On Tue, 20 Nov 2018, Baoquan He wrote: > On 11/20/18 at 02:38pm, Vlastimil Babka wrote: > > On 11/20/18 6:44 AM, Hugh Dickins wrote: > > > [PATCH] mm: put_and_wait_on_page_locked() while page is migrated > > > > > > We have all assumed that it is essential to hold a page reference while > > > waiting on a page lock: partly to guarantee that there is still a struct > > > page when MEMORY_HOTREMOVE is configured, but also to protect against > > > reuse of the struct page going to someone who then holds the page locked > > > indefinitely, when the waiter can reasonably expect timely unlocking. > > > > > > But in fact, so long as wait_on_page_bit_common() does the put_page(), > > > and is careful not to rely on struct page contents thereafter, there is > > > no need to hold a reference to the page while waiting on it. That does > > > > So there's still a moment where refcount is elevated, but hopefully > > short enough, right? Let's see if it survives Baoquan's stress testing. > > Yes, I applied Hugh's patch 8 hours ago, then our QE Ping operated on > that machine, after many times of hot removing/adding, the endless > looping during mirgrating is not seen any more. The test result for > Hugh's patch is positive. I even suggested Ping increasing the memory > pressure to "stress -m 250", it still succeeded to offline and remove. > > So I think this patch works to solve the issue. Thanks a lot for your > help, all of you. Very good to hear, thanks a lot for your quick feedback. > > High, will you post a formal patch in a separate thread? Yes, I promise that I shall do so in the next few days, but not today: some other things have to take priority. And Vlastimil has raised an excellent point about the interaction with PSI "thrashing": I need to read up and decide which way to go on that (and add Johannes to the Cc when I post). I think I shall probably post it directly to Linus (lists and other people Cc'ed of course): not because I think it should be rushed in too quickly, nor to sidestep Andrew, but because Linus was very closely involved in both the PG_waiters and WQ_FLAG_BOOKMARK discussions: it is an area of special interest to him. Hugh
