Re: [PATCH 4/4] fanotify: Expose the file changes to the user

[Tvrtko Ursulin <tvrtko.ursulin@sophos.com>]

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

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