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Thu, 17 Jun 2021 06:33:02 +0000 (UTC) Received: from [192.168.0.100] (unknown [49.204.182.129]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) (Authenticated sender: faiyazm) by smtp.codeaurora.org (Postfix) with ESMTPSA id 4F8ECC433F1; Thu, 17 Jun 2021 06:32:55 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 smtp.codeaurora.org 4F8ECC433F1 Authentication-Results: aws-us-west-2-caf-mail-1.web.codeaurora.org; dmarc=none (p=none dis=none) header.from=codeaurora.org Authentication-Results: aws-us-west-2-caf-mail-1.web.codeaurora.org; spf=fail smtp.mailfrom=faiyazm@codeaurora.org Subject: Re: [PATCH v12] mm: slub: move sysfs slab alloc/free interfaces to debugfs To: Vlastimil Babka , Qian Cai , cl@linux.com, penberg@kernel.org, rientjes@google.com, iamjoonsoo.kim@lge.com, akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, greg@kroah.com, glittao@gmail.com, andy.shevchenko@gmail.com Cc: vinmenon@codeaurora.org, Catalin Marinas , Alexander Viro , Linux-FSDevel References: <1623438200-19361-1-git-send-email-faiyazm@codeaurora.org> <8c821abf-8fa6-b78b-cea4-b7d3b3b74a69@quicinc.com> <25d59ad1-4d21-181c-afc2-8f396672bfd1@codeaurora.org> <21ccb5c6-2aee-f223-cd45-52b78e1f8640@suse.cz> From: Faiyaz Mohammed Message-ID: <042f92b2-041d-b4c4-5419-2c4c4799e6d5@codeaurora.org> Date: Thu, 17 Jun 2021 12:02:53 +0530 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 MIME-Version: 1.0 In-Reply-To: <21ccb5c6-2aee-f223-cd45-52b78e1f8640@suse.cz> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 6/16/2021 9:47 PM, Vlastimil Babka wrote: > On 6/16/21 5:50 PM, Faiyaz Mohammed wrote: >> >> >> On 6/16/2021 4:35 PM, Vlastimil Babka wrote: >>> On 6/15/21 5:58 PM, Qian Cai wrote: >>>> >>>> >>>> On 6/11/2021 3:03 PM, Faiyaz Mohammed wrote: >>>>> alloc_calls and free_calls implementation in sysfs have two issues, >>>>> one is PAGE_SIZE limitation of sysfs and other is it does not adhere >>>>> to "one value per file" rule. >>>>> >>>>> To overcome this issues, move the alloc_calls and free_calls >>>>> implementation to debugfs. >>>>> >>>>> Debugfs cache will be created if SLAB_STORE_USER flag is set. >>>>> >>>>> Rename the alloc_calls/free_calls to alloc_traces/free_traces, >>>>> to be inline with what it does. >>>>> >>>>> Reviewed-by: Vlastimil Babka >>>>> Reviewed-by: Greg Kroah-Hartman >>>>> Signed-off-by: Faiyaz Mohammed >>>> >>>> Reverting this commit on today's linux-next fixed all leaks (hundreds) reported by kmemleak like below, >>>> >>>> unreferenced object 0xffff00091ae1b540 (size 64): >>>> comm "lsbug", pid 1607, jiffies 4294958291 (age 1476.340s) >>>> hex dump (first 32 bytes): >>>> 02 00 00 00 00 00 00 00 6b 6b 6b 6b 6b 6b 6b 6b ........kkkkkkkk >>>> 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk >>>> backtrace: >>>> [] slab_post_alloc_hook+0xa0/0x418 >>>> [] kmem_cache_alloc_trace+0x1e4/0x378 >>>> [] slab_debugfs_start+0x30/0x50 >>>> slab_debugfs_start at /usr/src/linux-next/mm/slub.c:5831 >>>> [] seq_read_iter+0x214/0xd50 >>>> [] seq_read+0x28c/0x418 >>>> [] full_proxy_read+0xdc/0x148 >>>> [] vfs_read+0x104/0x340 >>>> [] ksys_read+0xf8/0x1e0 >>>> [] __arm64_sys_read+0x74/0xa8 >>>> [] invoke_syscall.constprop.0+0xdc/0x1d8 >>>> [] do_el0_svc+0xe4/0x298 >>>> [] el0_svc+0x20/0x30 >>>> [] el0t_64_sync_handler+0xb0/0xb8 >>>> [] el0t_64_sync+0x178/0x17c >>>> >>> >>> I think the problem is here: >>> >>>>> +static void slab_debugfs_stop(struct seq_file *seq, void *v) >>>>> +{ >>>>> + kfree(v); >>>>> +} >>>>> + >>>>> +static void *slab_debugfs_next(struct seq_file *seq, void *v, loff_t *ppos) >>>>> +{ >>>>> + loff_t *spos = v; >>>>> + struct loc_track *t = seq->private; >>>>> + >>>>> + if (*ppos < t->count) { >>>>> + *ppos = ++*spos; >>>>> + return spos; >>>>> + } >>>>> + *ppos = ++*spos; >>>>> + return NULL; >>>>> +} >>> >>> If we return NULL, then NULL is passed to slab_debugfs_stop and thus we don't >>> kfree ppos. kfree(NULL) is silently ignored. >>> >> I think yes, if NULL passed to kfree, it simply do return. >>> I think as we have private struct loc_track, we can add a pos field there and >>> avoid the kmaloc/kfree altogether. >>> >> Hmm, yes we can add pos field "or" we can use argument "v" mean we can >> update v with pos in ->next() and use in ->show() to avoid the leak >> (kmalloc/kfree). > > Can you explain the "or" part more. It's exactly what we already do, no?I am thinking if we simplly do ppos return from slab_debugfs_start() and in slab_debugfs_next() assign ppos to "v", update it and return if records are there. something like below (approach 1): ... static void *slab_debugfs_next(struct seq_file *seq, void *v, loff_t *ppos) { ... v = ppos; if (*ppos < t->count) { ++*ppos; return v; } ++*ppos; return NULL; } ... static void *slab_debugfs_start(struct seq_file *seq, loff_t *ppos) { return ppos; } ... > "v" as you said. The problem is, if next(); returns NULL, then stop() gets the > NULL as "v". It's just what I see in the code of seq_read_iter() and traverse() > in fs/seq_file.c. I don't see another way to say there are no more records to > print - only to return NULL in next(). > Ah, ok so we could maybe do the kfree() in next() then before returning NULL, > which is the last moment we have the pointer. But really, if we already have a > loc_track in private, why kmalloc an additional loff_t. > Yes, we can do kfree() before returning NULL, but better to add ppos in lock_track. (approach 2) > Anyway it seems to me also that > Documentation/filesystems/seq_file.rst should be updated, as the kfree() in > stop() is exactly what it suggests, and it doesn't show how next() indicates > that there are no more records by returning NULL, and what to do about kfree() then. Can you please suggest me which approach would be good to avoid the leak?. I will update in next patch version.