From mboxrd@z Thu Jan 1 00:00:00 1970 Subject: Re: [discuss] [PATCH] Inconsistent mmap()/mremap() flags From: Thayne Harbaugh Reply-To: thayne@c2.net In-Reply-To: References: <1190958393.5128.85.camel@phantasm.home.enterpriseandprosperity.com> <1191308772.5200.66.camel@phantasm.home.enterpriseandprosperity.com> <200710021545.32556.ak@suse.de> Content-Type: text/plain Date: Tue, 02 Oct 2007 09:21:26 -0600 Message-Id: <1191338486.5200.71.camel@phantasm.home.enterpriseandprosperity.com> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org Return-Path: To: Hugh Dickins Cc: Andi Kleen , discuss@x86-64.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org List-ID: On Tue, 2007-10-02 at 15:16 +0100, Hugh Dickins wrote: > On Tue, 2 Oct 2007, Andi Kleen wrote: > > > > > First call mmap with a low hint address, the new size you'll be wanting > > > from the mremap, PROT_NONE, MAP_ANONYMOUS, -1, 0. Then call mremap with > > > old address, old size, new size, MREMAP_MAYMOVE|MREMAP_FIXED, and new > > > address as returned by the preparatory mmap. > > > > That's racy unfortunately in a multithreaded process. They would need to loop. > > Perhaps. Though I don't see what your loop would be doing; > and the mapping established by the first thread would only > be vulnerable to another thread if that were really set on > interfering (an un-FIXED mmap by another thread will keep > away from the area assigned to the first). > > Certainly a two-stage procedure has to be weaker than one stage, > but it is just how MAP_FIXED is normally used (isn't it?): first > stake out an arena for all that's needed without MAP_FIXED, then > fit into it the actual mappings required using MAP_FIXED. Blind > use of MAP_FIXED is always in danger of unmapping something vital. > > But whether the two-stage procedure is good enough for Thayne's > purpose, he'll have to judge for himself. I think my eyes have been opened enough so that I can get things to work - it's certainly better in many respects than using MAP_32BIT with its many limitations. Thank you. -- 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: email@kvack.org