Re: [PATCH v2] man2/fallocate.2: add doc for FALLOC_FL_WRITE_ZEROES

Alejandro Colomar <[email protected]> Sat, 22 Aug 2026 14:41:34 +0200
Newsgroups	org.kernel.vger.linux-xfs,org.kernel.vger.linux-ext4,org.kernel.vger.linux-man
Message-ID	<aomV6Jt7udtAwwcw@devuan>
Hi Pankaj, Christoph,

> Date: 2026-08-20 11:04:00+0200
> From: Pankaj Raghav <[email protected]>
> Subject: Re: [PATCH v2] man2/fallocate.2: add doc for FALLOC_FL_WRITE_ZEROES

We use the full path in the prefix.  That is, 'man/man2/...: ...'.
Also, we use regular sentence conventions for the first letter after
the ':', so s/add/Add/.

> This flag was added to the kernel in v6.17. As of now, only ext4 and

Please use two spaces after period.
<https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING.d/patches/description#n44>

> block device nodes support this flag.
> 
> Signed-off-by: Pankaj Raghav <[email protected]>

Please include the CC list here too.

> ---
> Changes since v1:
> - Change the phrasing around zeroing and its condition (Zhang Yi)
> - Update ext4 support only for extent-based files (Zhang Yi)
> 
> Note:
> This feature was recently added to XFS and it is expected to land in
> v7.13. I will update this man page again once v7.13 is released.
> 
>  man/man2/fallocate.2 | 38 ++++++++++++++++++++++++++++++++++++++
>  1 file changed, 38 insertions(+)
> 
> diff --git a/man/man2/fallocate.2 b/man/man2/fallocate.2
> index ef6586c77..48a8c3468 100644
> --- a/man/man2/fallocate.2
> +++ b/man/man2/fallocate.2
> @@ -244,6 +244,43 @@ SMB3 (since Linux 3.17)
>  .IP \[bu]
>  Btrfs (since Linux 4.16)
>  .\" commit f27451f229966874a8793995b8e6b74326d125df
> +.SS Writing zeroes in file space
> +Specifying the
> +.B FALLOC_FL_WRITE_ZEROES
> +flag (available since Linux 6.17)
> +.\" commit 7bd43cc79cab3850f34da0a31d5b042b701590ef
> +in
> +.I mode
> +physically zeros space in the byte range starting at
> +.I offset
> +and continuing for
> +.I size
> +bytes.
> +Within the specified range,
> +blocks are preallocated for the regions that span the holes

What does it mean that "blocks are preallocated"?  That they must be
allocated prior to this call?  I suspect not.  Otherwise, we'd be saying
'must be' instead of 'are'.

I suspect it means that this call allocates blocks for those regions,
but then I'm not sure we want 'pre' there.

> +in the file.
> +After a successful call,
> +subsequent reads from this range will return zeros and
> +subsequent writes to that range do not require further
> +changes to the file mapping metadata.
> +.P
> +Zeroing is done within the filesystem.
> +The filesystem may use a hardware-accelerated zeroing command
> +or may submit regular writes.
> +The behavior depends on the filesystem design and the
> +available hardware.
> +.P
> +No other flags may be specified in
> +.I mode
> +in conjunction with
> +.BR FALLOC_FL_WRITE_ZEROES .
> +.P
> +Not all filesystems support
> +.BR FALLOC_FL_WRITE_ZEROES ;
> +if a filesystem doesn't support the operation, an error is returned.
> +The operation is supported only in ext4 (only for extent-based files)
> +since Linux 6.17.

Christoph, is this correct?  I remember you said something about block
device nodes in v1.


Have a lovely day!
Alex

> +.\" commit f4265b8d32c49ff95711e6fef7d05245a2905b30
>  .SS Increasing file space
>  Specifying the
>  .B FALLOC_FL_INSERT_RANGE
> @@ -385,6 +422,7 @@ or
>  is
>  .BR FALLOC_FL_COLLAPSE_RANGE ,
>  .BR FALLOC_FL_ZERO_RANGE ,
> +.BR FALLOC_FL_WRITE_ZEROES ,
>  or
>  .BR FALLOC_FL_INSERT_RANGE ,
>  but the file referred to by
> 
> base-commit: d926b42e0ea0406d5c2556e18205eced92d053f1
> -- 
> 2.51.2
> 

-- 
<https://www.alejandro-colomar.es>
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