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

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Message-ID: <20210506175918.GC388843@casper.infradead.org>
Date:   Thu, 6 May 2021 18:59:18 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Mike Rapoport <rppt@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        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,
        dan.j.williams@...el.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC 5/9] x86, mm: Use cache of page tables

On Wed, May 05, 2021 at 03:09:09PM +0300, Mike Rapoport wrote:
> On Wed, May 05, 2021 at 10:51:55AM +0200, Peter Zijlstra wrote:
> > 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?
> 
> We call __ClearPageTable() at pgtable_pxy_page_dtor(), so at least for pte
> and pmd we need to somehow tell release_pages() what kind of page it was.

One of the things I've been thinking about doing is removing the pgtable
dtors and instead calling the pgtable dtor in __put_page() if PageTable().
Might work nicely with this ...