From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Sat, 17 Apr 2004 00:10:09 +0100 From: Jamie Lokier Subject: Re: msync() needed before munmap() when writing to shared mapping? Message-ID: <20040416231009.GA27775@mail.shareable.org> References: <20040416220223.GA27084@mail.shareable.org> <20040416154652.7ab27e79.akpm@osdl.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20040416154652.7ab27e79.akpm@osdl.org> Sender: owner-linux-mm@kvack.org Return-Path: To: Andrew Morton Cc: linux-kernel@vger.kernel.org, linux-mm List-ID: Andrew Morton wrote: > Jamie Lokier 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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: aart@kvack.org