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Wed, 22 Sep 2021 00:37:06 -0700 (PDT) MIME-Version: 1.0 References: <20210917034815.80264-1-songmuchun@bytedance.com> <20210917034815.80264-2-songmuchun@bytedance.com> In-Reply-To: From: Barry Song <21cnbao@gmail.com> Date: Wed, 22 Sep 2021 19:36:55 +1200 Message-ID: Subject: Re: [PATCH RESEND v2 1/4] mm: hugetlb: free the 2nd vmemmap page associated with each HugeTLB page To: Muchun Song Cc: Mike Kravetz , Andrew Morton , Oscar Salvador , Michal Hocko , Barry Song , David Hildenbrand , Chen Huang , "Bodeddula, Balasubramaniam" , Jonathan Corbet , Matthew Wilcox , Xiongchun duan , fam.zheng@bytedance.com, Muchun Song , Qi Zheng , linux-doc@vger.kernel.org, LKML , Linux-MM Content-Type: text/plain; charset="UTF-8" X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 4922430000AF X-Stat-Signature: dmf4yuqg74qwxyrgz7ych8hzim6cg3pd Authentication-Results: imf08.hostedemail.com; dkim=pass header.d=gmail.com header.s=20210112 header.b=J0+Atfxh; spf=pass (imf08.hostedemail.com: domain of 21cnbao@gmail.com designates 209.85.208.52 as permitted sender) smtp.mailfrom=21cnbao@gmail.com; dmarc=pass (policy=none) header.from=gmail.com X-HE-Tag: 1632296228-621779 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Wed, Sep 22, 2021 at 2:39 PM Muchun Song wrote: > > On Wed, Sep 22, 2021 at 4:43 AM Barry Song <21cnbao@gmail.com> wrote: > > > > On Wed, Sep 22, 2021 at 1:46 AM Muchun Song wrote: > > > > > > On Tue, Sep 21, 2021 at 8:11 PM Barry Song <21cnbao@gmail.com> wrote: > > > > > > > > On Tue, Sep 21, 2021 at 10:23 PM Muchun Song wrote: > > > > > > > > > > On Sat, Sep 18, 2021 at 6:06 PM Muchun Song wrote: > > > > > > > > > > > > On Sat, Sep 18, 2021 at 12:39 PM Barry Song <21cnbao@gmail.com> wrote: > > > > > > > > > > > > > > On Sat, Sep 18, 2021 at 12:08 AM Muchun Song wrote: > > > > > > > > > > > > > > > > Currently, we only free 6 vmemmap pages associated with a 2MB HugeTLB > > > > > > > > page. However, we can remap all tail vmemmap pages to the page frame > > > > > > > > mapped to with the head vmemmap page. Finally, we can free 7 vmemmap > > > > > > > > pages for a 2MB HugeTLB page. It is a fine gain (e.g. we can save > > > > > > > > extra 2GB memory when there is 1TB HugeTLB pages in the system > > > > > > > > compared with the current implementation). > > > > > > > > > > > > > > > > But the head vmemmap page is not freed to the buddy allocator and all > > > > > > > > tail vmemmap pages are mapped to the head vmemmap page frame. So we > > > > > > > > can see more than one struct page struct with PG_head (e.g. 8 per 2 MB > > > > > > > > HugeTLB page) associated with each HugeTLB page. We should adjust > > > > > > > > compound_head() to make it returns the real head struct page when the > > > > > > > > parameter is the tail struct page but with PG_head flag. > > > > > > > > > > > > > > > > Signed-off-by: Muchun Song > > > > > > > > --- > > > > > > > > Documentation/admin-guide/kernel-parameters.txt | 2 +- > > > > > > > > include/linux/page-flags.h | 75 +++++++++++++++++++++++-- > > > > > > > > mm/hugetlb_vmemmap.c | 60 +++++++++++--------- > > > > > > > > mm/sparse-vmemmap.c | 21 +++++++ > > > > > > > > 4 files changed, 126 insertions(+), 32 deletions(-) > > > > > > > > > > > > > > > > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt > > > > > > > > index bdb22006f713..a154a7b3b9a5 100644 > > > > > > > > --- a/Documentation/admin-guide/kernel-parameters.txt > > > > > > > > +++ b/Documentation/admin-guide/kernel-parameters.txt > > > > > > > > @@ -1606,7 +1606,7 @@ > > > > > > > > [KNL] Reguires CONFIG_HUGETLB_PAGE_FREE_VMEMMAP > > > > > > > > enabled. > > > > > > > > Allows heavy hugetlb users to free up some more > > > > > > > > - memory (6 * PAGE_SIZE for each 2MB hugetlb page). > > > > > > > > + memory (7 * PAGE_SIZE for each 2MB hugetlb page). > > > > > > > > Format: { on | off (default) } > > > > > > > > > > > > > > > > on: enable the feature > > > > > > > > diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h > > > > > > > > index 8e1d97d8f3bd..7b1a918ebd43 100644 > > > > > > > > --- a/include/linux/page-flags.h > > > > > > > > +++ b/include/linux/page-flags.h > > > > > > > > @@ -184,13 +184,64 @@ enum pageflags { > > > > > > > > > > > > > > > > #ifndef __GENERATING_BOUNDS_H > > > > > > > > > > > > > > > > +#ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP > > > > > > > > +extern bool hugetlb_free_vmemmap_enabled; > > > > > > > > + > > > > > > > > +/* > > > > > > > > + * If the feature of freeing some vmemmap pages associated with each HugeTLB > > > > > > > > + * page is enabled, the head vmemmap page frame is reused and all of the tail > > > > > > > > + * vmemmap addresses map to the head vmemmap page frame (furture details can > > > > > > > > + * refer to the figure at the head of the mm/hugetlb_vmemmap.c). In other > > > > > > > > + * word, there are more than one page struct with PG_head associated with each > > > > > > > > + * HugeTLB page. We __know__ that there is only one head page struct, the tail > > > > > > > > + * page structs with PG_head are fake head page structs. We need an approach > > > > > > > > + * to distinguish between those two different types of page structs so that > > > > > > > > + * compound_head() can return the real head page struct when the parameter is > > > > > > > > + * the tail page struct but with PG_head. > > > > > > > > + * > > > > > > > > + * The page_head_if_fake() returns the real head page struct iff the @page may > > > > > > > > + * be fake, otherwise, returns the @page if it cannot be a fake page struct. > > > > > > > > + */ > > > > > > > > +static __always_inline const struct page *page_head_if_fake(const struct page *page) > > > > > > > > +{ > > > > > > > > + if (!hugetlb_free_vmemmap_enabled) > > > > > > > > + return page; > > > > > > > > + > > > > > > > > + /* > > > > > > > > + * Only addresses aligned with PAGE_SIZE of struct page may be fake head > > > > > > > > + * struct page. The alignment check aims to avoid access the fields ( > > > > > > > > + * e.g. compound_head) of the @page[1]. It can avoid touch a (possibly) > > > > > > > > + * cold cacheline in some cases. > > > > > > > > + */ > > > > > > > > + if (IS_ALIGNED((unsigned long)page, PAGE_SIZE) && > > > > > > > > + test_bit(PG_head, &page->flags)) { > > > > > > > > + /* > > > > > > > > + * We can safely access the field of the @page[1] with PG_head > > > > > > > > + * because the @page is a compound page composed with at least > > > > > > > > + * two contiguous pages. > > > > > > > > + */ > > > > > > > > + unsigned long head = READ_ONCE(page[1].compound_head); > > > > > > > > + > > > > > > > > + if (likely(head & 1)) > > > > > > > > + return (const struct page *)(head - 1); > > > > > > > > + } > > > > > > > > + > > > > > > > > + return page; > > > > > > > > +} > > > > > > > > +#else > > > > > > > > +static __always_inline const struct page *page_head_if_fake(const struct page *page) > > > > > > > > +{ > > > > > > > > + return page; > > > > > > > > +} > > > > > > > > +#endif > > > > > > > > + > > > > > > > > static inline unsigned long _compound_head(const struct page *page) > > > > > > > > { > > > > > > > > unsigned long head = READ_ONCE(page->compound_head); > > > > > > > > > > > > > > > > if (unlikely(head & 1)) > > > > > > > > return head - 1; > > > > > > > > - return (unsigned long)page; > > > > > > > > + return (unsigned long)page_head_if_fake(page); > > > > > > > > > > > > > > hard to read. page_head_if_fake, what is the other side of > > > > > > > page_head_if_not_fake? > > > > > > > > > > > > 1) return itself if the @page is not a fake head page. > > > > > > 2) return head page if @page is a fake head page. > > > > > > > > > > > > So I want to express that page_head_if_fake returns a > > > > > > head page only and only if the parameter of @page is a > > > > > > fake head page. Otherwise, it returns itself. > > > > > > > > > > > > > I would expect something like > > > > > > > page_to_page_head() > > > > > > > or > > > > > > > get_page_head() > > > > > > > > > > > > > > > > > > > Those names seem to be not appropriate as well, because > > > > > > its functionality does not make sure it can return a head > > > > > > page. If the parameter is a head page, it definitely > > > > > > returns a head page, otherwise, it may return itself which > > > > > > may be a tail page. > > > > > > > > > > > > From this point of view, I still prefer page_head_if_fake. > > > > > > > > > > > > > Anyway, I am not quite sure what is the best name. but page_head_if_fake(page) > > > > > > > sounds odd to me. just like the things have two sides, but if_fake presents > > > > > > > one side only. > > > > > > > > > > > > If others have any ideas, comments are welcome. > > > > > > > > > > > > > > > > > > > > > } > > > > > > > > > > > > > > > > #define compound_head(page) ((typeof(page))_compound_head(page)) > > > > > > > > @@ -225,12 +276,14 @@ static inline unsigned long _compound_head(const struct page *page) > > > > > > > > > > > > > > > > static __always_inline int PageTail(struct page *page) > > > > > > > > { > > > > > > > > - return READ_ONCE(page->compound_head) & 1; > > > > > > > > + return READ_ONCE(page->compound_head) & 1 || > > > > > > > > + page_head_if_fake(page) != page; > > > > > > > > > > > > > > i would expect a wrapper like: > > > > > > > page_is_fake_head() > > > > > > > > > > > > Good point. Will do. > > > > > > > > > > > > > > > > > > > > and the above page_to_page_head() can leverage the wrapper. > > > > > > > here too. > > > > > > > > > > > > > > > } > > > > > > > > > > > > > > > > static __always_inline int PageCompound(struct page *page) > > > > > > > > { > > > > > > > > - return test_bit(PG_head, &page->flags) || PageTail(page); > > > > > > > > + return test_bit(PG_head, &page->flags) || > > > > > > > > + READ_ONCE(page->compound_head) & 1; > > > > > > > > > > > > > > hard to read. could it be something like the below? > > > > > > > return PageHead(page) || PageTail(page); > > > > > > > > > > > > > > or do we really need to change this function? even a fake head still has > > > > > > > the true test_bit(PG_head, &page->flags), though it is not a real head, it > > > > > > > is still a pagecompound, right? > > > > > > > > > > > > Right. PageCompound() can not be changed. It is odd but > > > > > > efficient because calling page_head_if_fake is eliminated. > > > > > > So I select performance not readability. I'm not sure if it's > > > > > > worth it. > > > > > > > > > > In order to improve readability, I'll introduce 3 helpers as follows. > > > > > > > > > > 1) page_head_or_fake(), which returns true for the head page > > > > > or fake head page. > > > > > 2) page_head_is_fake(), which returns true for fake head page. > > > > > 3) page_tail_not_fake_head(), which returns true for the tail page > > > > > except the fake head page. > > > > > > > > > > In the end, PageHead(), PageTail() and PageCompound() become > > > > > the following. > > > > > > > > > > static __always_inline int PageHead(struct page *page) > > > > > { > > > > > return page_head_or_fake(page) && !page_head_is_fake(page); > > > > > } > > > > > > > > > > static __always_inline int PageTail(struct page *page) > > > > > { > > > > > return page_tail_not_fake_head(page) || page_head_is_fake(page); > > > > > } > > > > > > > > > > static __always_inline int PageCompound(struct page *page) > > > > > { > > > > > return page_head_or_fake(page) || page_tail_not_fake_head(page); > > > > > } > > > > > > > > > > Do those look more readable? > > > > > > > > > > > > > still not good enough. After a second thought, page_head_if_fake seems > > > > to have the best performance though this function returns an odd value. > > > > i just made a little bit refine on your code in doc: > > > > > > Right. page_head_if_fake is the choice for performance. > > > > > > > > > > > diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h > > > > index 2c0d11e71e26..240c2fca13c7 100644 > > > > --- a/include/linux/page-flags.h > > > > +++ b/include/linux/page-flags.h > > > > @@ -197,8 +197,9 @@ extern bool hugetlb_free_vmemmap_enabled; > > > > * compound_head() can return the real head page struct when the parameter is > > > > * the tail page struct but with PG_head. > > > > * > > > > - * The page_head_if_fake() returns the real head page struct iff the @page may > > > > - * be fake, otherwise, returns the @page if it cannot be a fake page struct. > > > > + * The page_head_if_fake() returns the real head page struct if the @page is > > > > + * fake page_head, otherwise, returns @page which can either be a true page_ > > > > + * head or tail. > > > > */ > > > > > > Good annotation. > > > > > > > static __always_inline const struct page *page_head_if_fake(const > > > > struct page *page) > > > > { > > > > @@ -226,6 +227,12 @@ static __always_inline const struct page > > > > *page_head_if_fake(const struct page *p > > > > > > > > return page; > > > > } > > > > + > > > > +static __always_inline const struct page *page_is_fake_head(const > > > > struct page *page) > > > > +{ > > > > + return page_head_if_fake(page) != page; > > > > +} > > > > + > > > > #else > > > > static __always_inline const struct page *page_head_if_fake(const > > > > struct page *page) > > > > { > > > > @@ -247,7 +254,7 @@ static inline unsigned long _compound_head(const > > > > struct page *page) > > > > static __always_inline int PageTail(struct page *page) > > > > { > > > > return READ_ONCE(page->compound_head) & 1 || > > > > - page_head_if_fake(page) != page; > > > > + page_is_fake_head(page); > > > > } > > > > > > Yeah, this makes PageTail more readable. In your previous thread, > > > you proposed that why not use PageTail in PageCompound directly > > > to improve code readability. So I want to introduce 2 more helpers > > > besides page_is_fake_head(). > > > > > > static __always_inline int page_tail_not_fake_head(struct page *page) > > > { > > > return READ_ONCE(page->compound_head) & 1; > > > } > > > > > > static __always_inline int page_head_or_fake(struct page *page) > > > { > > > return test_bit(PG_head, &page->flags); > > > } > > > > > > Then PageTail() and PageCompound() change to the following. > > > > > > static __always_inline int PageTail(struct page *page) > > > { > > > return page_tail_not_fake_head(page) || page_is_fake_head(page); > > > } > > > > > > static __always_inline int PageCompound(struct page *page) > > > { > > > return page_head_or_fake(page) || page_tail_not_fake_head(page); > > > } > > > > > > From the point of names of helpers, they act as self-annotation. > > > So I think PageTail and PageCompound become readable > > > as well. But you said "still not good enough". Is it because of > > > the names of helpers or introducing more complexity? > > > > I really don't think it is worth this complexity. If there is anything to make > > Got it. > > > the code more readable, I would rename page_head_if_fake() to > > page_fixed_dup_head(). > > Here means page_fixed_up_head, right? Is it a typo? I actually meant "duplicated", but in your case, it is "fake". It doesn't matter too much. Both are ok. > > Thanks. > > > > > this function fixes up the page: > > 1. if the page is a fake head, we need to return its true head (things > > get fixed.) > > 2. if the page is not a fake head, in other words, it is either true > > head or tail, no need to fix anything. > > > > > > > > Thanks. Thanks barry