From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1033358AbbKEQIr (ORCPT ); Thu, 5 Nov 2015 11:08:47 -0500 Received: from foss.arm.com ([217.140.101.70]:37856 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1033233AbbKEQIo (ORCPT ); Thu, 5 Nov 2015 11:08:44 -0500 Date: Thu, 5 Nov 2015 16:08:40 +0000 From: Catalin Marinas To: Andrew Morton Cc: linux-kernel@vger.kernel.org, Pekka Enberg , linux-mm@kvack.org, Geert Uytterhoeven , David Rientjes , Christoph Lameter , Joonsoo Kim , linux-arm-kernel@lists.infradead.org Subject: Re: [PATCH] mm: slab: Only move management objects off-slab for sizes larger than KMALLOC_MIN_SIZE Message-ID: <20151105160839.GR7637@e104818-lin.cambridge.arm.com> References: <20151105043155.GA20374@js1304-P5Q-DELUXE> <1446724235-31400-1-git-send-email-catalin.marinas@arm.com> <20151105053139.e38214a9.akpm@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20151105053139.e38214a9.akpm@linux-foundation.org> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Nov 05, 2015 at 05:31:39AM -0800, Andrew Morton wrote: > On Thu, 5 Nov 2015 11:50:35 +0000 Catalin Marinas wrote: > > > Commit 8fc9cf420b36 ("slab: make more slab management structure off the > > slab") enables off-slab management objects for sizes starting with > > PAGE_SIZE >> 5. This means 128 bytes for a 4KB page configuration. > > However, on systems with a KMALLOC_MIN_SIZE of 128 (arm64 in 4.4), such > > optimisation does not make sense since the slab management allocation > > would take 128 bytes anyway (even though freelist_size is 32) with the > > additional overhead of another allocation. > > > > This patch introduces an OFF_SLAB_MIN_SIZE macro which takes > > KMALLOC_MIN_SIZE into account. It also solves a slab bug on arm64 where > > the first kmalloc_cache to be initialised after slab_early_init = 0, > > "kmalloc-128", fails to allocate off-slab management objects from the > > same "kmalloc-128" cache. > > That all seems to be quite minor stuff. Apart from "it also solves a bug on arm64...". But I agree, the initial commit log doesn't give any justification for cc stable. > > Fixes: 8fc9cf420b36 ("slab: make more slab management structure off the slab") > > Cc: # 3.15+ > > Yet you believe the fix should be backported. > > So, the usual refrain: when fixing a bug, please describe the end-user > visible effects of that bug. What about (unless you prefer this slightly more intrusive fix: http://article.gmane.org/gmane.linux.ports.sh.devel/50303): ------------------8<-------------------------- >>From fda8f306b6941f4ddbefcbcfaa59fedef4a679a3 Mon Sep 17 00:00:00 2001 From: Catalin Marinas Date: Thu, 5 Nov 2015 11:14:48 +0000 Subject: [PATCH] mm: slab: Only move management objects off-slab for sizes larger than KMALLOC_MIN_SIZE On systems with a KMALLOC_MIN_SIZE of 128 (arm64, some mips and powerpc configurations defining ARCH_DMA_MINALIGN to 128), the first kmalloc_caches[] entry to be initialised after slab_early_init = 0 is "kmalloc-128" with index 7. Depending on the debug kernel configuration, sizeof(struct kmem_cache) can be larger than 128 resulting in an INDEX_NODE of 8. Commit 8fc9cf420b36 ("slab: make more slab management structure off the slab") enables off-slab management objects for sizes starting with PAGE_SIZE >> 5 (128 bytes for a 4KB page configuration) and the creation of the "kmalloc-128" cache would try to place the management objects off-slab. However, since KMALLOC_MIN_SIZE is already 128 and freelist_size == 32 in __kmem_cache_create(), kmalloc_slab(freelist_size) returns NULL (kmalloc_caches[7] not populated yet). This triggers the following bug on arm64: [ 0.000000] kernel BUG at /work/Linux/linux-2.6-aarch64/mm/slab.c:2283! [ 0.000000] Internal error: Oops - BUG: 0 [#1] SMP [ 0.000000] Modules linked in: [ 0.000000] CPU: 0 PID: 0 Comm: swapper Not tainted 4.3.0-rc4+ #540 [ 0.000000] Hardware name: Juno (DT) [ 0.000000] task: ffffffc0006962b0 ti: ffffffc00068c000 task.ti: ffffffc00068c000 [ 0.000000] PC is at __kmem_cache_create+0x21c/0x280 [ 0.000000] LR is at __kmem_cache_create+0x210/0x280 [...] [ 0.000000] Call trace: [ 0.000000] [] __kmem_cache_create+0x21c/0x280 [ 0.000000] [] create_boot_cache+0x48/0x80 [ 0.000000] [] create_kmalloc_cache+0x50/0x88 [ 0.000000] [] create_kmalloc_caches+0x4c/0xf4 [ 0.000000] [] kmem_cache_init+0x100/0x118 [ 0.000000] [] start_kernel+0x214/0x33c This patch introduces an OFF_SLAB_MIN_SIZE definition to avoid off-slab management objects for sizes equal to or smaller than KMALLOC_MIN_SIZE. Fixes: 8fc9cf420b36 ("slab: make more slab management structure off the slab") Cc: # 3.15+ Reported-by: Geert Uytterhoeven Cc: Christoph Lameter Cc: Pekka Enberg Cc: David Rientjes Cc: Joonsoo Kim Cc: Andrew Morton Signed-off-by: Catalin Marinas --- mm/slab.c | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-) diff --git a/mm/slab.c b/mm/slab.c index 4fcc5dd8d5a6..419b649b395e 100644 --- a/mm/slab.c +++ b/mm/slab.c @@ -282,6 +282,7 @@ static void kmem_cache_node_init(struct kmem_cache_node *parent) #define CFLGS_OFF_SLAB (0x80000000UL) #define OFF_SLAB(x) ((x)->flags & CFLGS_OFF_SLAB) +#define OFF_SLAB_MIN_SIZE (max_t(size_t, PAGE_SIZE >> 5, KMALLOC_MIN_SIZE + 1)) #define BATCHREFILL_LIMIT 16 /* @@ -2212,7 +2213,7 @@ __kmem_cache_create (struct kmem_cache *cachep, unsigned long flags) * it too early on. Always use on-slab management when * SLAB_NOLEAKTRACE to avoid recursive calls into kmemleak) */ - if ((size >= (PAGE_SIZE >> 5)) && !slab_early_init && + if (size >= OFF_SLAB_MIN_SIZE && !slab_early_init && !(flags & SLAB_NOLEAKTRACE)) /* * Size is large, assume best to place the slab management obj @@ -2276,7 +2277,7 @@ __kmem_cache_create (struct kmem_cache *cachep, unsigned long flags) /* * This is a possibility for one of the kmalloc_{dma,}_caches. * But since we go off slab only for object size greater than - * PAGE_SIZE/8, and kmalloc_{dma,}_caches get created + * OFF_SLAB_MIN_SIZE, and kmalloc_{dma,}_caches get created * in ascending order,this should not happen at all. * But leave a BUG_ON for some lucky dude. */