On Fri, Jan 11, 2019 at 7:17 PM Luis Henriques <lhenriques@xxxxxxxx> wrote:
>
> 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?
>
we have already encodeed src_name and src_oloc into osd_op.indata. I
think we can add more fields to it
> > 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
> >
