linux-kernel - Re: [PATCH 08/17] prmem: struct page: track vmap

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Date:   Tue, 23 Oct 2018 20:12:00 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Igor Stoppa <igor.stoppa@...il.com>
Cc:     Mimi Zohar <zohar@...ux.vnet.ibm.com>,
        Kees Cook <keescook@...omium.org>,
        Dave Chinner <david@...morbit.com>,
        James Morris <jmorris@...ei.org>,
        Michal Hocko <mhocko@...nel.org>,
        kernel-hardening@...ts.openwall.com,
        linux-integrity@...r.kernel.org,
        linux-security-module@...r.kernel.org, igor.stoppa@...wei.com,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Jonathan Corbet <corbet@....net>,
        Laura Abbott <labbott@...hat.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Pavel Tatashin <pasha.tatashin@...cle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 08/17] prmem: struct page: track vmap_area

On Wed, Oct 24, 2018 at 12:34:55AM +0300, Igor Stoppa wrote:
> The connection between each page and its vmap_area avoids more expensive
> searches through the btree of vmap_areas.

Typo -- it's an rbtree.

> +++ b/include/linux/mm_types.h
> @@ -87,13 +87,24 @@ struct page {
>  			/* See page-flags.h for PAGE_MAPPING_FLAGS */
>  			struct address_space *mapping;
>  			pgoff_t index;		/* Our offset within mapping. */
> -			/**
> -			 * @private: Mapping-private opaque data.
> -			 * Usually used for buffer_heads if PagePrivate.
> -			 * Used for swp_entry_t if PageSwapCache.
> -			 * Indicates order in the buddy system if PageBuddy.
> -			 */
> -			unsigned long private;
> +			union {
> +				/**
> +				 * @private: Mapping-private opaque data.
> +				 * Usually used for buffer_heads if
> +				 * PagePrivate.
> +				 * Used for swp_entry_t if PageSwapCache.
> +				 * Indicates order in the buddy system if
> +				 * PageBuddy.
> +				 */
> +				unsigned long private;
> +				/**
> +				 * @area: reference to the containing area
> +				 * For pages that are mapped into a virtually
> +				 * contiguous area, avoids performing a more
> +				 * expensive lookup.
> +				 */
> +				struct vmap_area *area;
> +			};

Not like this.  Make it part of a different struct in the existing union,
not a part of the pagecache struct.  And there's no need to use ->private
explicitly.

> @@ -1747,6 +1750,10 @@ void *__vmalloc_node_range(unsigned long size, unsigned long align,
>  	if (!addr)
>  		return NULL;
>  
> +	va = __find_vmap_area((unsigned long)addr);
> +	for (i = 0; i < va->vm->nr_pages; i++)
> +		va->vm->pages[i]->area = va;

I don't like it that you're calling this for _every_ vmalloc() caller
when most of them will never use this.  Perhaps have page->va be initially
NULL and then cache the lookup in it when it's accessed for the first time.