| Message ID | 20210226091718.2927291-4-yuzhao@google.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | trim the uses of compound_head() | expand |
On Fri, Feb 26, 2021 at 02:17:18AM -0700, Yu Zhao wrote: > All places but one test, set or clear PG_active and PG_unevictable on > small or head pages. Use compound_head() explicitly for that singleton > so the rest can rid of redundant compound_head(). How do you know it's only one place? I really wish you'd work with me on folios. They make the compiler prove that it's not a tail page.
On Fri, Feb 26, 2021 at 12:13:14PM +0000, Matthew Wilcox wrote: > On Fri, Feb 26, 2021 at 02:17:18AM -0700, Yu Zhao wrote: > > All places but one test, set or clear PG_active and PG_unevictable on > > small or head pages. Use compound_head() explicitly for that singleton > > so the rest can rid of redundant compound_head(). > > How do you know it's only one place? I really wish you'd work with me > on folios. They make the compiler prove that it's not a tail page. I hasn't been following the effort closely, so I'm rereading the very first discussion "Are THPs the right model for the pagecache?" and then I need to rewatch the recorded Zoom meeting. As I said I'm on board with the idea, but I can't create a roadmap based on my current rough understanding, unless you prefer me to just randomly throw some reviewed-bys at your patches in the next few days. (Our long-term plan for folios is to support arbitrary anon page sizes because anon memory is more than 90% of total user memory on Android, Chrome OS and in our data centers.) That said, if you have something ready to test, we could do it for you in our runtime environments immediately.
On Fri, Feb 26, 2021 at 12:49:41PM -0700, Yu Zhao wrote: > On Fri, Feb 26, 2021 at 12:13:14PM +0000, Matthew Wilcox wrote: > > On Fri, Feb 26, 2021 at 02:17:18AM -0700, Yu Zhao wrote: > > > All places but one test, set or clear PG_active and PG_unevictable on > > > small or head pages. Use compound_head() explicitly for that singleton > > > so the rest can rid of redundant compound_head(). > > > > How do you know it's only one place? I really wish you'd work with me > > on folios. They make the compiler prove that it's not a tail page. > > I hasn't been following the effort closely, so I'm rereading the very > first discussion "Are THPs the right model for the pagecache?" and > then I need to rewatch the recorded Zoom meeting. As I said I'm on > board with the idea, but I can't create a roadmap based on my current > rough understanding, unless you prefer me to just randomly throw some > reviewed-bys at your patches in the next few days. (Our long-term plan > for folios is to support arbitrary anon page sizes because anon memory > is more than 90% of total user memory on Android, Chrome OS and in our > data centers.) > > That said, if you have something ready to test, we could do it for you > in our runtime environments immediately. I don't have anything ready to test for anonymous memory; indeed I have no plans to work on anonymous memory myself. My focus is on the page cache. But, once we get the folio _concept_ into the kernel (ie a struct page which is definitely not a tail page), it can be used more widely than the page cache, and independently from anything I'm working on. The biggest risk is that we end up duplicating work ...
On Fri, Feb 26, 2021 at 12:13:14PM +0000, Matthew Wilcox wrote: > On Fri, Feb 26, 2021 at 02:17:18AM -0700, Yu Zhao wrote: > > All places but one test, set or clear PG_active and PG_unevictable on > > small or head pages. Use compound_head() explicitly for that singleton > > so the rest can rid of redundant compound_head(). > > How do you know it's only one place? I really wish you'd work with me > on folios. They make the compiler prove that it's not a tail page. +1 to this. The problem with compound_head() is systemic and ad-hoc solution to few page flags will only complicate the picture.
On Mon, Mar 01, 2021 at 02:50:07PM +0300, Kirill A. Shutemov wrote: > On Fri, Feb 26, 2021 at 12:13:14PM +0000, Matthew Wilcox wrote: > > On Fri, Feb 26, 2021 at 02:17:18AM -0700, Yu Zhao wrote: > > > All places but one test, set or clear PG_active and PG_unevictable on > > > small or head pages. Use compound_head() explicitly for that singleton > > > so the rest can rid of redundant compound_head(). > > > > How do you know it's only one place? I really wish you'd work with me > > on folios. They make the compiler prove that it's not a tail page. > > +1 to this. > > The problem with compound_head() is systemic and ad-hoc solution to few > page flags will only complicate the picture. Well, I call it an incremental improvement, and how exactly does it complicate the picture? I see your point: you prefer a complete replacement. But my point is not about the preference; it's about presenting an option: I'm not saying we have to go with this series; I'm saying if you don't want to wait, here is something quick but not perfect.
On Mon, 1 Mar 2021, Yu Zhao wrote: > On Mon, Mar 01, 2021 at 02:50:07PM +0300, Kirill A. Shutemov wrote: > > On Fri, Feb 26, 2021 at 12:13:14PM +0000, Matthew Wilcox wrote: > > > On Fri, Feb 26, 2021 at 02:17:18AM -0700, Yu Zhao wrote: > > > > All places but one test, set or clear PG_active and PG_unevictable on > > > > small or head pages. Use compound_head() explicitly for that singleton > > > > so the rest can rid of redundant compound_head(). > > > > > > How do you know it's only one place? I really wish you'd work with me > > > on folios. They make the compiler prove that it's not a tail page. > > > > +1 to this. > > > > The problem with compound_head() is systemic and ad-hoc solution to few > > page flags will only complicate the picture. > > Well, I call it an incremental improvement, and how exactly does it > complicate the picture? > > I see your point: you prefer a complete replacement. But my point is > not about the preference; it's about presenting an option: I'm not > saying we have to go with this series; I'm saying if you don't want > to wait, here is something quick but not perfect. +1 to this. Hugh
On Mon, Mar 01, 2021 at 12:16:19PM -0800, Hugh Dickins wrote: > On Mon, 1 Mar 2021, Yu Zhao wrote: > > On Mon, Mar 01, 2021 at 02:50:07PM +0300, Kirill A. Shutemov wrote: > > > On Fri, Feb 26, 2021 at 12:13:14PM +0000, Matthew Wilcox wrote: > > > > On Fri, Feb 26, 2021 at 02:17:18AM -0700, Yu Zhao wrote: > > > > > All places but one test, set or clear PG_active and PG_unevictable on > > > > > small or head pages. Use compound_head() explicitly for that singleton > > > > > so the rest can rid of redundant compound_head(). > > > > > > > > How do you know it's only one place? I really wish you'd work with me > > > > on folios. They make the compiler prove that it's not a tail page. > > > > > > +1 to this. > > > > > > The problem with compound_head() is systemic and ad-hoc solution to few > > > page flags will only complicate the picture. > > > > Well, I call it an incremental improvement, and how exactly does it > > complicate the picture? > > > > I see your point: you prefer a complete replacement. But my point is > > not about the preference; it's about presenting an option: I'm not > > saying we have to go with this series; I'm saying if you don't want > > to wait, here is something quick but not perfect. > > +1 to this. page folios are here and ready to go. I'm doing another pass on them, quantifying the improvements to text with each patch. So far I'm at 4357 bytes of text saved, in the first 10 patches (many of which look as if they're not going to produce any savings). Yu Zhao's patches seem risky. The only way to know if any places have been missed is by enabling CONFIG_DEBUG_VM_PGFLAGS, which we do not recommend for production environments.
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 c9626e54df8d..b7fe855a6ee9 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_NO_TAIL) __CLEARPAGEFLAG(LRU, lru, PF_NO_TAIL) TESTCLEARFLAG(LRU, lru, PF_NO_TAIL) -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)
All places but one test, set or clear PG_active and PG_unevictable on small or head pages. Use compound_head() explicitly for that singleton so the rest can rid of redundant compound_head(). bloat-o-meter result: add/remove: 0/0 grow/shrink: 3/38 up/down: 388/-4270 (-3882) Signed-off-by: Yu Zhao <yuzhao@google.com> --- fs/proc/task_mmu.c | 3 ++- include/linux/page-flags.h | 10 +++++----- 2 files changed, 7 insertions(+), 6 deletions(-)