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From:   Daeho Jeong <daeho43@gmail.com>
Date:   Wed, 21 Jul 2021 20:59:28 -0700
Message-ID: <CACOAw_w_5YD3GZGrrdMK_hK6KWbyQOfEMo-JwT2CgWLHAMCBrg@mail.gmail.com>
Subject: Re: [f2fs-dev] [PATCH] f2fs: change fiemap way in printing
 compression chunk
To:     Eric Biggers <ebiggers@kernel.org>
Cc:     linux-kernel@vger.kernel.org,
        linux-f2fs-devel@lists.sourceforge.net, kernel-team@android.com,
        Daeho Jeong <daehojeong@google.com>
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On Wed, Jul 21, 2021 at 6:56 PM Eric Biggers <ebiggers@kernel.org> wrote:
>
> On Wed, Jul 21, 2021 at 06:40:00PM -0700, Daeho Jeong wrote:
> > On Wed, Jul 21, 2021 at 6:15 PM Eric Biggers <ebiggers@kernel.org> wrote:
> > >
> > > On Wed, Jul 21, 2021 at 06:04:22PM -0700, Daeho Jeong wrote:
> > > > >
> > > > > How f2fs stores the mapping information doesn't matter.  That's an
> > > > > implementation detail that shouldn't be exposed to userspace.  The only thing
> > > > > that should be exposed is the actual mapping, and for that it seems natural to
> > > > > report the physical blocks first.
> > > > >
> > > > > There is no perfect solution for how to handle the remaining logical blocks,
> > > > > given that the fiemap API was not designed for compressed files, but I think we
> > > > > should just go with extending the length of the last compressed extent in the
> > > > > cluster to cover the remaining logical blocks, i.e.:
> > > > >
> > > > >   [0..31]: 2683128..2683159 flag(0x1009) -> merged, encoded, last_extent
> > > > >
> > > > > That's what btrfs does on compressed files.
> > > > >
> > > > > - Eric
> > > >
> > > > I also agree that that's an implementation detail that shouldn't be
> > > > exposed to userspace.
> > > >
> > > > I want to make it more clear for better appearance.
> > > >
> > > > Do you think we have to remove "unwritten" information below? I also
> > > > think it might be unnecessary information for the user.
> > > > [0..31]: 2683128..2683159 flag(0x1009) -> merged, encoded, last_extent
> > > > (unwritten?)
> > >
> > > FIEMAP_EXTENT_UNWRITTEN already has a specific meaning; see
> > > Documentation/filesystems/fiemap.rst.  It means that the data is all zeroes, and
> > > the disk space is preallocated but the data hasn't been written to disk yet.
> > >
> > > In this case, the data is *not* necessarily all zeroes.  So I think
> > > FIEMAP_EXTENT_UNWRITTEN shouldn't be used here.
> > >
> > > > Do you want f2fs to print out the info on a cluster basis, even when
> > > > the user asks for one block information?
> > > > Like
> > > > If the user asks for the info of [8..15], f2fs will return the info of [0..31]?
> > >
> > > Yes, since that's how FS_IOC_FIEMAP is supposed to work; see
> > > Documentation/filesystems/fiemap.rst:
> > >
> > >         All offsets and lengths are in bytes and mirror those on disk.  It is
> > >         valid for an extents logical offset to start before the request or its
> > >         logical length to extend past the request.
> > >
> > > (That being said, the f2fs compression+encryption tests I've written don't
> > > exercise this case; they only map the whole file at once.)
> > >
> > > - Eric
> >
> > My last question is.
> > How about a discontinuous cluster like [0..31] maps to discontinuous
> > three blocks like physical address 0x4, 0x14 and 0x24.
> > I think we have to return three extents for the one logical region
> > like the below. What do you think?
> > [0..31] -> 0x4 (merged, encoded)
> > [0..31] -> 0x14 (merged, encoded)
> > [0..31] -> 0x24 (merged, encoded, last_extent)
>
> No, please don't do that.  struct fiemap_extent only has a single length field,
> not separate lengths for fe_logical and fe_physical, so with your proposal there
> would be no way to know how many physical blocks to take from each extent.  It
> would be reporting the same part of the file in contradictory ways.
>
> Like I suggested originally, I think this case should be reported like:
>
>        fe_logical=0    fe_physical=16384  length=4096
>        fe_logical=4096 fe_physical=81920  length=4096
>        fe_logical=8192 fe_physical=147456 length=8192
>
> It's not perfect, but I think it's the least bad option, for the reasons I've
> explained previously...
>
> - Eric

Ok, I got your point. Let me try it again.

Thank you,