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

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Date:   Wed, 21 Jul 2021 15:30:46 -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

Hi Eric,

On Wed, Jul 21, 2021 at 2:35 PM Eric Biggers <ebiggers@...nel.org> wrote:
>
> On Wed, Jul 21, 2021 at 12:20:48AM -0700, Daeho Jeong wrote:
> > From: Daeho Jeong <daehojeong@...gle.com>
> >
> > When we print out a discontinuous compression chunk, it shows like a
> > continuous chunk now. To show it more correctly, I've changed the way of
> > printing fiemap info like below. Plus, eliminated NEW_ADDR(-1) in fiemap
> > info, since it is not in fiemap user api manual.
> >
> > 0: 0000000000000000 0000000000000000 0000000000001000 1008 (M/E)
> > 1: 0000000000001000 0000000f15c0f000 0000000000001000 1008 (M/E)
> > 2: 0000000000002000 0000000000000000 0000000000002000 1808 (M/U/E)
> > 3: 0000000000004000 0000000000000000 0000000000001000 1008 (M/E)
> > 4: 0000000000005000 0000000f15c10000 0000000000001000 1008 (M/E)
> > 5: 0000000000006000 0000000000000000 0000000000002000 1808 (M/U/E)
> > 6: 0000000000008000 0000000000000000 0000000000001000 1008 (M/E)
>
> Please label these columns.
>
> Anyway, this doesn't appear to work quite in the way I had in mind.  With this
> patch, what I'm seeing is:
>
> $ head -c 16384 /dev/zero > file; xfs_io -c "fiemap -v" file
> file:
>  EXT: FILE-OFFSET      BLOCK-RANGE      TOTAL FLAGS
>    0: [0..7]:          0..7                 8 0x1008
>    1: [8..15]:         2683128..2683135     8 0x1008
>    2: [16..31]:        0..15               16 0x1809
>
> So, working in 512-byte sectors, the logical sectors 0-31 are stored as one
> compressed cluster in the 8 physical sectors 2683128-2683135.
>
> The problem is, with this patch these physical sectors are reported at logical
> sectors 8-15 instead of 0-7.  Obviously, this isn't particularly well-defined,
> but I thought it was logical for the physical blocks to be associated with the
> first logical blocks.  That is what the tests I've written (xfstest f2fs/002,
> and the Android vts_kernel_encryption_test) assume.
>
> Is there any particular reason why you wouldn't report instead:
>
>    0: [0..7]:         2683128..2683135     8 0x1008
>    1: [8..31]:        0..23                8 0x1809
>
> - Eric

The reason is related to how F2FS stores the mapping information in
the mapping table.
Actually, the mapping inside of the table is like this.
[0..7]:  COMPR_ADDR flag(0x1008) -> merged, encoded
[8..15]: 2683128..2683135 flag(0x1008) -> merged, encoded
[16..31]: NEW_ADDR flag(0x1809) -> merged, unwritten(!), last_extent

I understand what you mean.
But, if we adapt to your way, how do you think we can print out when
we ask for f2fs to print out only the [8..15] area? Zero address? How
about flags?
I think the current way explains the layout of the f2fs metadata more exactly.

Thank you,