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

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Date:	Fri, 26 Nov 2010 14:21:18 +0300
From:	Alexey Zaytsev <alexey.zaytsev@...il.com>
To:	Tvrtko Ursulin <tvrtko.ursulin@...hos.com>
Cc:	Eric Paris <eparis@...hat.com>, Scott Hassan <hassan@...funk.com>,
	Jan Kara <jack@...e.cz>,
	"agruen@...bit.com" <agruen@...bit.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"stefan@...ttcher.org" <stefan@...ttcher.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH 4/4] fanotify: Expose the file changes to the user

On Fri, Nov 26, 2010 at 13:11, Tvrtko Ursulin <tvrtko.ursulin@...hos.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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