Re: reliably detect writes to a file: mmap, mtime, ...

[Andy Lutomirski <luto@amacapital.net>]

On Dec 30, 2015 4:24 AM, "Martin Uecker" <muecker@gwdg.de> wrote:
>
>
>
> Hi all,
>
> I want to reliably detect changes to a file even when
> written to using mmap. Surprisingly, there seems to be
> no API which would make this possible. Or at least I
> haven't found a way to do it...
>
>
> I looked at:
>
> - mtime. What is missing here is an API which would
> force mtime to be updated if there are dirty PTEs
> in some mapping (which need to be cleared/transferred
> to struct page at this point). This would allow to
> reliably detect changes to the file. If I understand it
> correctly, there was patch from Andy Lutomirski which
> made msync(ASYNC) do exactly this:
>
> http://oss.sgi.com/archives/xfs/2013-08/msg00748.html
>
> But it seems this never got in. The other problem with
> this is that mtime has limited granularity.
> (but maybe that could be worked around by having some
> kind of counter + API which tells how often mtime has
> been updated without changing its nominal value)

Those patches plus nanosecond granularity should do it, I think.  I
keep meaning to dust them off.  You could do it :)

--Andy

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