Re: [PATCH v17 14/21] mm/compaction: do page isolation first in compaction

[Alex Shi]

在 2020/8/10 下午10:41, Alexander Duyck 写道:
> On Mon, Aug 10, 2020 at 6:10 AM Alex Shi <alex.shi@xxxxxxxxxxxxxxxxx> wrote:
>>
>>
>>
>> 在 2020/8/7 下午10:51, Alexander Duyck 写道:
>>> I wonder if this entire section shouldn't be restructured. This is the
>>> only spot I can see where we are resetting the LRU flag instead of
>>> pulling the page from the LRU list with the lock held. Looking over
>>> the code it seems like something like that should be possible. I am
>>> not sure the LRU lock is really protecting us in either the
>>> PageCompound check nor the skip bits. It seems like holding a
>>> reference on the page should prevent it from switching between
>>> compound or not, and the skip bits are per pageblock with the LRU bits
>>> being per node/memcg which I would think implies that we could have
>>> multiple LRU locks that could apply to a single skip bit.
>>
>> Hi Alexander,
>>
>> I don't find problem yet on compound or skip bit usage. Would you clarify the
>> issue do you concerned?
>>
>> Thanks!
> 
> The point I was getting at is that the LRU lock is being used to
> protect these and with your changes I don't think that makes sense
> anymore.
> 
> The skip bits are per-pageblock bits. With your change the LRU lock is
> now per memcg first and then per node. As such I do not believe it
> really provides any sort of exclusive access to the skip bits. I still
> have to look into this more, but it seems like you need a lock per
> either section or zone that can be used to protect those bits and deal
> with this sooner rather than waiting until you have found an LRU page.
> The one part that is confusing though is that the definition of the
> skip bits seems to call out that they are a hint since they are not
> protected by a lock, but that is exactly what has been happening here.
> 

The skip bits are safe here, since even it race with other skip action,
It will still skip out. The skip action is try not to compaction too much,
not a exclusive action needs avoid race.


> The point I was getting at with the PageCompound check is that instead
> of needing the LRU lock you should be able to look at PageCompound as
> soon as you call get_page_unless_zero() and preempt the need to set
> the LRU bit again. Instead of trying to rely on the LRU lock to
> guarantee that the page hasn't been merged you could just rely on the
> fact that you are holding a reference to it so it isn't going to
> switch between being compound or order 0 since it cannot be freed. It
> spoils the idea I originally had of combining the logic for
> get_page_unless_zero and TestClearPageLRU into a single function, but
> the advantage is you aren't clearing the LRU flag unless you are
> actually going to pull the page from the LRU list.

Sorry, I still can not follow you here. Compound code part is unchanged
and follow the original logical. So would you like to pose a new code to
see if its works?

Thanks
Alex

> 
> My main worry is that this is the one spot where we appear to be
> clearing the LRU bit without ever actually pulling the page off of the
> LRU list, and I am thinking we would be better served by addressing
> the skip and PageCompound checks earlier rather than adding code to
> set the bit again if either of those cases are encountered. This way
> we don't pseudo-pin pages in the LRU if they are compound or supposed
> to be skipped.
> 
> Thanks.
> 
> - Alex
>