linux-hardening - Re: [PATCH RFC 5/9] x86, mm: Use cache of page tables

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Date:   Wed, 5 May 2021 10:51:55 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Rick Edgecombe <rick.p.edgecombe@...el.com>
Cc:     dave.hansen@...el.com, luto@...nel.org, linux-mm@...ck.org,
        x86@...nel.org, akpm@...ux-foundation.org,
        linux-hardening@...r.kernel.org,
        kernel-hardening@...ts.openwall.com, ira.weiny@...el.com,
        rppt@...nel.org, dan.j.williams@...el.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC 5/9] x86, mm: Use cache of page tables

On Tue, May 04, 2021 at 05:30:28PM -0700, Rick Edgecombe wrote:
> @@ -54,6 +98,8 @@ void ___pte_free_tlb(struct mmu_gather *tlb, struct page *pte)
>  {
>  	pgtable_pte_page_dtor(pte);
>  	paravirt_release_pte(page_to_pfn(pte));
> +	/* Set Page Table so swap knows how to free it */
> +	__SetPageTable(pte);
>  	paravirt_tlb_remove_table(tlb, pte);
>  }
>  
> @@ -70,12 +116,16 @@ void ___pmd_free_tlb(struct mmu_gather *tlb, pmd_t *pmd)
>  	tlb->need_flush_all = 1;
>  #endif
>  	pgtable_pmd_page_dtor(page);
> +	/* Set Page Table so swap nows how to free it */
> +	__SetPageTable(virt_to_page(pmd));
>  	paravirt_tlb_remove_table(tlb, page);
>  }
>  
>  #if CONFIG_PGTABLE_LEVELS > 3
>  void ___pud_free_tlb(struct mmu_gather *tlb, pud_t *pud)
>  {
> +	/* Set Page Table so swap nows how to free it */
> +	__SetPageTable(virt_to_page(pud));
>  	paravirt_release_pud(__pa(pud) >> PAGE_SHIFT);
>  	paravirt_tlb_remove_table(tlb, virt_to_page(pud));
>  }
> @@ -83,6 +133,8 @@ void ___pud_free_tlb(struct mmu_gather *tlb, pud_t *pud)
>  #if CONFIG_PGTABLE_LEVELS > 4
>  void ___p4d_free_tlb(struct mmu_gather *tlb, p4d_t *p4d)
>  {
> +	/* Set Page Table so swap nows how to free it */
> +	__SetPageTable(virt_to_page(p4d));
>  	paravirt_release_p4d(__pa(p4d) >> PAGE_SHIFT);
>  	paravirt_tlb_remove_table(tlb, virt_to_page(p4d));
>  }

This, to me, seems like a really weird place to __SetPageTable(), why
can't we do that on allocation?

> @@ -888,6 +889,12 @@ void release_pages(struct page **pages, int nr)
>  			continue;
>  		}
>  
> +		if (PageTable(page)) {
> +			__ClearPageTable(page);
> +			free_table(page);
> +			continue;
> +		}
> +
>  		if (!put_page_testzero(page))
>  			continue;
>  
> diff --git a/mm/swap_state.c b/mm/swap_state.c
> index 3cdee7b11da9..a60ec3d4ab21 100644
> --- a/mm/swap_state.c
> +++ b/mm/swap_state.c
> @@ -22,6 +22,7 @@
>  #include <linux/swap_slots.h>
>  #include <linux/huge_mm.h>
>  #include <linux/shmem_fs.h>
> +#include <asm/pgalloc.h>
>  #include "internal.h"
>  
>  /*
> @@ -310,6 +311,11 @@ static inline void free_swap_cache(struct page *page)
>  void free_page_and_swap_cache(struct page *page)
>  {
>  	free_swap_cache(page);
> +	if (PageTable(page)) {
> +		__ClearPageTable(page);
> +		free_table(page);
> +		return;
> +	}
>  	if (!is_huge_zero_page(page))
>  		put_page(page);
>  }

And then free_table() can __ClearPageTable() and all is nice and
symmetric and all this weirdness goes away, no?