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

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

[Pankaj Raghav]

This flag was added to the kernel in v6.17. As of now, only ext4 and
block device nodes support this flag.

Signed-off-by: Pankaj Raghav <p.raghav@samsung.com>
---
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
+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.
+.\" 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

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

[Alejandro Colomar]

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Hi Pankaj,

> Date: 2026-08-20 11:04:00+0200
> From: Pankaj Raghav <p.raghav@samsung.com>
>
> This flag was added to the kernel in v6.17. As of now, only ext4 and
> block device nodes support this flag.
> 
> Signed-off-by: Pankaj Raghav <p.raghav@samsung.com>
> ---
> 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.

Do you really mean Linux v7.13?


Have a lovely day!
Alex

> 
>  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
> +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.
> +.\" 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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Re: [PATCH v2] man2/fallocate.2: add doc for FALLOC_FL_WRITE_ZEROES

[Pankaj Raghav]

On 8/20/26 11:28, Alejandro Colomar wrote:
> Hi Pankaj,
> 
>> Date: 2026-08-20 11:04:00+0200
>> From: Pankaj Raghav <p.raghav@samsung.com>
>>
>> This flag was added to the kernel in v6.17. As of now, only ext4 and
>> block device nodes support this flag.
>>
>> Signed-off-by: Pankaj Raghav <p.raghav@samsung.com>
>> ---
>> 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.
> 
> Do you really mean Linux v7.13?
> 
> 

Oops. I mean v7.3 :)

--
Pankaj