linux-kernel - Re: [PATCH v12] mm: slub: move sysfs slab alloc/free interfaces to debugfs

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Message-ID: <ce1b3c14-ec88-c957-0694-834051d4d39e@suse.cz>
Date:   Wed, 16 Jun 2021 13:05:45 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Qian Cai <quic_qiancai@...cinc.com>,
        Faiyaz Mohammed <faiyazm@...eaurora.org>, cl@...ux.com,
        penberg@...nel.org, rientjes@...gle.com, iamjoonsoo.kim@....com,
        akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, greg@...ah.com, glittao@...il.com,
        andy.shevchenko@...il.com
Cc:     vinmenon@...eaurora.org, Catalin Marinas <catalin.marinas@....com>
Subject: Re: [PATCH v12] mm: slub: move sysfs slab alloc/free interfaces to
 debugfs

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 <vbabka@...e.cz>
>> Reviewed-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
>> Signed-off-by: Faiyaz Mohammed <faiyazm@...eaurora.org>
> 
> 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:
>     [<ffff8000106b06b8>] slab_post_alloc_hook+0xa0/0x418
>     [<ffff8000106b5c7c>] kmem_cache_alloc_trace+0x1e4/0x378
>     [<ffff8000106b5e40>] slab_debugfs_start+0x30/0x50
>     slab_debugfs_start at /usr/src/linux-next/mm/slub.c:5831
>     [<ffff8000107b3dbc>] seq_read_iter+0x214/0xd50
>     [<ffff8000107b4b84>] seq_read+0x28c/0x418
>     [<ffff8000109560b4>] full_proxy_read+0xdc/0x148
>     [<ffff800010738f24>] vfs_read+0x104/0x340
>     [<ffff800010739ee0>] ksys_read+0xf8/0x1e0
>     [<ffff80001073a03c>] __arm64_sys_read+0x74/0xa8
>     [<ffff8000100358d4>] invoke_syscall.constprop.0+0xdc/0x1d8
>     [<ffff800010035ab4>] do_el0_svc+0xe4/0x298
>     [<ffff800011138528>] el0_svc+0x20/0x30
>     [<ffff800011138b08>] el0t_64_sync_handler+0xb0/0xb8
>     [<ffff80001001259c>] 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 as we have private struct loc_track, we can add a pos field there and
avoid the kmaloc/kfree altogether.