On Fri, Sep 11, 2020 at 11:37:50AM +0800, Alex Shi wrote: > > > 在 2020/9/10 下午9:49, Matthew Wilcox 写道: > > On Mon, Aug 24, 2020 at 08:54:39PM +0800, Alex Shi wrote: > >> lru_lock and page cache xa_lock have no reason with current sequence, > >> put them together isn't necessary. let's narrow the lru locking, but > >> left the local_irq_disable to block interrupt re-entry and statistic update. > > > > What stats are you talking about here? > > Hi Matthew, > > Thanks for comments! > > like __dec_node_page_state(head, NR_SHMEM_THPS); will have preemptive warning... OK, but those stats are guarded by 'if (mapping)', so this patch doesn't produce that warning because we'll have taken the xarray lock and disabled interrupts. > > How about this patch instead? It occurred to me we already have > > perfectly good infrastructure to track whether or not interrupts are > > already disabled, and so we should use that instead of ensuring that > > interrupts are disabled, or tracking that ourselves. > > So your proposal looks like; > 1, xa_lock_irq(&mapping->i_pages); (optional) > 2, spin_lock_irqsave(&ds_queue->split_queue_lock, flags); > 3, spin_lock_irqsave(&pgdat->lru_lock, flags); > > Is there meaningful for the 2nd and 3rd flags? Yes. We want to avoid doing: if (mapping) spin_lock(&ds_queue->split_queue_lock); else spin_lock_irq(&ds_queue->split_queue_lock); ... if (mapping) spin_unlock(&ds_queue->split_queue_lock); else spin_unlock_irq(&ds_queue->split_queue_lock); Just using _irqsave has the same effect and is easier to reason about. > IIRC, I had a similar proposal as your, the flags used in xa_lock_irqsave(), > but objected by Hugh. I imagine Hugh's objection was that we know it's safe to disable/enable interrupts here because we're in a sleepable context. But for the other two locks, we'd rather not track whether we've already disabled interrupts or not. Maybe you could dig up the email from Hugh? I can't find it.
