linux-kernel - Re: [PATCH mm] mm: fix BUG with kvzalloc+GFP

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Date:   Mon, 26 Sep 2022 17:03:48 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Uladzislau Rezki <urezki@...il.com>,
        Florian Westphal <fw@...len.de>
Cc:     Michal Hocko <mhocko@...e.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
        netdev@...r.kernel.org, netfilter-devel@...r.kernel.org,
        Martin Zaharinov <micron10@...il.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: Re: [PATCH mm] mm: fix BUG with kvzalloc+GFP_ATOMIC

On 9/23/22 17:10, Uladzislau Rezki wrote:
> On Fri, Sep 23, 2022 at 04:54:09PM +0200, Florian Westphal wrote:
>> Uladzislau Rezki <urezki@...il.com> wrote:
>>> On Fri, Sep 23, 2022 at 03:35:12PM +0200, Florian Westphal wrote:
>>>> Michal Hocko <mhocko@...e.com> wrote:
>>>>> On Fri 23-09-22 12:38:58, Florian Westphal wrote:
>>>>>> Martin Zaharinov reports BUG() in mm land for 5.19.10 kernel:
>>>>>>   kernel BUG at mm/vmalloc.c:2437!
>>>>>>   invalid opcode: 0000 [#1] SMP
>>>>>>   CPU: 28 PID: 0 Comm: swapper/28 Tainted: G        W  O      5.19.9 #1
>>>>>>   [..]
>>>>>>   RIP: 0010:__get_vm_area_node+0x120/0x130
>>>>>>    __vmalloc_node_range+0x96/0x1e0
>>>>>>    kvmalloc_node+0x92/0xb0
>>>>>>    bucket_table_alloc.isra.0+0x47/0x140
>>>>>>    rhashtable_try_insert+0x3a4/0x440
>>>>>>    rhashtable_insert_slow+0x1b/0x30
>>>>>>   [..]
>>>>>>
>>>>>> bucket_table_alloc uses kvzallocGPF_ATOMIC).  If kmalloc fails, this now
>>>>>> falls through to vmalloc and hits code paths that assume GFP_KERNEL.
>>>>>>
>>>>>> Revert the problematic change and stay with slab allocator.
>>>>>
>>>>> Why don't you simply fix the caller?
>>>>
>>>> Uh, not following?
>>>>
>>>> kvzalloc(GFP_ATOMIC) was perfectly fine, is this illegal again?
>>>>
>>> <snip>
>>> static struct vm_struct *__get_vm_area_node(unsigned long size,
>>> 		unsigned long align, unsigned long shift, unsigned long flags,
>>> 		unsigned long start, unsigned long end, int node,
>>> 		gfp_t gfp_mask, const void *caller)
>>> {
>>> 	struct vmap_area *va;
>>> 	struct vm_struct *area;
>>> 	unsigned long requested_size = size;
>>>
>>> 	BUG_ON(in_interrupt());
>>> ...
>>> <snip>
>>>
>>> vmalloc is not supposed to be called from the IRQ context.
>>
>> It uses kvzalloc, not vmalloc api.
>>
>> Before 2018, rhashtable did use kzalloc OR kvzalloc, depending on gfp_t.
>>
>> Quote from 93f976b5190df327939 changelog:
>>    As of ce91f6ee5b3b ("mm: kvmalloc does not fallback to vmalloc for
>>    incompatible gfp flags") we can simplify the caller
>>    and trust kvzalloc() to just do the right thing.
>>
>> I fear that if this isn't allowed it will result in hard-to-spot bugs
>> because things will work fine until a fallback to vmalloc happens.
>>
>> rhashtable may not be the only user of kvmalloc api that rely on
>> ability to call it from (soft)irq.
>>
> Doing the "p = kmalloc(sizeof(*p), GFP_ATOMIC);" from an atomic context
> is also a problem nowadays. Such code should be fixed across the kernel
> because of PREEMPT_RT support.

But the "atomic context" here is different, no? Calling kmalloc() from 
IRQ handlers AFAIK is ok as IRQ handlers are threaded on PREEMPT_RT. 
Calling it inside an local_irq_disable() would be a problem on the other 
hand. But then under e.g. spin_lock_irqsave() could be ok as those don't 
really disable irqs on RT.

> --
> Uladzislau Rezki