linux-kernel - Re: [RFC PATCH 3/4] mm/page_alloc: introduce __GFP_PTE

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Date:   Tue, 24 Aug 2021 18:12:56 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Mike Rapoport <rppt@...nel.org>, linux-mm@...ck.org
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Kees Cook <keescook@...omium.org>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 3/4] mm/page_alloc: introduce __GFP_PTE_MAPPED flag to
 allocate pte-mapped pages

On 8/23/21 15:25, Mike Rapoport wrote:
> @@ -3283,5 +3283,7 @@ static inline int seal_check_future_write(int seals, struct vm_area_struct *vma)
>  	return 0;
>  }
>  
> +void pte_mapped_cache_init(void);
> +
>  #endif /* __KERNEL__ */
>  #endif /* _LINUX_MM_H */
> diff --git a/include/linux/pageblock-flags.h b/include/linux/pageblock-flags.h
> index 973fd731a520..4faf8c542b00 100644
> --- a/include/linux/pageblock-flags.h
> +++ b/include/linux/pageblock-flags.h
> @@ -21,6 +21,8 @@ enum pageblock_bits {
>  			/* 3 bits required for migrate types */
>  	PB_migrate_skip,/* If set the block is skipped by compaction */
>  
> +	PB_pte_mapped, /* If set the block is mapped with PTEs in direct map */
> +
>  	/*
>  	 * Assume the bits will always align on a word. If this assumption
>  	 * changes then get/set pageblock needs updating.

You have broken this assumption :)
> @@ -536,7 +558,7 @@ void set_pfnblock_flags_mask(struct page *page, unsigned long flags,
>  	unsigned long bitidx, word_bitidx;
>  	unsigned long old_word, word;
>  
> -	BUILD_BUG_ON(NR_PAGEBLOCK_BITS != 4);
> +	BUILD_BUG_ON(NR_PAGEBLOCK_BITS != 5);

This is not sufficient to satisfy the "needs updating" part. We would now need
NR_PAGEBLOCK_BITS == 8.