Re: Any method to limit page cache usage in test cases?

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

On 2018/11/14 下午3:21, Nikolay Borisov wrote:
> 
> 
> On 14.11.18 г. 9:17 ч., Qu Wenruo wrote:
>>
>>
>> On 2018/11/14 下午3:00, Qu Wenruo wrote:
>>>
>>>
>>> On 2018/11/14 下午2:47, Nikolay Borisov wrote:
>>>>
>>>>
>>>> On 14.11.18 г. 2:31 ч., Qu Wenruo wrote:
>>>>> Hi,
>>>>>
>>>>> Is there any (easy) method for a fstests test case to limit the page
>>>>> cache usage?
>>>>>
>>>>> I triggered btrfs/139 failure with 2G vRAM VM, and located the root
>>>>> cause of the problem.
>>>>
>>>> You can always size your test vm properly. Otherwise what about the
>>>> various sysctl tuning knobs? I.e Documentation/sysctl/vm.txt explains
>>>> some of them: dirty_bytes, dirty_background_bytes,
>>>> dirty_background_ratio, dirty_expire_centisecs
>>>
>>> Thanks for the hint about vm.txt!
>>>
>>> I just realized we could just drop_caches to force dirty page writeback,
>>> without the need to tweaking the complex memory pressure/watermark
>>> mechanism.
>>
>> Well, this doesn't work as expected.
>>
>> It will cause transaction commit, seems that kernel is trying too hard
>> to free page cache.
>>
>> Is there any way to only flush dirty pages of a file?
> 
> well fdatasync causes ->fsync to run, in btrfs that will be
> btrfs_sync_file.

Yes, that's the why we can't use fsync()/fdatasync(), and I'm trying to
use drop_caches.

> One of the first thing it does is start_ordered_ops
> which does btrfs_fdatawrite_range, which in turn is filemap_fdatawrite_range

So I'm afraid I have to go the complex memory pressure/watermark method.

Thanks,
Qu

> 
> 
>>
>> Thanks,
>> Qu
>>
>>>
>>> Thanks,
>>> Qu
>>>>
>>>>
>>>> So with a 2g machine the default settings are using only a fraction of
>>>> the ram. If you adjust the same settings for the larger ram size you
>>>> should get almost identical behavior.>
>>>>
>>>>
>>>>>
>>>>> However it's only really reproducible for small ram VM, since it could
>>>>> trigger dirty page writeback due to memory pressure.
>>>>>
>>>>> So I'm wondering if we could do such thing even for large RAM test machine.
>>>>>
>>>>> Thanks,
>>>>> Qu
>>>>>
>>