On Fri, Sep 09, 2016 at 09:38:06AM +1000, Dave Chinner wrote:
> On Tue, Aug 30, 2016 at 12:09:49PM -0700, Darrick J. Wong wrote:
> > > I recall for FIEMAP that some filesystems may not have files aligned
> > > to sector offsets, and we just used byte offsets. Storage like
> > > NVDIMMs are cacheline granular, so I don't think it makes sense to
> > > tie this to old disk sector sizes. Alternately, the units could be
> > > in terms of fs blocks as returned by statvfs.st_bsize, but mixing
> > > units for fmv_block, fmv_offset, fmv_length is uneeded complexity.
> >
> > Ugh. I'd rather just change the units to bytes rather than force all
> > the users to multiply things. :)
>
> Yup, units need to be either in disk addresses (i.e. 512 byte units)
> or bytes. If people can't handle disk addresses (seems to be the
> case), the bytes it should be.
<nod>
> > I'd much rather just add more special owner codes for any other
> > filesystem that has distinguishable metadata types that are not
> > covered by the existing OWN_ codes. We /do/ have 2^64 possible
> > values, so it's not like we're going to run out.
>
> This is diagnositc information as much as anything, just like
> fiemap is diagnostic information. So if we have specific type
> information, it needs to be reported accurately to be useful.
>
> Hence I really don't care if the users and developers of other fs
> types don't understand what the special owner codes that a specific
> filesystem returns mean. i.e. it's not useful user information -
> only a tool that groks the specific filesystem is going to be able
> to anything useful with special owner codes. So, IMO, there's little
> point trying to make them generic or to even trying to define and
> explain them in the man page....
<shrug> I'm ok with describing generally what each special owner code
means. Maybe the manpage could be more explicit about "None of these
codes are useful unless you're a low level filesystem tool"?
> > > It seems like there are several fields in the structure that are used for
> > > only input or only output? Does it make more sense to have one structure
> > > used only for the input request, and then the array of values returned be
> > > in a different structure? I'm not necessarily requesting that it be changed,
> > > but it definitely is something I noticed a few times while reading this doc.
> >
> > I've been thinking about rearranging this a bit, since the flags
> > handling is very awkward with the current array structure. Each
> > rmap has its own flags; we may someday want to pass operation flags
> > into the ioctl; and we currently have one operation flag to pass back
> > to userspace. Each of those flags can be a separate field. I think
> > people will get confused about FMV_OF_* and FMV_HOF_* being referenced
> > in oflags, and iflags has no meaning for returned records.
>
> Yup, that's what I initially noticed when I glanced at this. The XFS
> getbmap interface is just plain nasty, and we shouldn't be copying
> that API pattern if we can help it.
Lol ok. :)
> > So, this instead?
> >
> > struct getfsmap_rec {
> > u32 device; /* device id */
> > u32 flags; /* mapping flags */
> > u64 block; /* physical addr, bytes */
> > u64 owner; /* inode or special owner code */
> > u64 offset; /* file offset of mapping, bytes */
> > u64 length; /* length of segment, bytes */
> > u64 reserved; /* will be set to zero */
> > }; /* 48 bytes */
> >
> > struct getfsmap_head {
> > u32 iflags; /* none defined yet */
> > u32 oflags; /* FMV_HOF_DEV_T */
> > u32 count; /* # entries in recs array */
> > u32 entries; /* # entries filled in (output) */
> > u64 reserved[2]; /* must be zero */
> >
> > struct getfsmap_rec keys[2]; /* low and high keys for the mapping search */
> > struct getfsmap_rec recs[0];
> > }; /* 32 bytes + 2*48 = 128 bytes */
> >
> > #define XFS_IOC_GETFSMAP _IOWR('X', 59, struct getfsmap_head)
> >
> > This also means that userspace can set up for the next ioctl
> > invocation with memcpy(&head->keys[0], &head->recs[head->entries - 1]).
> >
> > Yes, I think I like this better. Everyone else, please chime in. :)
>
> That's pretty much the structure I was going to suggest - it matches
> the fiemap pattern. i.e control parameters are separated from record
> data. I'd dump a bit more reserved space in the structure, though;
> we've got heaps of flag space for future expansion, but if we need
> to pass new parameters into/out of the kernel we'll quickly use the
> reserved space.
I padded struct fsmap with enough reserved space to make it an even 64 bytes,
and padded struct fsmap_head so that the space before keys is 64 bytes in
length. See v3 patch of the ioctl manpage.
--D
>
> Cheers,
>
> Dave.
> --
> Dave Chinner
> david@xxxxxxxxxxxxx
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