linux-kernel - Re: [f2fs-dev] [PATCH] f2fs: change fiemap way in printing compression chunk

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Date:   Wed, 21 Jul 2021 18:40:00 -0700
From:   Daeho Jeong <daeho43@...il.com>
To:     Eric Biggers <ebiggers@...nel.org>
Cc:     linux-kernel@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net, kernel-team@...roid.com,
        Daeho Jeong <daehojeong@...gle.com>
Subject: Re: [f2fs-dev] [PATCH] f2fs: change fiemap way in printing
 compression chunk

On Wed, Jul 21, 2021 at 6:15 PM Eric Biggers <ebiggers@...nel.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)