Re: adding linux syscall fallocate

[Maciej <maciej.grochowski%protonmail.com@localhost>]

Hi r0ller,

A couple of weeks ago I also run to the issue when I found lack of fallocate or POSIX_FALLOCATE(2) (to be precise) a little bit sad.

From the very first view typical usage of POSIX_FALLOCATE(2) seems straight forward, comparing to the Linux fallocate(2) where different modes have to be supported. However, things can go a little bit more complicated if you are dealing with an existing file with a more complex structure.

Because of that, I found difficult to provide good quality implementation without a proper way to test it.

Before EuroBSD 2019 as a part of work about fuzzing the FFS, I decided to bring some well known FS test (namely speaking "XFS tests') suit to make sure that bugs that we fix did not introduce a regression.

The same thing applies to the new features of FS, is relatively easy to port implementation from other OS/FS, but without a proper way to test them, I would be very careful to introduce such things too quickly to the end-users.

One thing that I was missing for XFS tests, and going to publish part of it soon, is a way to view the internal structure of inodes and other metadata of Filesystem. My primary use case was to debug mount issues, in the example the issue that I showed during my presentation about the fuzzing. But also same apply to the code that manipulates inode blocks.

Hopefully, we can elaborate on that, and as I pointed earlier I would be careful with merging new FS features especially such wide used as POSIX_FALLOCATE(2) without a proper FS test suit or extensive enough testing that would require proper too like i.e. FSDB.

Thanks

Maciej

‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐

On Sunday, November 3, 2019 6:06 PM, r0ller <r0ller%freemail.hu@localhost> wrote:

Hi Jaromir,

Indeed. That's bad news but thanks for your answer! I've even found this: https://wiki.netbsd.org/projects/project/ffs-fallocate/

Are there any details for this project besides that page? I don't know anything about NetBSD internals though if it's not meant for gurus, I'd have a look at it and give it a try.

Best regards,

r0ller

-------- Eredeti levél --------

Feladó: Jaromír Doleček < jaromir.dolecek%gmail.com@localhost (Link -> mailto:jaromir.dolecek%gmail.com@localhost) >

Dátum: 2019 november 3 15:16:34

Tárgy: Re: adding linux syscall fallocate

Címzett: r0ller < r0ller%freemail.hu@localhost (Link -> mailto:r0ller%freemail.hu@localhost) >

Le dim. 3 nov. 2019 à 08:57, r0ller <r0ller%freemail.hu@localhost> a écrit :

> As you can see on the attached screenshot, "line 4741" gets printed out. So I went on to check what happens in VOP_FALLOCATE but it gets really internal there.

>

> Does anyone have any hint?

fallocate VOP is not implemented for FFS:

> grep fallocate *

ffs_vnops.c: { &vop_fallocate_desc, genfs_eopnotsupp }, /* fallocate */

ffs_vnops.c: { &vop_fallocate_desc, spec_fallocate }, /* fallocate */

ffs_vnops.c: { &vop_fallocate_desc, vn_fifo_bypass }, /* fallocate */

Jaromir