On Feb 7, 2019, at 10:02 AM, Darrick J. Wong <darrick.wong@xxxxxxxxxx> wrote:
> 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:
>>>
>>> ...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.
>>>
>>> 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.
Do we really need to be this way, about reserving a single flag for Lustre,
which will likely also be useful for other filesystems? It's not like
Lustre is some closed-source binary module for which we need to make life
difficult, it is used by many thousands of the largest computers at labs
and universities and companies around the world. We are working to clean
up the code outside the staging tree and resubmit it. Not reserving a flag
just means we will continue to use random values in Lustre before it can
be merged, which will make life harder when we try to merge again.
In the case of Lustre, the proposed DEV_COOKIE would indicate fe_device is
the integer index number of the server on which each extent of the file is
located (Darrick's "replica number" term is not really correct). The server
index is familiar to all Lustre users, so having filefrag print out the
device number of "0000" or "0009" is totally clear to them.
For pNFS or Ceph or other network filesystems (if they implement filefrag)
it could be an index or some other number (e.g. the IP address of the server
or low bits of the UUID or whatever). Reading back in the archives of the
original FIEMAP discussion, it seems BtrFS would prefer to use DEV_COOKIE
instead of DEV_T, because it uses internal RAID encoding and not plain block
devices, but I'm not familiar with the details there.
Alternately, or in addition to, a DEV_COOKIE flag which indicates that the
same fe_device field is "not a device", it would be possible to add:
#define FIEMAP_NO_DIRECT 0x40000000
and/or:
#define FIEMAP_EXTENT_NET 0x80000000 /* Data stored remotely.
* Sets NO_DIRECT flag */
returned by the filesystem that indicates the extent blocks are not local
to the node, so FIBMAP should return an error (-EOPNOTSUP or -EREMOTE or
whatever) because the file can't be booted from. In that case, we could
return FIEMAP_EXTENT_DEVICE to indicate the fe_device field is valid, and
return FIEMAP_EXTENT_NET to indicate the values in fe_device are not local
block devices, just filesystem-specific values to distinguish devices.
However, I'm open to both _DEV_COOKIE and _NET flag if that is preferred,
since I think the two are somewhat complementary.
>> 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.
Just a reminder here, you should set FIEMAP_FLAG_SYNC when mapping FIBMAP
to FIEMAP so that the data on that file is flushed to disk before returning,
since the block mapping may not be assigned yet or may be unstable, which
could lead to an unbootable system if used for LILO.
Cheers, Andreas
>>> 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>
>>>
>>
>> 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
Cheers, Andreas
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