From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Fri, 16 Apr 2004 16:59:36 -0700 From: Andrew Morton Subject: Re: msync() needed before munmap() when writing to shared mapping? Message-Id: <20040416165936.7fd9f5e1.akpm@osdl.org> In-Reply-To: <20040416231009.GA27775@mail.shareable.org> References: <20040416220223.GA27084@mail.shareable.org> <20040416154652.7ab27e79.akpm@osdl.org> <20040416231009.GA27775@mail.shareable.org> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org Return-Path: To: Jamie Lokier Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org List-ID: Jamie Lokier wrote: > > ... > 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? Sure. If some other process is using the same pages we don't go toss them away. > 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). Seems that way - MS_INVALIDATE will simply propagate pte dirtiness into page dirtiness. For non-file-backed mappings it is a no-op. > Is there a call which does what the command about MS_INVALIDATE says, > i.e. free clean pages and flush dirty ones? Not really. What is a clean anonymous page? If it's ever been written to, it's conceptually dirty, whether or not it is physically dirty. ie: if you invalidate it, you've lost your data. I guess you could get a similar result by munmap() and then mmapping it again. -- 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