On Wed, 9 Sep 2020, Alexander Duyck wrote: > On Tue, Sep 8, 2020 at 4:41 PM Hugh Dickins <hughd@xxxxxxxxxx> wrote: > > [PATCH v18 28/32] mm/compaction: Drop locked from isolate_migratepages_block > > Most of this consists of replacing "locked" by "lruvec", which is good: > > but please fold those changes back into 20/32 (or would it be 17/32? > > I've not yet looked into the relationship between those two), so we > > can then see more clearly what change this 28/32 (will need renaming!) > > actually makes, to use lruvec_holds_page_lru_lock(). That may be a > > good change, but it's mixed up with the "locked"->"lruvec" at present, > > and I think you could have just used lruvec for locked all along > > (but of course there's a place where you'll need new_lruvec too). > > I am good with my patch being folded in. No need to keep it separate. Thanks. Though it was only the "locked"->"lruvec" changes I was suggesting to fold back, to minimize the diff, so that we could see your use of lruvec_holds_page_lru_lock() more clearly - you had not introduced that function at the stage of the earlier patches. But now that I stare at it again, using lruvec_holds_page_lru_lock() there doesn't look like an advantage to me: when it decides no, the same calculation is made all over again in mem_cgroup_page_lruvec(), whereas the code before only had to calculate it once. So, the code before looks better to me: I wonder, do you think that rcu_read_lock() is more expensive than I think it? There can be debug instrumentation that makes it heavier, but by itself it is very cheap (by design) - not worth branching around. > > > [PATCH v18 29/32] mm: Identify compound pages sooner in isolate_migratepages_block > > NAK. I agree that isolate_migratepages_block() looks nicer this way, but > > take a look at prep_new_page() in mm/page_alloc.c: post_alloc_hook() is > > where set_page_refcounted() changes page->_refcount from 0 to 1, allowing > > a racing get_page_unless_zero() to succeed; then later prep_compound_page() > > is where PageHead and PageTails get set. So there's a small race window in > > which this patch could deliver a compound page when it should not. > > So the main motivation for the patch was to avoid the case where we > are having to reset the LRU flag. That would be satisfying. Not necessary, but I agree satisfying. Maybe depends also on your "skip" change, which I've not looked at yet? > One question I would have is what if > we swapped the code block with the __isolate_lru_page_prepare section? > WIth that we would be taking a reference on the page, then verifying > the LRU flag is set, and then testing for compound page flag bit. > Would doing that close the race window since the LRU flag being set > should indicate that the allocation has already been completed has it > not? Yes, I think that would be safe, and would look better. But I am very hesitant to give snap assurances here (I've twice missed out a vital PageLRU check from this sequence myself): it is very easy to deceive myself and only see it later. If you can see a bug in what's there before these patches, certainly we need to fix it. But adding non-essential patches to the already overlong series risks delaying it. Hugh
