Re: [PATCHSET v6 4/8] fuse: allow servers to use iomap for better file IO performance

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From: Demi Marie Obenour <[email protected]>
To: Gao Xiang <[email protected]>,
 "Darrick J. Wong" <[email protected]>
Cc: [email protected], [email protected], [email protected],
 [email protected], [email protected], [email protected],
 [email protected], [email protected],
 [email protected]
Message-ID: <[email protected]>
Subject: Re: [PATCHSET v6 4/8] fuse: allow servers to use iomap for better
 file IO performance
References: <176169810144.1424854.11439355400009006946.stgit@frogsfrogsfrogs>
 <[email protected]>
 <20251119180449.GS196358@frogsfrogsfrogs>
 <[email protected]>
 <[email protected]>
In-Reply-To: <[email protected]>

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On 11/19/25 16:51, Gao Xiang wrote:
> On 2025/11/20 05:00, Gao Xiang wrote:
>> On 2025/11/20 02:04, Darrick J. Wong wrote:
>>> On Wed, Nov 19, 2025 at 04:19:36AM -0500, Demi Marie Obenour wrote:
>>>>> By keeping the I/O path mostly within the kernel, we can dramatical=
ly
>>>>> increase the speed of disk-based filesystems.
>>>>
>>>> ZFS, BTRFS, and bcachefs all support compression, checksumming,
>>>> and RAID.=C2=A0 ZFS and bcachefs also support encryption, and f2fs a=
nd
>>>> ext4 support fscrypt.
>>>>
>>>> Will this patchset be able to improve FUSE implementations of these
>>>> filesystems?=C2=A0 I'd rather not be in the situation where one can =
have
>>>> a FUSE filesystem that is fast, but only if it doesn't support moder=
n
>>>> data integrity or security features.
>>>
>>> Not on its own, no.
>>>
>>>> I'm not a filesystem developer, but here are some ideas (that you
>>>> can take or leave):
>>>>
>>>> 1. Keep the compression, checksumming, and/or encryption in-kernel,
>>>> =C2=A0=C2=A0=C2=A0 and have userspace tell the kernel what algorithm=
 and/or encryption
>>>> =C2=A0=C2=A0=C2=A0 key to use.=C2=A0 These algorithms are generally =
well-known and secure
>>>> =C2=A0=C2=A0=C2=A0 against malicious input.=C2=A0 It might be necess=
ary to make an extra
>>>> =C2=A0=C2=A0=C2=A0 data copy, but ideally that copy could just stay =
within the
>>>> =C2=A0=C2=A0=C2=A0 CPU caches.
>>>
>>> I think this is easily doable for fscrypt and compression since (IIRC=
)
>>> the kernel filesystems already know how to transform data for I/O, an=
d
>>> nowadays iomap allows hooking of bios before submission and/or after
>>> endio.=C2=A0 Obviously you'd have to store encryption keys in the ker=
nel
>>> somewhere.
>>
>> I think it depends, since (this way) it tries to reuse some of the
>> existing kernel filesystem implementations (and assuming the code is
>> safe), so at least it still needs to load a dedicated kernel module
>> for such usage at least.
>>
>> I think it's not an issue for userspace ext4 of course (because ext4
>> and fscrypt is almost builtin for all kernels), but for out-of-tree
>> fses even pure userspace fses, I'm not sure it's doable to load the
>> module in a container context.
>=20
> Two examples for reference:
>=20
>   - For compression, in-tree f2fs already has a compression header
>     in data of each compressed extent:
>     https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/=
tree/fs/f2fs/f2fs.h?h=3Dv6.17#n1497
>=20
>     while other fs may store additional metadata in extent metadata
>     or other place.

Extent metadata shouldn't be a problem, as that is already available
during reads and written asynchronously for writes.  The headers are
awkward, though, and might need some special-casing.
>   - gocryptfs (a pure userspace FUSE fs) uses a different format
>     from fscrypt (encrypted data seems even unaligned on disk):
>     https://github.com/rfjakob/gocryptfs/blob/master/Documentation/file=
-format.md

This is probably an anti-pattern in general, as I expect it precludes
the use of inline encryption hardware via blk-crypto.
--=20
Sincerely,
Demi Marie Obenour (she/her/hers)
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