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

From: Eric Biggers <ebiggers@kernel.org>
To: Daeho Jeong <daeho43@gmail.com>
Cc: linux-kernel@vger.kernel.org,
	linux-f2fs-devel@lists.sourceforge.net, kernel-team@android.com,
	Daeho Jeong <daehojeong@google.com>
Subject: Re: [f2fs-dev] [PATCH] f2fs: change fiemap way in printing compression chunk
Date: Wed, 21 Jul 2021 18:15:29 -0700	[thread overview]
Message-ID: <YPjGsSEdsoSsCJlB@sol.localdomain> (raw)
In-Reply-To: <CACOAw_xeTSa8J_9=+6thXvFT75u734D5asNRogUxt+DC-tPhxg@mail.gmail.com>

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