block);
> > > > > > > +	else if (inode->i_mapping->a_ops->bmap)
> > > > > > > +		*block = inode->i_mapping->a_ops->bmap(inode->i_mapping,
> > > > > > > +						       *block);
> > > > > > > +	else
> > > > > > >  		return -EINVAL;
> > > > > > 
> > > > > > Waitaminute.  btrfs currently supports fiemap but not bmap, and now
> > > > > > suddenly it will support this legacy interface they've never supported
> > > > > > before.  Are they on board with this?
> > > > > > 
> > > > > > --D
> > > > > > 
> > > > > > >  
> > > > > > > -	*block = inode->i_mapping->a_ops->bmap(inode->i_mapping, *block);
> > > > > > >  	return 0;
> > > > > > >  }
> > > > > > >  EXPORT_SYMBOL(bmap);
> > > > > > > diff --git a/fs/ioctl.c b/fs/ioctl.c
> > > > > > > index 6086978fe01e..bfa59df332bf 100644
> > > > > > > --- a/fs/ioctl.c
> > > > > > > +++ b/fs/ioctl.c
> > > > > > > @@ -116,6 +116,38 @@ int fiemap_fill_user_extent(struct fiemap_extent_info *fieinfo, u64 logical,
> > > > > > >  	return (flags & FIEMAP_EXTENT_LAST) ? 1 : 0;
> > > > > > >  }
> > > > > > >  
> > > > > > > +int fiemap_fill_kernel_extent(struct fiemap_extent_info *fieinfo, u64 logical,
> > > > > > > +			    u64 phys, u64 len, u32 flags)
> > > > > > > +{
> > > > > > > +	struct fiemap_extent *extent = fieinfo->fi_extents_start;
> > > > > > > +
> > > > > > > +	/* only count the extents */
> > > > > > > +	if (fieinfo->fi_extents_max == 0) {
> > > > > > > +		fieinfo->fi_extents_mapped++;
> > > > > > > +		return (flags & FIEMAP_EXTENT_LAST) ? 1 : 0;
> > > > > > > +	}
> > > > > > > +
> > > > > > > +	if (fieinfo->fi_extents_mapped >= fieinfo->fi_extents_max)
> > > > > > > +		return 1;
> > > > > > > +
> > > > > > > +	if (flags & SET_UNKNOWN_FLAGS)
> > > > > > > +		flags |= FIEMAP_EXTENT_UNKNOWN;
> > > > > > > +	if (flags & SET_NO_UNMOUNTED_IO_FLAGS)
> > > > > > > +		flags |= FIEMAP_EXTENT_ENCODED;
> > > > > > > +	if (flags & SET_NOT_ALIGNED_FLAGS)
> > > > > > > +		flags |= FIEMAP_EXTENT_NOT_ALIGNED;
> > > > > > > +
> > > > > > > +	extent->fe_logical = logical;
> > > > > > > +	extent->fe_physical = phys;
> > > > > > > +	extent->fe_length = len;
> > > > > > > +	extent->fe_flags = flags;
> > > > > > > +
> > > > > > > +	fieinfo->fi_extents_mapped++;
> > > > > > > +
> > > > > > > +	if (fieinfo->fi_extents_mapped == fieinfo->fi_extents_max)
> > > > > > > +		return 1;
> > > > > > > +	return (flags & FIEMAP_EXTENT_LAST) ? 1 : 0;
> > > > > > > +}
> > > > > > >  /**
> > > > > > >   * fiemap_fill_next_extent - Fiemap helper function
> > > > > > >   * @fieinfo:	Fiemap context passed into ->fiemap
> > > > > > > diff --git a/include/linux/fs.h b/include/linux/fs.h
> > > > > > > index 7a434979201c..28bb523d532a 100644
> > > > > > > --- a/include/linux/fs.h
> > > > > > > +++ b/include/linux/fs.h
> > > > > > > @@ -1711,6 +1711,8 @@ struct fiemap_extent_info {
> > > > > > >  	fiemap_fill_cb	fi_cb;
> > > > > > >  };
> > > > > > >  
> > > > > > > +int fiemap_fill_kernel_extent(struct fiemap_extent_info *info, u64 logical,
> > > > > > > +			      u64 phys, u64 len, u32 flags);
> > > > > > >  int fiemap_fill_next_extent(struct fiemap_extent_info *info, u64 logical,
> > > > > > >  			    u64 phys, u64 len, u32 flags);
> > > > > > >  int fiemap_check_flags(struct fiemap_extent_info *fieinfo, u32 fs_flags);
> > > > > > > -- 
> > > > > > > 2.17.2
> > > > > > > 
> > > > > 
> > > > > -- 
> > > > > Carlos
> > > 
> > > -- 
> > > Carlos
> 
> -- 
> Carlos