Re: [PATCH v7 2/7] mm: swap: free_swap_and_cache_nr() as batched free_swap_and_cache()

[David Hildenbrand <david@redhat.com>]

On 08.04.24 20:39, Ryan Roberts wrote:
> Now that we no longer have a convenient flag in the cluster to determine
> if a folio is large, free_swap_and_cache() will take a reference and
> lock a large folio much more often, which could lead to contention and
> (e.g.) failure to split large folios, etc.
> 
> Let's solve that problem by batch freeing swap and cache with a new
> function, free_swap_and_cache_nr(), to free a contiguous range of swap
> entries together. This allows us to first drop a reference to each swap
> slot before we try to release the cache folio. This means we only try to
> release the folio once, only taking the reference and lock once - much
> better than the previous 512 times for the 2M THP case.
> 
> Contiguous swap entries are gathered in zap_pte_range() and
> madvise_free_pte_range() in a similar way to how present ptes are
> already gathered in zap_pte_range().
> 
> While we are at it, let's simplify by converting the return type of both
> functions to void. The return value was used only by zap_pte_range() to
> print a bad pte, and was ignored by everyone else, so the extra
> reporting wasn't exactly guaranteed. We will still get the warning with
> most of the information from get_swap_device(). With the batch version,
> we wouldn't know which pte was bad anyway so could print the wrong one.
> 
> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
> ---
>   include/linux/pgtable.h | 29 ++++++++++++
>   include/linux/swap.h    | 12 +++--
>   mm/internal.h           | 63 ++++++++++++++++++++++++++
>   mm/madvise.c            | 12 +++--
>   mm/memory.c             | 13 +++---
>   mm/swapfile.c           | 97 +++++++++++++++++++++++++++++++++--------
>   6 files changed, 195 insertions(+), 31 deletions(-)
> 
> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> index a3fc8150b047..75096025fe52 100644
> --- a/include/linux/pgtable.h
> +++ b/include/linux/pgtable.h
> @@ -708,6 +708,35 @@ static inline void pte_clear_not_present_full(struct mm_struct *mm,
>   }
>   #endif
>   
> +#ifndef clear_not_present_full_ptes
> +/**
> + * clear_not_present_full_ptes - Clear multiple not present PTEs which are
> + *				 consecutive in the pgtable.
> + * @mm: Address space the ptes represent.
> + * @addr: Address of the first pte.
> + * @ptep: Page table pointer for the first entry.
> + * @nr: Number of entries to clear.
> + * @full: Whether we are clearing a full mm.
> + *
> + * May be overridden by the architecture; otherwise, implemented as a simple
> + * loop over pte_clear_not_present_full().
> + *
> + * Context: The caller holds the page table lock.  The PTEs are all not present.
> + * The PTEs are all in the same PMD.
> + */
> +static inline void clear_not_present_full_ptes(struct mm_struct *mm,
> +		unsigned long addr, pte_t *ptep, unsigned int nr, int full)
> +{
> +	for (;;) {
> +		pte_clear_not_present_full(mm, addr, ptep, full);
> +		if (--nr == 0)
> +			break;
> +		ptep++;
> +		addr += PAGE_SIZE;
> +	}
> +}
> +#endif
> +
>   #ifndef __HAVE_ARCH_PTEP_CLEAR_FLUSH
>   extern pte_t ptep_clear_flush(struct vm_area_struct *vma,
>   			      unsigned long address,
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index f6f78198f000..5737236dc3ce 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -471,7 +471,7 @@ extern int swap_duplicate(swp_entry_t);
>   extern int swapcache_prepare(swp_entry_t);
>   extern void swap_free(swp_entry_t);
>   extern void swapcache_free_entries(swp_entry_t *entries, int n);
> -extern int free_swap_and_cache(swp_entry_t);
> +extern void free_swap_and_cache_nr(swp_entry_t entry, int nr);
>   int swap_type_of(dev_t device, sector_t offset);
>   int find_first_swap(dev_t *device);
>   extern unsigned int count_swap_pages(int, int);
> @@ -520,8 +520,9 @@ static inline void put_swap_device(struct swap_info_struct *si)
>   #define free_pages_and_swap_cache(pages, nr) \
>   	release_pages((pages), (nr));
>   
> -/* used to sanity check ptes in zap_pte_range when CONFIG_SWAP=0 */
> -#define free_swap_and_cache(e) is_pfn_swap_entry(e)
> +static inline void free_swap_and_cache_nr(swp_entry_t entry, int nr)
> +{
> +}
>   
>   static inline void free_swap_cache(struct folio *folio)
>   {
> @@ -589,6 +590,11 @@ static inline int add_swap_extent(struct swap_info_struct *sis,
>   }
>   #endif /* CONFIG_SWAP */
>   
> +static inline void free_swap_and_cache(swp_entry_t entry)
> +{
> +	free_swap_and_cache_nr(entry, 1);
> +}
> +
>   #ifdef CONFIG_MEMCG
>   static inline int mem_cgroup_swappiness(struct mem_cgroup *memcg)
>   {
> diff --git a/mm/internal.h b/mm/internal.h
> index 3bdc8693b54f..de68705624b0 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -11,6 +11,8 @@
>   #include <linux/mm.h>
>   #include <linux/pagemap.h>
>   #include <linux/rmap.h>
> +#include <linux/swap.h>
> +#include <linux/swapops.h>
>   #include <linux/tracepoint-defs.h>
>   
>   struct folio_batch;
> @@ -189,6 +191,67 @@ static inline int folio_pte_batch(struct folio *folio, unsigned long addr,
>   
>   	return min(ptep - start_ptep, max_nr);
>   }
> +
> +/**
> + * pte_next_swp_offset - Increment the swap entry offset field of a swap pte.
> + * @pte: The initial pte state; is_swap_pte(pte) must be true.

Likely we also want non_swap_entry() to be false.

> + *
> + * Increments the swap offset, while maintaining all other fields, including
> + * swap type, and any swp pte bits. The resulting pte is returned.
> + */
> +static inline pte_t pte_next_swp_offset(pte_t pte)
> +{
> +	swp_entry_t entry = pte_to_swp_entry(pte);
> +	pte_t new = __swp_entry_to_pte(__swp_entry(swp_type(entry),
> +						   swp_offset(entry) + 1));
> +
> +	if (pte_swp_soft_dirty(pte))
> +		new = pte_swp_mksoft_dirty(new);
> +	if (pte_swp_exclusive(pte))
> +		new = pte_swp_mkexclusive(new);
> +	if (pte_swp_uffd_wp(pte))
> +		new = pte_swp_mkuffd_wp(new);
> +
> +	return new;
> +}


Acked-by: David Hildenbrand <david@redhat.com>

-- 
Cheers,

David / dhildenb