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

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

On Thu, Feb 07, 2019 at 01:36:41PM +0100, Carlos Maiolino wrote:
> Apologies, I forgot to mention another thing..
> 
> 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()).
> 
> Here, I believe you are forgetting that filesystems do not touch fiemap_extent
> directly. We call fiemap_fell_next_extent() helper to fill each extent found by
> fiemap. So, in either way, we'd need to modify fiemap_fill_next_extent() and the
> callbacks being used to accommodate this new field or create a new helper to
> modify the device which doesn't sound reasonable. So, either way, we will end up
> needing to modify all filesystems.

Yep.  Drat.  I guess you could add a bdev parameter to
fiemap_fill_next_extent, and we'd use that to encode fe_device.  If the
fs passes NULL then we just get it from the superblock or something.

> So, although I really like the idea of improving the FIEMAP interface, I'm
> starting to consider another patchset for it. I think it requires an interface
> change big enough to fit in this patchset, which actually has a different
> purpose. Or, maybe, address this at the end of this patchset, leaving different
> interface changes in different patchsets, instead of making many changes all at
> once, mixed together.

<nod> I think you're right, fiemap upgrades as one series and then
fibmap-via-fiemap as the second one.

--D

> > 
> > Some clustered filesystem or whatever could set FIEMAP_EXTENT_DEV_COOKIE
> > and encode the replica number in fe_device.
> > 
> > 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;
> > 
> > /*
> >  * 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>
> > 
> > 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