linux-kernel - Re: [PATCH] Shrink fanotify_event

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Message-ID: <AANLkTi=iVVUCpskSfwFaxPdseft+vLwmfpUhAEjP3RXV@mail.gmail.com>
Date:	Sun, 21 Nov 2010 00:03:53 +0300
From:	Alexey Zaytsev <alexey.zaytsev@...il.com>
To:	Eric Paris <eparis@...hat.com>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Shrink fanotify_event_metadata by 32 bits

Ok, just so you know, the real motivation is to use the '__u8
reserved' for an offset to the first option's header. This way we
should be able to add a reasonable number of new core fields to struct
fanotify_event_metadata, and still maintain backwards compatibility.
I'd have to add an other __u32 for that purpose, so we are actually
saving 64 bits per event with this patch.

On Sat, Nov 20, 2010 at 08:08, Alexey Zaytsev <alexey.zaytsev@...il.com> wrote:
> There seems to be no point wasting 32 bits for either the event version or length
>
> Signed-off-by: Alexey Zaytsev <alexey.zaytsev@...il.com>
> ---
>
> Hi.
>
> While I'm working on the file modification events,
> please consider this patch. It obviously breaks
> the userspace ABI. But maybe that's acceptable, given
> that no kernel has ever been released with the old ABI.
>
> And the users only need to recompile their code, without
> changing anything.
>
> If we agree that this change is desirable, please make sure
> it makes it to Linus before the release. ;)
>
>
>  include/linux/fanotify.h |    7 ++++---
>  1 files changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/fanotify.h b/include/linux/fanotify.h
> index 0f01214..9a7986f 100644
> --- a/include/linux/fanotify.h
> +++ b/include/linux/fanotify.h
> @@ -86,10 +86,11 @@
>  #define FANOTIFY_METADATA_VERSION      2
>
>  struct fanotify_event_metadata {
> -       __u32 event_len;
> -       __u32 vers;
> -       __aligned_u64 mask;
> +       __u16 event_len;
> +       __u8 vers;
> +       __u8 reserved;
>        __s32 fd;
> +       __aligned_u64 mask;
>        __s32 pid;
>  };
>
>
>
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