linux-kernel - Re: [PATCH] f2fs: set deadline to drop expired inmem pages

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Date:   Tue, 17 Apr 2018 14:44:22 +0800
From:   Chao Yu <yuchao0@...wei.com>
To:     Jaegeuk Kim <jaegeuk@...nel.org>
CC:     <linux-f2fs-devel@...ts.sourceforge.net>,
        <linux-kernel@...r.kernel.org>, <chao@...nel.org>
Subject: Re: [PATCH] f2fs: set deadline to drop expired inmem pages

On 2018/4/17 4:16, Jaegeuk Kim wrote:
> On 04/13, Chao Yu wrote:
>> On 2018/4/13 12:05, Jaegeuk Kim wrote:
>>> On 04/13, Chao Yu wrote:
>>>> On 2018/4/13 9:04, Jaegeuk Kim wrote:
>>>>> On 04/10, Chao Yu wrote:
>>>>>> Hi Jaegeuk,
>>>>>>
>>>>>> On 2018/4/8 16:13, Chao Yu wrote:
>>>>>>> f2fs doesn't allow abuse on atomic write class interface, so except
>>>>>>> limiting in-mem pages' total memory usage capacity, we need to limit
>>>>>>> start-commit time as well, otherwise we may run into infinite loop
>>>>>>> during foreground GC because target blocks in victim segment are
>>>>>>> belong to atomic opened file for long time.
>>>>>>>
>>>>>>> Now, we will check the condition with f2fs_balance_fs_bg in
>>>>>>> background threads, once if user doesn't commit data exceeding 30
>>>>>>> seconds, we will drop all cached data, so I expect it can keep our
>>>>>>> system running safely to prevent Dos attack.
>>>>>>
>>>>>> Is it worth to add this patch to avoid abuse on atomic write interface by user?
>>>>>
>>>>> Hmm, hope to see a real problem first in this case.
>>>>
>>>> I think this can be a more critical security leak instead of a potential issue
>>>> which we can wait for someone reporting that can be too late.
>>>>
>>>> For example, user can simply write a huge file whose data spread in all f2fs
>>>> segments, once user open that file as atomic, foreground GC will suffer
>>>> deadloop, causing denying any further service of f2fs.
>>>
>>> How can you guarantee it won't happen within 30sec? If you want to avoid that,
>>
>> Now the value is smaller than generic hang task threshold in order to avoid
>> foreground GC helding gc_mutex too long, we can tune that parameter?
>>
>>> you have to take a look at foreground gc.
>>
>> What do you mean? let GC moves blocks of atomic write opened file?
> 
> I thought that we first need to detect when foreground GC is stuck by such the
> huge number of atomic writes. Then, we need to do something like dropping all
> the atomic writes.

Yup, that will be reasonable. :)

Thanks,

> 
>>
>> Thanks,
>>
>>>
>>>>
>>>> Thanks,
>>>>
>>>>>
>>>>>> Thanks,
>>>>>
>>>>> .
>>>>>
>>>
>>> .
>>>
> 
> .
>