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 >
