Received-SPF: pass (google.com: domain of linux-kernel+bounces-173460-linux.lists.archive=gmail.com@vger.kernel.org designates 147.75.199.223 as permitted sender) client-ip=147.75.199.223;
Message-ID: <90a127e5-d884-44b5-bb76-773a7485c2f1@linux.alibaba.com>
Date: Wed, 8 May 2024 23:10:46 +0800
Precedence: bulk
MIME-Version: 1.0
User-Agent: Mozilla Thunderbird
Subject: Re: [RFC PATCH] mm/vmalloc: fix vmalloc which may return null if
 called with __GFP_NOFAIL
To: Hailong Liu <hailong.liu@oppo.com>
Cc: akpm@linux-foundation.org, Michal Hocko <mhocko@suse.com>,
 urezki@gmail.com, hch@infradead.org, lstoakes@gmail.com, 21cnbao@gmail.com,
 linux-mm@kvack.org, linux-kernel@vger.kernel.org, xiang@kernel.org,
 chao@kernel.org, Oven <liyangouwen1@oppo.com>
References: <20240508125808.28882-1-hailong.liu@oppo.com>
 <404add29-2d3f-45db-9103-0c5b66fb254e@linux.alibaba.com>
 <20240508144321.ymnhn54daaabalhe@oppo.com>
From: Gao Xiang <hsiangkao@linux.alibaba.com>
In-Reply-To: <20240508144321.ymnhn54daaabalhe@oppo.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit

Hi,

On 2024/5/8 22:43, Hailong Liu wrote:
> On Wed, 08. May 21:41, Gao Xiang wrote:
>>
>> +Cc Michal,
>>
>> On 2024/5/8 20:58, hailong.liu@oppo.com wrote:
>>> From: "Hailong.Liu" <hailong.liu@oppo.com>
>>>
>>> Commit a421ef303008 ("mm: allow !GFP_KERNEL allocations for kvmalloc")
>>> includes support for __GFP_NOFAIL, but it presents a conflict with
>>> commit dd544141b9eb ("vmalloc: back off when the current task is
>>> OOM-killed"). A possible scenario is as belows:
>>>
>>> process-a
>>> kvcalloc(n, m, GFP_KERNEL | __GFP_NOFAIL)
>>>       __vmalloc_node_range()
>>> 	__vmalloc_area_node()
>>> 	    vm_area_alloc_pages()
>>>               --> oom-killer send SIGKILL to process-a
>>>               if (fatal_signal_pending(current)) break;
>>> --> return NULL;
>>>
>>> to fix this, do not check fatal_signal_pending() in vm_area_alloc_pages()
>>> if __GFP_NOFAIL set.
>>>
>>> Reported-by: Oven <liyangouwen1@oppo.com>
>>> Signed-off-by: Hailong.Liu <hailong.liu@oppo.com>
>>
>> Why taging this as RFC here?  It seems a corner-case fix of
>> commit a421ef303008
>>
>> Thanks,
>> Gao Xiang
>>
> 
> Hi Gao Xiang:
> 
> RFC here to wait for a better way to handle this case :).
> IMO, if vmalloc support __GFP_NOFAIL it should not return
> null even system is deadlock on memory.

The starting point is that kmalloc doesn't support __GFP_NOFAIL
if order > 1 (even for very short temporary uses), see:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/mm/page_alloc.c?h=v6.8#n2896

but it is possible if we have such page pointer array (since two
(order-1) pages can only keep 1024 8-byte entries, it can happen
if compression ratios are high), and kvmalloc(__GFP_NOFAIL) has
already been supported for almost two years, it will fallback to
order-0 allocation as described in commit e9c3cda4d86e
("mm, vmalloc: fix high order __GFP_NOFAIL allocations").

With my limited understanding, I'm not sure why it can cause
deadlock here since it will fallback to order-0 allocation then,
and such allocation is just for short temporary uses again
because kmalloc doesn't support order > 1 short memory
allocation strictly.

Thanks,
Gao Xiang

> 
> --
> 
> Best Regards,
> Hailong.