On Thu, Jul 22, 2021 at 10:36 AM Eric Biggers <[email protected]> wrote:
>
> On Thu, Jul 22, 2021 at 09:33:59AM -0700, Daeho Jeong wrote:
> > On Thu, Jul 22, 2021 at 9:26 AM Eric Biggers <[email protected]> wrote:
> > >
> > > On Thu, Jul 22, 2021 at 09:17:54AM -0700, Eric Biggers wrote:
> > > > > I think it matches with FIEMAP_EXTENT_UNWRITTEN. Otherwise, we should
> > > > > shorten the last extent like below.
> > > > >
> > > > > fe_logical=0 fe_physical=16384 length=4096
> > > > > fe_logical=4096 fe_physical=81920 length=4096
> > > > > fe_logical=8192 fe_physical=147456 length=4096
> > > > >
> > > > >
> > > > > Unwritten extent - the extent is allocated but its data has not been
> > > > > initialized. This indicates the extent's data will be all zero if
> > > > > read
> > > > > through the filesystem but the contents are undefined if read
> > > > > directly from
> > > > > the device.
> > > >
> > > > Well, as I said before, using UNWRITTEN isn't appropriate because it
> > > > indicates
> > > > that the data is all zeroes, which in this case it is not. Similarly,
> > > > reporting
> > > > a hole isn't appropriate because it also indicates that the data is all
> > > > zeroes
> > > > and also that it has no space allocated on-disk at all.
> > > >
> > > > I think we should just over-report the physical length of the last
> > > > extent in the
> > > > cluster, which is what btrfs does...
> > > >
> > >
> > > Also keep in mind that as far as fiemap is concerned, when
> > > FIEMAP_EXTENT_ENCODED
> > > is set (indicating that the data is compressed or encrypted), then
> > > reading the
> > > data from disk will have "undefined results"; see
> > > Documentation/filesystems/fiemap.rst. As such, when someone decides to
> > > do so
> > > anyway (which is necessary for encryption testing), they really need to
> > > know
> > > *exactly* what they're doing. So I think it's less bad to bend the rules
> > > on
> > > extents where FIEMAP_EXTENT_ENCODED is already set.
> > >
> > > In contrast, your suggestion would incorreectly report some parts of the
> > > file as
> > > standard extents (or holes) without FIEMAP_EXTENT_ENCODED, so it would be
> > > expected that the standard meaning would apply to those parts.
> > >
> > > - Eric
> >
> > I also think it is okay with the FIEMAP_EXTENT_ENCODED flag.
> > It is actually all zero in a view of the filesystem internal and still
> > undefined if read directly from the device.
> > If we remove this extent, it might be confusing to understand the layout of
> > it.
>
> FIEMAP_EXTENT_ENCODED | FIEMAP_EXTENT_UNWRITTEN is contradictory, so that
> doesn't seem like a good option. And no, the range is *not* zero when read
> from
> the file by userspace, which is what UNWRITTEN is supposed to indicate.
>
> Compressed data is terminated by an end-of-stream marker, so it is possible to
> decompress even if extra data is appended to it. So that's another reason
> why I
> feel that my suggestion is not as bad as the other options.
>
> > Plus, I think we need to remove the last extent, when we return back
> > the block to the free space pool to filesystem using releasing
> > reserved space ioctl.
>
> This seems to be a filesystem implementation detail. Again, FS_IOC_FIEMAP is
> just about returning the actual extent mapping, not about other filesystem
> implementation details.
>
> - Eric
Understood. For confirmation, is this your final suggestion?
fe_logical=0 fe_physical=16384 length=4096
fe_logical=4096 fe_physical=81920 length=4096
fe_logical=8192 fe_physical=147456 length=8192
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