linux-kernel - Re: [PATCH] zram: Using GFP_ATOMIC instead of GFP_KERNEL to allocate bitmap memory in backing_dev

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Date:   Sat, 2 Dec 2023 21:54:38 +0800
From:   Dongyun Liu <dongyun.liu3@...il.com>
To:     Jens Axboe <axboe@...nel.dk>, minchan@...nel.org,
        senozhatsky@...omium.org
Cc:     linux-kernel@...r.kernel.org, linux-block@...r.kernel.org,
        lincheng.yang@...nssion.com, jiajun.ling@...nssion.com,
        ldys2014@...mail.com, Dongyun Liu <dongyun.liu@...nssion.com>
Subject: Re: [PATCH] zram: Using GFP_ATOMIC instead of GFP_KERNEL to allocate
 bitmap memory in backing_dev_store



On 2023/12/1 22:19, Jens Axboe wrote:
> On 11/30/23 11:51 PM, Dongyun Liu wrote:
>>
>>
>> On 2023/11/30 23:37, Jens Axboe wrote:
>>> On 11/30/23 8:20 AM, Dongyun Liu wrote:
>>>> diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
>>>> index d77d3664ca08..ee6c22c50e09 100644
>>>> --- a/drivers/block/zram/zram_drv.c
>>>> +++ b/drivers/block/zram/zram_drv.c
>>>> @@ -514,7 +514,7 @@ static ssize_t backing_dev_store(struct device *dev,
>>>>          nr_pages = i_size_read(inode) >> PAGE_SHIFT;
>>>>        bitmap_sz = BITS_TO_LONGS(nr_pages) * sizeof(long);
>>>> -    bitmap = kvzalloc(bitmap_sz, GFP_KERNEL);
>>>> +    bitmap = kmalloc(bitmap_sz, GFP_ATOMIC);
>>>>        if (!bitmap) {
>>>>            err = -ENOMEM;
>>>>            goto out;
>>>
>>> Outside of this moving from a zeroed alloc to one that does not, the
>>> change looks woefully incomplete. Why does this allocation need to be
>>> GFP_ATOMIC, and:
>>
>> By using GFP_ATOMIC, it indicates that the caller cannot reclaim or
>> sleep, although we can prevent the risk of  deadlock when acquiring
>> the zram->lock again in zram_bvec_write.
> 
> Yes, I am very much aware of how gfp allocation flags work and how why
> it's broken. It was a rhetorical question as to why you think you could
> get away with just fixing one of them.
> 
>>> 1) file_name = kmalloc(PATH_MAX, GFP_KERNEL); does not
>>
>> There is no zram->init_lock held here, so there is no need to use
>> GFP_ATOMIC.
> 
> True
> 
>>> 2) filp_open() -> getname_kernel() -> __getname() does not
>>> 3) filp_open() -> getname_kernel() does not
>>> 4) bdev_open_by_dev() does not
>>
>> Missing the use of GFP_ATOMIC.
> 
> Indeed!
> 
>>> IOW, you have a slew of GFP_KERNEL allocations in there, and you
>>> probably just patched the largest one. But the core issue remains.
>>>
>>> The whole handling of backing_dev_store() looks pretty broken.
>>>
>>
>> Indeed, this patch only solves the biggest problem and does not
>> fundamentally solve it, because there are many processes for holding
>> zram->init_lock before allocation memory in backing_dev_store that
>> need to be fully modified, and I did not consider it thoroughly.
>> Obviously, a larger and better patch is needed to eliminate this risk,
>> but it is currently not necessary.
> 
> You agree that it doesn't fix the issue, it just happens to fix the one
> that you hit. And then you jump to the conclusion that this is all
> that's needed to fix it. Ehm, confused?
> 

Hi, Jens, Maybe there's something wrong with my expression. You can 
think of it this way: I agree with you that it doesn't fix the issue.