linux-kernel - Re: [RFC PATCH 4/4] x86/mm: write protect (most) page tables

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Message-Id: <8b50991d-2ab5-4577-83e9-a2d74135c5f5@www.fastmail.com>
Date:   Mon, 23 Aug 2021 20:34:05 -0700
From:   "Andy Lutomirski" <luto@...nel.org>
To:     "Dave Hansen" <dave.hansen@...el.com>,
        "Mike Rapoport" <rppt@...nel.org>, linux-mm@...ck.org
Cc:     "Andrew Morton" <akpm@...ux-foundation.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 (Intel)" <peterz@...radead.org>,
        "Rick P Edgecombe" <rick.p.edgecombe@...el.com>,
        "Vlastimil Babka" <vbabka@...e.cz>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 4/4] x86/mm: write protect (most) page tables



On Mon, Aug 23, 2021, at 4:50 PM, Dave Hansen wrote:
> On 8/23/21 6:25 AM, Mike Rapoport wrote:
> >  void ___pte_free_tlb(struct mmu_gather *tlb, struct page *pte)
> >  {
> > +	enable_pgtable_write(page_address(pte));
> >  	pgtable_pte_page_dtor(pte);
> >  	paravirt_release_pte(page_to_pfn(pte));
> >  	paravirt_tlb_remove_table(tlb, pte);
> > @@ -69,6 +73,7 @@ void ___pmd_free_tlb(struct mmu_gather *tlb, pmd_t *pmd)
> >  #ifdef CONFIG_X86_PAE
> >  	tlb->need_flush_all = 1;
> >  #endif
> > +	enable_pgtable_write(pmd);
> >  	pgtable_pmd_page_dtor(page);
> >  	paravirt_tlb_remove_table(tlb, page);
> >  }
> 
> I would expected this to have leveraged the pte_offset_map/unmap() code
> to enable/disable write access.  Granted, it would enable write access
> even when only a read is needed, but that could be trivially fixed with
> having a variant like:
> 
> 	pte_offset_map_write()
> 	pte_offset_unmap_write()

I would also like to see a discussion of how races in which multiple threads or CPUs access ptes in the same page at the same time.

> 
> in addition to the existing (presumably read-only) versions:
> 
> 	pte_offset_map()
> 	pte_offset_unmap()
> 
> Although those only work for the leaf levels, it seems a shame not to to
> use them.
> 
> I'm also cringing a bit at hacking this into the page allocator.   A
> *lot* of what you're trying to do with getting large allocations out and
> splitting them up is done very well today by the slab allocators.  It
> might take some rearrangement of 'struct page' metadata to be more slab
> friendly, but it does seem like a close enough fit to warrant investigating.
>