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    SubjectRe: [PATCH 4/4] fanotify: Expose the file changes to the user
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    On Friday 26 Nov 2010 11:21:18 Alexey Zaytsev wrote:
    > 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?

    I don't but maybe it is just lack of imagination.

    My bias is that I am mostly thinking about synchronous events where large
    backlog is not a realistic scenario. How realistic you think is this with
    async events?

    > > 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.

    Assuming 2.6.37 release as starting point for ABI considerations?

    > 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 }

    I don't get why not, but maybe I am just slow today. There will always be 1:1
    mapping from version to your options_offset field, no? How does then removing
    options_offset change anything?

    Tvrtko

    Sophos Limited, The Pentagon, Abingdon Science Park, Abingdon, OX14 3YP, United Kingdom.
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