Re: msync() needed before munmap() when writing to shared mapping?

Jamie Lokier
2004-04-16T23:10:09Z

Andrew Morton wrote:
> Jamie Lokier <jamie@shareable.org> wrote:
> > I've followed the logic from do_munmap() and it looks good:
> > unmap_vmas->zap_pte_range->page_remove_rmap->set_page_dirty.
> >
> > Can someone confirm this is correct, please?
>
> yup, zap_pte_range() transfers pte dirtiness into pagecache dirtiness when
> tearing down the mapping, leaving the dirty page floating about in
> pagecache for kupdate/kswapd/fsync to catch. Longstanding behaviour.

Thanks.

A related question. The comment for MADV_DONTNEED says:

* NB: This interface discards data rather than pushes it out to swap,
* as some implementations do. This has performance implications for
* applications like large transactional databases which want to discard
* pages in anonymous maps after committing to backing store the data
* that was kept in them. There is no reason to write this data out to
* the swap area if the application is discarding it.
*
* An interface that causes the system to free clean pages and flush
* dirty pages is already available as msync(MS_INVALIDATE).

MADV_DONTNEED calls zap_page_range().
That propagates dirtiness into the pagecache.

So it *doesn't* "discard data rather than push it out to swap", if the
same dirty data is mapped elsewhere e.g. as a shared anonymous
mapping, does it?

The comment also mentions MS_INVALIDATE, but MS_INVALIDATE doesn't do
what the comment says and doesn't implement anything like POSIX
either. (Linux's MS_INVALIDATE is practically equivalent to MS_ASYNC).

Is there a call which does what the command about MS_INVALIDATE says,
i.e. free clean pages and flush dirty ones?

Thanks,
-- Jamie
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