在 2020/1/17 上午5:52, Johannes Weiner 写道:
> You simply cannot serialize on page->mem_cgroup->lruvec when
> page->mem_cgroup isn't stable. You need to serialize on the page
> itself, one way or another, to make this work.
>
>
> So here is a crazy idea that may be worth exploring:
>
> Right now, pgdat->lru_lock protects both PageLRU *and* the lruvec's
> linked list.
>
> Can we make PageLRU atomic and use it to stabilize the lru_lock
> instead, and then use the lru_lock only serialize list operations?
>
Hi Johannes,
I am trying to figure out the solution of atomic PageLRU, but is
blocked by the following sitations, when PageLRU and lru list was protected
together under lru_lock, the PageLRU could be a indicator if page on lru list
But now seems it can't be the indicator anymore.
Could you give more clues of stabilization usage of PageLRU?
__page_cache_release/release_pages/compaction __pagevec_lru_add
if (TestClearPageLRU(page)) if (!PageLRU())
lruvec_lock();
list_add();
lruvec_unlock();
SetPageLRU() //position 1
lock_page_lruvec_irqsave(page, &flags);
del_page_from_lru_list(page, lruvec, ..);
unlock_page_lruvec_irqrestore(lruvec, flags);
SetPageLRU() //position 2
Thanks a lot!
Alex
> I.e. in compaction, you'd do
>
> if (!TestClearPageLRU(page))
> goto isolate_fail;
> /*
> * We isolated the page's LRU state and thereby locked out all
> * other isolators, including cgroup page moving, page reclaim,
> * page freeing etc. That means page->mem_cgroup is now stable
> * and we can safely look up the correct lruvec and take the
> * page off its physical LRU list.
> */
> lruvec = mem_cgroup_page_lruvec(page);
> spin_lock_irq(&lruvec->lru_lock);
> del_page_from_lru_list(page, lruvec, page_lru(page));
>
> Putback would mostly remain the same (although you could take the
> PageLRU setting out of the list update locked section, as long as it's
> set after the page is physically linked):
>
> /* LRU isolation pins page->mem_cgroup */
> lruvec = mem_cgroup_page_lruvec(page)
> spin_lock_irq(&lruvec->lru_lock);
> add_page_to_lru_list(...);
> spin_unlock_irq(&lruvec->lru_lock);
>
> SetPageLRU(page);
>
> And you'd have to carefully review and rework other sites that rely on
> PageLRU: reclaim, __page_cache_release(), __activate_page() etc.
>
> Especially things like activate_page(), which used to only check
> PageLRU to shuffle the page on the LRU list would now have to briefly
> clear PageLRU and then set it again afterwards.
>
> However, aside from a bit more churn in those cases, and the
> unfortunate additional atomic operations, I currently can't think of a
> fundamental reason why this wouldn't work.
>
> Hugh, what do you think?
>
