Re: [PATCH] zram: Using GFP_ATOMIC instead of GFP_KERNEL to allocate bitmap memory in backing_dev_store

[Jens Axboe]

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?

-- 
Jens Axboe