Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id ; Tue, 17 Sep 2002 18:00:45 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id ; Tue, 17 Sep 2002 18:00:45 -0400 Received: from e3.ny.us.ibm.com ([32.97.182.103]:35061 "EHLO e3.ny.us.ibm.com") by vger.kernel.org with ESMTP id ; Tue, 17 Sep 2002 18:00:41 -0400 Date: Tue, 17 Sep 2002 15:02:15 -0700 From: "Martin J. Bligh" To: "Juan M. de la Torre" , linux-kernel@vger.kernel.org Subject: Re: Possible bug in __alloc_pages() ? Message-ID: <132900000.1032300135@flay> In-Reply-To: <20020917214804.GA891@apocalipsis> References: <20020917214804.GA891@apocalipsis> X-Mailer: Mulberry/2.1.2 (Linux/x86) MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Content-Disposition: inline Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3365 Lines: 99 This is deliberate, and is to discourage fallback. I'm not desperately fond of the method, but I'm told it's not an accidental typo / bug. M. --On Tuesday, September 17, 2002 23:48:04 +0200 "Juan M. de la Torre" wrote: > > Hi, this code appears at the beggining of __page_alloc() (kernel 2.4.19): > > min = 1UL << order; > for (;;) { > zone_t *z = *(zone++); > if (!z) > break; > > min += z->pages_low; > if (z->free_pages > min) { > page = rmqueue(z, order); > if (page) > return page; > } > } > > AFAIK, what this code does is to try to alloc the requested pages from > the first zone in a zone_list (passed as argument) which have enought free > pages. > > A zone is considered to have enought free pages if z->free_pages is greater > than (number_of_requested_pages + z->pages_low). > > In the loop shown, the first iteration is OK, but in the second iteration > (which only occurs if the first zone in the zone_list hasn't enought free > pages) the zone will only be considered to have enought free pages if > z->free_pages is greater that (number_of_requested_pages + z->pages_low > + PREV_ZONE->pages_low). > > I think this is a bug, but i'm not sure (i'm not a VM hacker). > > If it is a bug, there are other two loops in the same function which > are buggy. > > > Possible patch: > > --- linux/mm/page_alloc.c.orig Tue Sep 17 23:45:02 2002 > +++ linux/mm/page_alloc.c Tue Sep 17 23:46:45 2002 > @@ -330,8 +330,7 @@ > if (!z) > break; > > - min += z->pages_low; > - if (z->free_pages > min) { > + if (z->free_pages > min + z->pages_low) { > page = rmqueue(z, order); > if (page) > return page; > @@ -354,8 +353,8 @@ > local_min = z->pages_min; > if (!(gfp_mask & __GFP_WAIT)) > local_min >>= 2; > - min += local_min; > - if (z->free_pages > min) { > + > + if (z->free_pages > min + local_min) { > page = rmqueue(z, order); > if (page) > return page; > @@ -394,8 +393,7 @@ > if (!z) > break; > > - min += z->pages_min; > - if (z->free_pages > min) { > + if (z->free_pages > min + z->pages_min) { > page = rmqueue(z, order); > if (page) > return page; > > Regards, > Juanma > > -- > /jm > > - > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ > > - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/