From mboxrd@z Thu Jan 1 00:00:00 1970 From: Alex Shi Subject: Re: [PATCH v16 15/22] mm/compaction: do page isolation first in compaction Date: Fri, 17 Jul 2020 13:09:43 +0800 Message-ID: References: <1594429136-20002-1-git-send-email-alex.shi@linux.alibaba.com> <1594429136-20002-16-git-send-email-alex.shi@linux.alibaba.com> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: Sender: cgroups-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-ID: Content-Type: text/plain; charset="us-ascii" To: Alexander Duyck Cc: Andrew Morton , Mel Gorman , Tejun Heo , Hugh Dickins , Konstantin Khlebnikov , Daniel Jordan , Yang Shi , Matthew Wilcox , Johannes Weiner , kbuild test robot , linux-mm , LKML , cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Shakeel Butt , Joonsoo Kim , Wei Yang , "Kirill A. Shutemov" >> @@ -950,6 +951,21 @@ static bool too_many_isolated(pg_data_t *pgdat) >> if (!(cc->gfp_mask & __GFP_FS) && page_mapping(page)) >> goto isolate_fail; >> >> + /* >> + * Be careful not to clear PageLRU until after we're >> + * sure the page is not being freed elsewhere -- the >> + * page release code relies on it. >> + */ >> + if (unlikely(!get_page_unless_zero(page))) >> + goto isolate_fail; >> + >> + if (__isolate_lru_page_prepare(page, isolate_mode) != 0) >> + goto isolate_fail_put; >> + >> + /* Try isolate the page */ >> + if (!TestClearPageLRU(page)) >> + goto isolate_fail_put; >> + >> /* If we already hold the lock, we can skip some rechecking */ >> if (!locked) { >> locked = compact_lock_irqsave(&pgdat->lru_lock, > > Why not do the __isolate_lru_page_prepare before getting the page? > That way you can avoid performing an extra atomic operation on non-LRU > pages. > This change come from Hugh Dickins as mentioned from commit log: >> trylock_page() is not safe to use at this time: its setting PG_locked >> can race with the page being freed or allocated ("Bad page"), and can >> also erase flags being set by one of those "sole owners" of a freshly >> allocated page who use non-atomic __SetPageFlag(). Hi Hugh, would you like to show more details of the bug? ... >> + * sure the page is not being freed elsewhere -- the >> + * page release code relies on it. >> + */ >> + if (unlikely(!get_page_unless_zero(page))) >> + goto busy; >> + >> + if (!TestClearPageLRU(page)) { >> + /* >> + * This page may in other isolation path, >> + * but we still hold lru_lock. >> + */ >> + put_page(page); >> + goto busy; >> + } >> + > > I wonder if it wouldn't make sense to combine these two atomic ops > with tests and the put_page into a single inline function? Then it > could be possible to just do one check and if succeeds you do the > block of code below, otherwise you just fall-through into the -EBUSY > case. > Uh, since get_page changes page->_refcount, TestClearPageLRU changes page->flags, So I don't know how to combine them, could you make it more clear with code? Thanks Alex