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

Peter Zijlstra
2021-05-05T08:52:59Z

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?