On Thu, Feb 25, 2021 at 03:55:53AM +0000, Matthew Wilcox wrote: > On Wed, Feb 24, 2021 at 04:50:39PM -0700, Yu Zhao wrote: > > On Wed, Feb 24, 2021 at 10:48:46PM +0000, Matthew Wilcox wrote: > > > On Wed, Feb 24, 2021 at 03:34:16PM -0700, Yu Zhao wrote: > > > > > If only somebody were working on a patch series to get rid of > > > > > all those calls to compound_head()! Some reviews on > > > > > https://lore.kernel.org/linux-mm/20210128070404.1922318-2-willy@xxxxxxxxxxxxx/ > > > > > would be nice. > > > > > > > > I'm on board with the idea and have done some research in this > > > > direction. We've found that the ideal *anon* page size for Chrome OS > > > > is not 4KB or 2MB, but 32KB. I hope we could leverage the folio to > > > > support flexible anon page size to reduce the number of page faults > > > > (vs 4KB) or internal fragmentation (vs 2MB). > > > > > > > > That being said, it seems to me this is a long term plan and right > > > > now we need something smaller. So if you don't mind, I'll just go > > > > ahead and remove compound_head() from Page{LRU,Active,Unevictable, > > > > SwapBacked} first? > > > > > > It's really not a big change I'm suggesting here. You need > > > https://lore.kernel.org/linux-mm/20210128070404.1922318-2-willy@xxxxxxxxxxxxx/ > > > https://lore.kernel.org/linux-mm/20210128070404.1922318-5-willy@xxxxxxxxxxxxx/ > > > https://lore.kernel.org/linux-mm/20210128070404.1922318-8-willy@xxxxxxxxxxxxx/ > > > and then the patch I sent above to create folio_lru(). > > > > > > Then any changes you want to make to use folios more broadly will > > > incrementally move us towards your goal of 32kB anon pages. > > > > Well, these patches introduce a new concept which I'm on board with. > > It's not really a new concept ... it's a new type for an existing concept > (a head page). > > > Assume everybody else is too, it still seems to me it's an overkill > > to employee folio to just get rid of unnecessary compound_head() > > in page_lru() -- this is not a criticism but a compliment. > > It's not overkill, that really is the point of a folio! If you > think about it, only head pages can be on the LRU list (because the > compound_head is in the union with the lru list_head). So it > always makes sense to talk about folios on the LRU list. > > > Let me work out something *conceptually* smaller first, and if you > > think folio is absolutely more suitable even for this specific issue, > > I'll go review and test the four patches you listed. Sounds good? > > Umm. It seems to me that no matter what you do, it'll be equivalent to > this, only without the type-safety? I'm thinking about something trivial but still very effective. So far I've only tested it with PG_{active,unevictable}, and I'm already seeing a 4KB gain less the 2KB loss from page_lru(). I didn't go with this at the beginning because it's also time- consuming. I need to go over every single use of PG_{active,unevictable,swapbacked,lru}. add/remove: 0/0 grow/shrink: 1/37 up/down: 4/-4129 (-4125) diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c index 3cec6fbef725..c866c363bb41 100644 --- a/fs/proc/task_mmu.c +++ b/fs/proc/task_mmu.c @@ -1712,6 +1712,7 @@ static void gather_stats(struct page *page, struct numa_maps *md, int pte_dirty, unsigned long nr_pages) { int count = page_mapcount(page); + struct page *head = compound_head(page); md->pages += nr_pages; if (pte_dirty || PageDirty(page)) @@ -1720,7 +1721,7 @@ static void gather_stats(struct page *page, struct numa_maps *md, int pte_dirty, if (PageSwapCache(page)) md->swapcache += nr_pages; - if (PageActive(page) || PageUnevictable(page)) + if (PageActive(head) || PageUnevictable(head)) md->active += nr_pages; if (PageWriteback(page)) diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h index db914477057b..35b3d272ab4c 100644 --- a/include/linux/page-flags.h +++ b/include/linux/page-flags.h @@ -335,8 +335,8 @@ PAGEFLAG(Dirty, dirty, PF_HEAD) TESTSCFLAG(Dirty, dirty, PF_HEAD) __CLEARPAGEFLAG(Dirty, dirty, PF_HEAD) PAGEFLAG(LRU, lru, PF_HEAD) __CLEARPAGEFLAG(LRU, lru, PF_HEAD) TESTCLEARFLAG(LRU, lru, PF_HEAD) -PAGEFLAG(Active, active, PF_HEAD) __CLEARPAGEFLAG(Active, active, PF_HEAD) - TESTCLEARFLAG(Active, active, PF_HEAD) +PAGEFLAG(Active, active, PF_ONLY_HEAD) __CLEARPAGEFLAG(Active, active, PF_ONLY_HEAD) + TESTCLEARFLAG(Active, active, PF_ONLY_HEAD) PAGEFLAG(Workingset, workingset, PF_HEAD) TESTCLEARFLAG(Workingset, workingset, PF_HEAD) __PAGEFLAG(Slab, slab, PF_NO_TAIL) @@ -407,9 +407,9 @@ CLEARPAGEFLAG(SwapCache, swapcache, PF_NO_TAIL) PAGEFLAG_FALSE(SwapCache) #endif -PAGEFLAG(Unevictable, unevictable, PF_HEAD) - __CLEARPAGEFLAG(Unevictable, unevictable, PF_HEAD) - TESTCLEARFLAG(Unevictable, unevictable, PF_HEAD) +PAGEFLAG(Unevictable, unevictable, PF_ONLY_HEAD) + __CLEARPAGEFLAG(Unevictable, unevictable, PF_ONLY_HEAD) + TESTCLEARFLAG(Unevictable, unevictable, PF_ONLY_HEAD) #ifdef CONFIG_MMU PAGEFLAG(Mlocked, mlocked, PF_NO_TAIL)
