Gregory Farnum <gfarnum@xxxxxxxxxx> writes:
> On Mon, Jan 7, 2019 at 7:01 AM Luis Henriques <lhenriques@xxxxxxxx> wrote:
>>
>> Hi,
>>
>> While working on implementing copy_file_range(2) for the kernel CephFS
>> client, I found an issue with truncated files that is described in [1].
>> The TL;DR is that, when executing a 'copy-from' OSD operation, both
>> truncate_seq and truncate_size are copied from the base object into the
>> target object. This, at least in the context of copy_file_range, doesn't
>> make sense and will cause problems if, for example, the target file had
>> previously been truncated, i.e. target.truncate_seq > base.truncate_seq
>> (see test case in [1]).
>>
>> I've proposed a fix [2] but after discussing it with Gregory it sounds
>> more like a hack than a real solution. Basically my patch simply adds a
>> new flag to the 'copy-from' operation which a client can use so that
>> truncate_{seq,size} aren't copied from the base object (and are *not*
>> changed with the copy operation).
>>
>> Having my PR [2] tagged as 'pending-discussion', I decided to try to
>> kick-off this discussion here in the mailing-list, maybe grabbing
>> attention from other people with a deeper understanding of the OSD
>> internals.
>>
>> Gregory's preferred solution would be to have the copy-from Op to allow
>> to set truncate_seq and truncate_size values directly. Unfortunately,
>> there seems to be no easy way of changing the interfaces to allow this
>> to happen as the ceph_osd_op union (in rados.h) doesn't seem to be able
>> to accommodate these 2 extra fields in copy_from. So, my initial
>> questions would be:
>>
>> - What would be the options for extending copy-from to include this 2
>> extra fields?
>
> It just occurred to me — and let's be clear, this is a TERRIBLE hack —
> that we might be able to handle this by generating compound
> operations: the first op in sequence uses copy-from either as it
> stands or in a changed one which doesn't copy the truncate values, and
> the second writes the correct new truncate values. I'm not sure if
> we've used compound ops with copy-from though so it'd take some
> testing to make sure that works correctly and I've not forgotten some
> new rule.
>
>> - I believe I understand the usage of truncate_{seq,size}, but it's
>> really not clear to me whether there are any scenarios where we *do*
>> want to modify truncate_{seq,size} while doing a copy-from. In the
>> case of CephFS I don't think a copy-from will ever truncate a file,
>> so the values could be left unchanged. But would the obvious solution
>> of simply *never* copying these fields be a valid solution? (I
>> suspect the answer is 'no' :-)
>
> This is all from memory and should be checked by someone, but the
> truncate_seq and truncate_size are used approximately as follows: (I'm
> definitely leaving out some of the manipulation for dealing with
> multiple objects)
>
> 1) client sends truncate op to MDS
> 2) MDS updates metadata and sends back new truncate_seq and
> truncate_size to client, which saves them
> 3) on a data op, the client sends the truncate_seq and truncate_size
> to the OSD along with the read or write request.
> 4) if the client has a newer truncate_seq than the OSD knows about,
> the OSD will respect the provided truncate_size, which may mean
> returning less data than it has, or performing a truncate on the
> object itself.
>
> But now I know I'm forgetting something important here, because I
> can't remember how we deal with existing truncate_seq = 1
> truncate_size = 10, then a truncate to 0 and a second truncate to 15
> (which results in the client providing truncate_seq 3 truncate_size 15
> to an OSD with a size 10 object that should be ignored).
>
> But leaving aside that issue, if we issue the OSD a copy-from op while
> we have a newer truncate-seq, it needs to update to our truncate-seq.
> Because we just gave it new data which it should return to any
> follow-on readers!
Aha! I was missing this obvious scenario. /me facepalms
Of course we _need_ to update truncate_seq in this Op!
> I am now wondering if maybe we're okay just having a second copy-from
> op that DOES fill in values the client is already sending — it looks
> like the existing copy-from op got bug-fixed to copy the truncate_seq
> in .94.4 because that's necessary for a CephFS cache pool to function,
> but obviously in this case we need different behavior.
If I understand you correctly, you're saying that a new struct should be
added to the union in ceph_osd_op, something like:
struct {
__le64 snapid;
__le64 src_version;
__u8 flags;
__le32 src_fadvise_flags;
__le64 truncate_size;
__le32 truncate_seq;
} __attribute__ ((packed)) copy_from2;
But this will cause problems, because it's increasing the union size,
right?
> I really don't know the details of the implementation around this
> enough any more to say with certainty what's going on. Hopefully you
> can investigate or one of the more full-time CephFS people knows the
> answer? :)
Having this issue sorted out is definitely something I would love to see
in nautilus. But I'm also not comfortable with the OSD internals, and
figuring out all the details of dealing with a new Op seems like a
*huge* task to me. I guess I could try, but it will take me a while :-/
Cheers,
--
Luis
> -Greg
>
>>
>> Another problem is that the client will also need to figure out if the
>> OSDs have this issue fixed so that it can decide whether to use the Op
>> or not. My PR adds a new OSD_COPY_FILE_RANGE feature, overlapping
>> SERVER_NAUTILUS, but Greg's preference is to use the osdmap. The kernel
>> client does have _some_ support for osdmap but I believe some extra bits
>> would be required to use this map in this case (although I would need to
>> look closer to figure out what and how to do that).
>>
>> Anyway, I'm looking for ideas on how to sort this out so that we can
>> have copy_file_range fixed in CephFS.
>>
>> [1] https://tracker.ceph.com/issues/37378
>> [2] https://github.com/ceph/ceph/pull/25374
>>
>> Cheers,
>> --
>> Luis
>
