From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Date: Tue, 3 Oct 2017 07:57:32 -0700 From: Christoph Hellwig To: Nicolas Pitre Cc: Richard Weinberger , Christoph Hellwig , Alexander Viro , "linux-mm@kvack.org" , linux-fsdevel , "linux-embedded@vger.kernel.org" , LKML , Chris Brandt Subject: Re: [PATCH v4 4/5] cramfs: add mmap support Message-ID: <20171003145732.GA8890@infradead.org> References: <20170927233224.31676-1-nicolas.pitre@linaro.org> <20170927233224.31676-5-nicolas.pitre@linaro.org> <20171001083052.GB17116@infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: owner-linux-mm@kvack.org List-ID: On Mon, Oct 02, 2017 at 07:33:29PM -0400, Nicolas Pitre wrote: > On Tue, 3 Oct 2017, Richard Weinberger wrote: > > > On Mon, Oct 2, 2017 at 12:29 AM, Nicolas Pitre wrote: > > > On Sun, 1 Oct 2017, Christoph Hellwig wrote: > > > > > >> up_read(&mm->mmap_sem) in the fault path is a still a complete > > >> no-go, > > >> > > >> NAK > > > > > > Care to elaborate? > > > > > > What about mm/filemap.c:__lock_page_or_retry() then? > > > > As soon you up_read() in the page fault path other tasks will race > > with you before > > you're able to grab the write lock. > > But I _know_ that. > > Could you highlight an area in my code where this is not accounted for? Existing users of lock_page_or_retry return VM_FAULT_RETRY right after up()ing mmap_sem, and they must already have a reference to the page which is the only thing touched until then. Your patch instead goes for an exclusive mmap_sem if it can, and even if there is nothing that breaks with that scheme right now there s nothing documenting that this actually safe, and we are way down in the complex page fault path. -- 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