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

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Message-ID: <YSdKsTX3EQQqgj0y@kernel.org>
Date:   Thu, 26 Aug 2021 11:02:57 +0300
From:   Mike Rapoport <rppt@...nel.org>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     linux-mm@...ck.org, 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>,
        Vlastimil Babka <vbabka@...e.cz>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 4/4] x86/mm: write protect (most) page tables

On Mon, Aug 23, 2021 at 04:50:10PM -0700, 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'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.

I thought more about using slab, but it seems to me the least suitable
option. The usecases at hand (page tables, secretmem, SEV/TDX) allocate in
page granularity and some of them use struct page metadata, so even its
rearrangement won't help. And adding support for 2M slabs to SLUB would be
quite intrusive.

I think that better options are moving such cache deeper into buddy or
using e.g. genalloc instead of a list to deal with higher order allocations. 

The choice between these two will mostly depend of the API selection, i.e.
a GFP flag or a dedicated alloc/free.

-- 
Sincerely yours,
Mike.