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Subject: Re: [PATCH 2/5] mm: introduce save_page_flags to cooperate with
 show_page_flags
To:     Muchun Song <songmuchun@bytedance.com>, akpm@linux-foundation.org,
        osalvador@suse.de, mhocko@suse.com, song.bao.hua@hisilicon.com,
        david@redhat.com, chenhuang5@huawei.com, bodeddub@amazon.com,
        corbet@lwn.net
Cc:     duanxiongchun@bytedance.com, fam.zheng@bytedance.com,
        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, zhengqi.arch@bytedance.com
References: <20210714091800.42645-1-songmuchun@bytedance.com>
 <20210714091800.42645-3-songmuchun@bytedance.com>
From:   Mike Kravetz <mike.kravetz@oracle.com>
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Date:   Mon, 26 Jul 2021 16:18:02 -0700
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On 7/14/21 2:17 AM, Muchun Song wrote:
> Introduce save_page_flags to return the page flags which can cooperate
> with show_page_flags. If we want to hihe some page flags from users, it
> will be useful to alter save_page_flags directly. This is a preparation
> for the next patch to hide some page flags from users.
> 
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> ---
>  include/trace/events/mmflags.h  | 3 +++
>  include/trace/events/page_ref.h | 8 ++++----
>  2 files changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/include/trace/events/mmflags.h b/include/trace/events/mmflags.h
> index 390270e00a1d..69cb84b1257e 100644
> --- a/include/trace/events/mmflags.h
> +++ b/include/trace/events/mmflags.h
> @@ -121,6 +121,9 @@ IF_HAVE_PG_IDLE(PG_idle,		"idle"		)		\
>  IF_HAVE_PG_ARCH_2(PG_arch_2,		"arch_2"	)		\
>  IF_HAVE_PG_SKIP_KASAN_POISON(PG_skip_kasan_poison, "skip_kasan_poison")
>  
> +#define save_page_flags(page)						\
> +	(((page)->flags & ~PAGEFLAGS_MASK))
> +

Looking ahead to the next patch, this is changed to hide the PG_head
flag for 'fake' head pages.

IIRC, all vmemmap pages except the first will be mapped read only.  So,
all vmemmap pages with 'fake' head pages will be read only.

It seems that all the modified trace events below are associated with
updates to page structs.  Therefore, it seems these events will never
experience a 'fake' head page.  Am I missing something?

-- 
Mike Kravetz


>  #define show_page_flags(flags)						\
>  	(flags) ? __print_flags(flags, "|",				\
>  	__def_pageflag_names						\
> diff --git a/include/trace/events/page_ref.h b/include/trace/events/page_ref.h
> index 643b1b4e9f27..53d303048d27 100644
> --- a/include/trace/events/page_ref.h
> +++ b/include/trace/events/page_ref.h
> @@ -28,7 +28,7 @@ DECLARE_EVENT_CLASS(page_ref_mod_template,
>  
>  	TP_fast_assign(
>  		__entry->pfn = page_to_pfn(page);
> -		__entry->flags = page->flags;
> +		__entry->flags = save_page_flags(page);
>  		__entry->count = page_ref_count(page);
>  		__entry->mapcount = page_mapcount(page);
>  		__entry->mapping = page->mapping;
> @@ -38,7 +38,7 @@ DECLARE_EVENT_CLASS(page_ref_mod_template,
>  
>  	TP_printk("pfn=0x%lx flags=%s count=%d mapcount=%d mapping=%p mt=%d val=%d",
>  		__entry->pfn,
> -		show_page_flags(__entry->flags & ~PAGEFLAGS_MASK),
> +		show_page_flags(__entry->flags),
>  		__entry->count,
>  		__entry->mapcount, __entry->mapping, __entry->mt,
>  		__entry->val)
> @@ -77,7 +77,7 @@ DECLARE_EVENT_CLASS(page_ref_mod_and_test_template,
>  
>  	TP_fast_assign(
>  		__entry->pfn = page_to_pfn(page);
> -		__entry->flags = page->flags;
> +		__entry->flags = save_page_flags(page);
>  		__entry->count = page_ref_count(page);
>  		__entry->mapcount = page_mapcount(page);
>  		__entry->mapping = page->mapping;
> @@ -88,7 +88,7 @@ DECLARE_EVENT_CLASS(page_ref_mod_and_test_template,
>  
>  	TP_printk("pfn=0x%lx flags=%s count=%d mapcount=%d mapping=%p mt=%d val=%d ret=%d",
>  		__entry->pfn,
> -		show_page_flags(__entry->flags & ~PAGEFLAGS_MASK),
> +		show_page_flags(__entry->flags),
>  		__entry->count,
>  		__entry->mapcount, __entry->mapping, __entry->mt,
>  		__entry->val, __entry->ret)
>