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

[Andreas Dilger <adilger@dilger.ca>]

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On Feb 6, 2019, at 1:44 PM, Darrick J. Wong <darrick.wong@oracle.com> 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.

Exactly correct.

>> 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.
> 
> 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.

I like this better than my plain "FIEMAP_EXTENT_DEVICE" proposal, since it
allows userspace to distinguish between an actual dev_t a unique-but-
locally-meaninless identifier that is needed for network filesystems.

Cheers, Andreas

> 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


Cheers, Andreas






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