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Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.4.0 Subject: Re: [PATCH v6 1/4] mm/slub: enable debugging memory wasting of kmalloc To: Feng Tang Cc: John Thomson , Hyeonggon Yoo <42.hyeyoo@gmail.com>, Andrew Morton , Christoph Lameter , Pekka Enberg , David Rientjes , Joonsoo Kim , Roman Gushchin , Dmitry Vyukov , Jonathan Corbet , Andrey Konovalov , "Hansen, Dave" , "linux-mm@kvack.org" , "linux-kernel@vger.kernel.org" , "kasan-dev@googlegroups.com" , Robin Murphy , John Garry , Kefeng Wang , Thomas Bogendoerfer , "linux-mips@vger.kernel.org" References: <9b71ae3e-7f53-4c9e-90c4-79d3d649f94c@app.fastmail.com> <53b53476-bb1e-402e-9f65-fd7f0ecf94c2@app.fastmail.com> <29271a2b-cf19-4af9-bfe5-5bcff8a23fda@app.fastmail.com> <097d8fba-bd10-a312-24a3-a4068c4f424c@suse.cz> Content-Language: en-US From: Vlastimil Babka In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit ARC-Authentication-Results: i=1; imf11.hostedemail.com; dkim=pass header.d=suse.cz header.s=susede2_rsa header.b=mjIZMVd0; 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dkim=pass header.d=suse.cz header.s=susede2_rsa header.b=mjIZMVd0; dkim=pass header.d=suse.cz header.s=susede2_ed25519 header.b=mMWuZH57; spf=pass (imf11.hostedemail.com: domain of vbabka@suse.cz designates 195.135.220.28 as permitted sender) smtp.mailfrom=vbabka@suse.cz; dmarc=none X-Rspamd-Server: rspam01 X-HE-Tag: 1667464410-214164 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 11/3/22 06:54, Feng Tang wrote: > On Wed, Nov 02, 2022 at 04:22:37PM +0800, Vlastimil Babka wrote: >> On 11/1/22 11:33, John Thomson wrote: > [...] >> > >> > [ 0.000000] Linux version 6.1.0-rc3+ (john@john) (mipsel-buildroot-linux-gnu-gcc.br_real (Buildroot 2021.11-4428-g6b6741b) 12.2.0, GNU ld (GNU Binutils) 2.39) #62 SMP Tue Nov 1 19:49:52 AEST 2022 >> > [ 0.000000] slub: __kmem_cache_alloc_lru called with kmem_cache ptr: 0x0 >> > [ 0.000000] CPU: 0 PID: 0 Comm: swapper Not tainted 6.1.0-rc3+ #62 >> > [ 0.000000] Stack : 810fff78 80084d98 80889d00 00000004 00000000 00000000 80889d5c 80c90000 >> > [ 0.000000] 80920000 807bd380 8089d368 80923bd3 00000000 00000001 80889d08 00000000 >> > [ 0.000000] 00000000 00000000 807bd380 8084bd51 00000002 00000002 00000001 6d6f4320 >> > [ 0.000000] 00000000 80c97ce9 80c97d14 fffffffc 807bd380 00000000 00000003 00000dc0 >> > [ 0.000000] 00000000 a0000000 80910000 8110a0b4 00000000 00000020 80010000 80010000 >> > [ 0.000000] ... >> > [ 0.000000] Call Trace: >> > [ 0.000000] [<80008260>] show_stack+0x28/0xf0 >> > [ 0.000000] [<8070cdc0>] dump_stack_lvl+0x60/0x80 >> > [ 0.000000] [<801c1428>] kmem_cache_alloc+0x5c0/0x740 >> > [ 0.000000] [<8092856c>] prom_soc_init+0x1fc/0x2b4 >> > [ 0.000000] [<80928060>] prom_init+0x44/0xf0 >> > [ 0.000000] [<80929214>] setup_arch+0x4c/0x6a8 >> > [ 0.000000] [<809257e0>] start_kernel+0x88/0x7c0 >> > [ 0.000000] >> > [ 0.000000] SoC Type: MediaTek MT7621 ver:1 eco:3 >> >> The stack means CONFIG_TRACING=n, is that right? > > Yes, from the kconfig, CONFIG_TRACING is not set. > >> That would mean >> prom_soc_init() >> soc_dev_init() >> kzalloc() -> kmalloc() >> kmalloc_trace() // after #else /* CONFIG_TRACING */ >> kmem_cache_alloc(s, flags); >> >> Looks like this path is a small bug in the wasting detection patch, as we >> throw away size there. > > Yes, from the code reading and log from John, it is. > > One strange thing is, I reset the code to v6.0, and found that > __kmem_cache_alloc_lru() also access the 's->object_size' > > void *__kmem_cache_alloc_lru(struct kmem_cache *s, struct list_lru *lru, > gfp_t gfpflags) > { > void *ret = slab_alloc(s, lru, gfpflags, _RET_IP_, s->object_size); > ... > } > > And from John's dump_stack() info, this call is also where the NULL pointer > happens, which I still can't figue out. > >> AFAICS before this patch, we "survive" "kmem_cache *s" being NULL as >> slab_pre_alloc_hook() will happen to return NULL and we bail out from >> slab_alloc_node(). But this is a side-effect, not an intended protection. >> Also the CONFIG_TRACING variant of kmalloc_trace() would have called >> trace_kmalloc dereferencing s->size anyway even before this patch. >> >> I don't think we should add WARNS in the slab hot paths just to prevent this >> rare error of using slab too early. At most VM_WARN... would be acceptable >> but still not necessary as crashing immediately from a NULL pointer is >> sufficient. >> >> So IMHO mips should fix their soc init, > > Yes, for the mips fix, John has proposed to defer the calling of prom_soc_init(), > which looks reasonable. > >> and we should look into the >> CONFIG_TRACING=n variant of kmalloc_trace(), to pass orig_size properly. > > You mean check if the pointer is NULL and bail out early. No I mean here: #else /* CONFIG_TRACING */ /* Save a function call when CONFIG_TRACING=n */ static __always_inline __alloc_size(3) void *kmalloc_trace(struct kmem_cache *s, gfp_t flags, size_t size) { void *ret = kmem_cache_alloc(s, flags); ret = kasan_kmalloc(s, ret, size, flags); return ret; } we call kmem_cache_alloc() and discard the size parameter, so it will assume s->object_size (and as the side-effect, crash if s is NULL). We shouldn't add "s is NULL?" checks, but fix passing the size - probably switch to __kmem_cache_alloc_node()? and in the following kmalloc_node_trace() analogically.