linux-kernel - Re: [PATCH] mm/page-flags: Check enforce parameter in PF_ONLY

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Date:   Wed, 27 Mar 2019 14:15:06 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Anshuman Khandual <anshuman.khandual@....com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        hannes@...xchg.org, david@...hat.com, vbabka@...e.cz,
        willy@...radead.org, akpm@...ux-foundation.org,
        Nicholas Piggin <npiggin@...il.com>
Subject: Re: [PATCH] mm/page-flags: Check enforce parameter in PF_ONLY_HEAD()

[Cc Nick]

On Wed 27-03-19 17:57:52, Anshuman Khandual wrote:
> Just check for enforce parameter in PF_ONLY_HEAD() wrapper before calling
> VM_BUG_ON_PGFLAGS() for tail pages.

Why is this an actual fix? Only TESTPAGEFLAG doesn't enforce the check
but I suspect that Nick just wanted the check to be _always_ performed
as the name suggests. What kind of problem are you trying to solve?

> Fixes: 62906027091f ("mm: add PageWaiters indicating tasks are waiting for a page bit")
> Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>
> ---
>  include/linux/page-flags.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index 9f8712a4b1a5..82539e287bc6 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -229,7 +229,7 @@ static inline void page_init_poison(struct page *page, size_t size)
>  #define PF_ANY(page, enforce)	PF_POISONED_CHECK(page)
>  #define PF_HEAD(page, enforce)	PF_POISONED_CHECK(compound_head(page))
>  #define PF_ONLY_HEAD(page, enforce) ({					\
> -		VM_BUG_ON_PGFLAGS(PageTail(page), page);		\
> +		VM_BUG_ON_PGFLAGS(enforce && PageTail(page), page);	\
>  		PF_POISONED_CHECK(page); })
>  #define PF_NO_TAIL(page, enforce) ({					\
>  		VM_BUG_ON_PGFLAGS(enforce && PageTail(page), page);	\
> -- 
> 2.20.1
> 

-- 
Michal Hocko
SUSE Labs