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

[Mike Rapoport]

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.
 
> > @@ -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?

-- 
Sincerely yours,
Mike.