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    SubjectRe: [PATCH 4/4] fanotify: Expose the file changes to the user
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    On Fri, Nov 26, 2010 at 13:11, Tvrtko Ursulin <tvrtko.ursulin@sophos.com> wrote:
    > On Monday 22 Nov 2010 00:37:21 Alexey Zaytsev wrote:
    >> +struct fanotify_opt_hdr {
    >> +       __u8 type;
    >> +       __u8 reserved;
    >> +       __u16 len;
    >> +       /* Payload goes here. */
    >> +};
    >> +
    >> +#define FANOTIFY_METADATA_VERSION      3
    >>
    >>  struct fanotify_event_metadata {
    >> -       __u16 event_len;
    >> +       __u16 event_len; /* Including the options */
    >>         __u8 vers;
    >> -       __u8 reserved;
    >> +       __u8 options_offset; /* Aka header length */
    >>         __s32 fd;
    >>         __aligned_u64 mask;
    >>         __s32 pid;
    >> +       /* Options go here. */
    >>  };
    >
    > I am not 100% comfortable with having 16 bits length fields because I am just
    > not sure there is a measurable performance difference versus just going with
    > 32 bits.

    I'm not concerned so much with the performance, as with the storage.
    If we are generating events for every access on a mount point, some
    consumers might build a considerable backlog over a period of high
    activity. Would be good if we could cut the event size by 1/3 for
    free. And I don't see an event growing 64k even with the options. Do
    you?


    >
    > Also, options_offset is, if I understood it correctly, basically the lenght of
    > fanotify_event_metadata. As such, isn't that field redundant since the lenght
    > is implied from the protocol version?

    There are two problems there.

    1) You lose backwards-compatibility. It's still an ABI breakage, even
    if you tell the users about it.

    2) You can't build a program to account for different fanotify versions:
    if (vers >= N) { use the cool stuff } else if {vers >= N-1} {
    still good }
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