Re: 2.4.18 no timestamp update on modified mmapped files

From: Hugh Dickins (
Date: Thu Jun 13 2002 - 04:58:39 EST

On Wed, 12 Jun 2002, jw schultz wrote:
> On Wed, Jun 12, 2002 at 03:52:34PM +0100, Hugh Dickins wrote:
> > On Wed, 12 Jun 2002, Andrew Morton wrote:
> > >
> > > 1: Map the page to disk at fault time, generate SIGBUS on
> > > ENOSPC (the standards don't seem to address this issue, and
> > > this is a non-standard overload of SIGBUS).
> >
> > I believe your option 1 is closest to the right direction; and SIGBUS
> > is entirely appropriate, I don't see it as a non-standard overload.
> I concur that #1 is closest. I'd prefer it to happen on a
> write fault rather read but the frequency with which
> this should occur is low enough i wouldn't sweat it.
> It is a non-standard overload of SIGBUS. SIGBUS is to
> indicate an unaligned memory access or otherwise malformed
> address. Many confuse SIGBUS with SIGSEGV because they are
> usually symptoms of the same problems but a SIGSEGV is to
> indicate memory protection violation (unresolvable page
> fault) which is not the same as a malformed address. I
> believe Linux, at least on x86 maps both errors to SIGSEGV.
> I would think SIGXFSZ might be a better fit.

No. I think you're looking back too far in UNIX history.
I imagine SIGBUS was originally defined as you describe,
but got hijacked by the inventors of the mmap system call
(only a limited number of signals available). That overload
has been enshrined in standards for ten(?) years.

SIGSEGV is used where mapping itself cannot be accessed (no mapping
or insufficient permission); SIGBUS where mapped object cannot be
accessed - I/O error or, more usually, beyond end of (last page of)
file. Linux just follows the standards on those.

It would be inappropriate to use anything but SIGBUS for no space.


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