From: Daeho Jeong <email@example.com> To: Eric Biggers <firstname.lastname@example.org> Cc: email@example.com, firstname.lastname@example.org, email@example.com, Daeho Jeong <firstname.lastname@example.org> Subject: Re: [f2fs-dev] [PATCH] f2fs: change fiemap way in printing compression chunk Date: Wed, 21 Jul 2021 20:59:28 -0700 [thread overview] Message-ID: <CACOAw_w_5YD3GZGrrdMK_hK6KWbyQOfEMo-JwT2CgWLHAMCBrg@mail.gmail.com> (raw) In-Reply-To: <YPjQQTjJ6rO2sxemail@example.com> On Wed, Jul 21, 2021 at 6:56 PM Eric Biggers <firstname.lastname@example.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 <email@example.com> 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,
next prev parent reply other threads:[~2021-07-22 3:59 UTC|newest] Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top 2021-07-21 7:20 Daeho Jeong 2021-07-21 21:35 ` [f2fs-dev] " Eric Biggers 2021-07-21 22:30 ` Daeho Jeong 2021-07-22 0:15 ` Eric Biggers 2021-07-22 1:04 ` Daeho Jeong 2021-07-22 1:15 ` Eric Biggers 2021-07-22 1:40 ` Daeho Jeong 2021-07-22 1:56 ` Eric Biggers 2021-07-22 3:59 ` Daeho Jeong [this message] 2021-07-22 6:24 ` Christoph Hellwig 2021-07-22 6:39 ` Daeho Jeong
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