Messages in this thread | | | Date | Thu, 14 Jun 2012 03:07:53 -0700 | From | Andrew Morton <> | Subject | Re: [PATCH 2/2] msync: start async writeout when MS_ASYNC |
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On Thu, 14 Jun 2012 11:02:02 +0200 Paolo Bonzini <pbonzini@redhat.com> wrote:
> Il 13/06/2012 23:29, Andrew Morton ha scritto: > > > If the application does not want to start I/O, it can simply call msync > > > with flags equal to MS_INVALIDATE. This one remains a no-op, as it should > > > be on a reasonable implementation. > > > > Means that people will find that their msync(MS_ASYNC) call will newly > > start IO. This may well be undesirable for some. > > If they knew it was a no-op, and relying on it, they might as well not > have called it at all and save a syscall.
Nope. Back in the day (3+ years ago?), msync(MS_ASYNC) would flush pte-dirty bits into page->flags:PG_Dirty. So it was used to make the filesystem aware that userspace had modified some MAP_SHARED memory. The fs would then perform writeback within (typically) 30 seconds. So a legitimate use would be for the app to periodically run msync(MS_ASYNC) to avoid having modified data floating about un-written-back for arbitrarily long periods.
Nowadays we mark the pte's read-only and take a fault on first write, and mark the page dirty at that time.
So it wasn't a no-op, and with this patch, such applications will newly find themselves to be starting I/O within the msync() call.
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