Re: [PATCH 1/2] mm:swap: clean up swap readahead

[Huang\, Ying]

Hi, Minchan,

Minchan Kim <minchan@xxxxxxxxxx> writes:

> When I see recent change of swap readahead, I am very unhappy
> about current code structure which diverges two swap readahead
> algorithm in do_swap_page. This patch is to clean it up.
>
> Main motivation is that fault handler doesn't need to be aware of
> readahead algorithms but just should call swapin_readahead.
>
> As first step, this patch cleans up a little bit but not perfect
> (I just separate for review easier) so next patch will make the goal
> complete.
>
> Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
> ---
>  include/linux/swap.h | 17 ++--------
>  mm/memory.c          | 17 +++-------
>  mm/swap_state.c      | 89 ++++++++++++++++++++++++++++------------------------
>  3 files changed, 55 insertions(+), 68 deletions(-)
>
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index 84255b3da7c1..7c7c8b344bc9 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -427,12 +427,8 @@ extern struct page *__read_swap_cache_async(swp_entry_t, gfp_t,
>  			bool *new_page_allocated);
>  extern struct page *swapin_readahead(swp_entry_t, gfp_t,
>  			struct vm_area_struct *vma, unsigned long addr);
> -
> -extern struct page *swap_readahead_detect(struct vm_fault *vmf,
> -					  struct vma_swap_readahead *swap_ra);
>  extern struct page *do_swap_page_readahead(swp_entry_t fentry, gfp_t gfp_mask,
> -					   struct vm_fault *vmf,
> -					   struct vma_swap_readahead *swap_ra);
> +					   struct vm_fault *vmf);
>  
>  /* linux/mm/swapfile.c */
>  extern atomic_long_t nr_swap_pages;
> @@ -551,15 +547,8 @@ static inline bool swap_use_vma_readahead(void)
>  	return false;
>  }
>  
> -static inline struct page *swap_readahead_detect(
> -	struct vm_fault *vmf, struct vma_swap_readahead *swap_ra)
> -{
> -	return NULL;
> -}
> -
> -static inline struct page *do_swap_page_readahead(
> -	swp_entry_t fentry, gfp_t gfp_mask,
> -	struct vm_fault *vmf, struct vma_swap_readahead *swap_ra)
> +static inline struct page *do_swap_page_readahead(swp_entry_t fentry,
> +				gfp_t gfp_mask, struct vm_fault *vmf)
>  {
>  	return NULL;
>  }
> diff --git a/mm/memory.c b/mm/memory.c
> index 8a0c410037d2..e955298e4290 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2849,21 +2849,14 @@ int do_swap_page(struct vm_fault *vmf)
>  	struct vm_area_struct *vma = vmf->vma;
>  	struct page *page = NULL, *swapcache = NULL;
>  	struct mem_cgroup *memcg;
> -	struct vma_swap_readahead swap_ra;
>  	swp_entry_t entry;
>  	pte_t pte;
>  	int locked;
>  	int exclusive = 0;
>  	int ret = 0;
> -	bool vma_readahead = swap_use_vma_readahead();
>  
> -	if (vma_readahead)
> -		page = swap_readahead_detect(vmf, &swap_ra);
> -	if (!pte_unmap_same(vma->vm_mm, vmf->pmd, vmf->pte, vmf->orig_pte)) {
> -		if (page)
> -			put_page(page);
> +	if (!pte_unmap_same(vma->vm_mm, vmf->pmd, vmf->pte, vmf->orig_pte))
>  		goto out;
> -	}

The page table holding PTE may be unmapped in pte_unmap_same(), so is it
safe for us to access page table after this in do_swap_page_readahead()?

Best Regards,
Huang, Ying

>  	entry = pte_to_swp_entry(vmf->orig_pte);
>  	if (unlikely(non_swap_entry(entry))) {
> @@ -2889,9 +2882,7 @@ int do_swap_page(struct vm_fault *vmf)
>  
>  
>  	delayacct_set_flag(DELAYACCT_PF_SWAPIN);
> -	if (!page)
> -		page = lookup_swap_cache(entry, vma_readahead ? vma : NULL,
> -					 vmf->address);
> +	page = lookup_swap_cache(entry, vma, vmf->address);
>  	if (!page) {
>  		struct swap_info_struct *si = swp_swap_info(entry);
>  
> @@ -2907,9 +2898,9 @@ int do_swap_page(struct vm_fault *vmf)
>  				swap_readpage(page, true);
>  			}
>  		} else {
> -			if (vma_readahead)
> +			if (swap_use_vma_readahead())
>  				page = do_swap_page_readahead(entry,
> -					GFP_HIGHUSER_MOVABLE, vmf, &swap_ra);
> +					GFP_HIGHUSER_MOVABLE, vmf);
>  			else
>  				page = swapin_readahead(entry,
>  				       GFP_HIGHUSER_MOVABLE, vma, vmf->address);
> diff --git a/mm/swap_state.c b/mm/swap_state.c
> index 6c017ced11e6..e3c535fcd2df 100644
> --- a/mm/swap_state.c
> +++ b/mm/swap_state.c
> @@ -331,32 +331,38 @@ struct page *lookup_swap_cache(swp_entry_t entry, struct vm_area_struct *vma,
>  			       unsigned long addr)
>  {
>  	struct page *page;
> -	unsigned long ra_info;
> -	int win, hits, readahead;
>  
>  	page = find_get_page(swap_address_space(entry), swp_offset(entry));
>  
>  	INC_CACHE_INFO(find_total);
>  	if (page) {
> +		bool vma_ra = swap_use_vma_readahead();
> +		bool readahead = TestClearPageReadahead(page);
> +
>  		INC_CACHE_INFO(find_success);
>  		if (unlikely(PageTransCompound(page)))
>  			return page;
> -		readahead = TestClearPageReadahead(page);
> -		if (vma) {
> -			ra_info = GET_SWAP_RA_VAL(vma);
> -			win = SWAP_RA_WIN(ra_info);
> -			hits = SWAP_RA_HITS(ra_info);
> +
> +		if (vma && vma_ra) {
> +			unsigned long ra_val;
> +			int win, hits;
> +
> +			ra_val = GET_SWAP_RA_VAL(vma);
> +			win = SWAP_RA_WIN(ra_val);
> +			hits = SWAP_RA_HITS(ra_val);
>  			if (readahead)
>  				hits = min_t(int, hits + 1, SWAP_RA_HITS_MAX);
>  			atomic_long_set(&vma->swap_readahead_info,
>  					SWAP_RA_VAL(addr, win, hits));
>  		}
> +
>  		if (readahead) {
>  			count_vm_event(SWAP_RA_HIT);
> -			if (!vma)
> +			if (!vma || !vma_ra)
>  				atomic_inc(&swapin_readahead_hits);
>  		}
>  	}
> +
>  	return page;
>  }
>  
> @@ -645,16 +651,15 @@ static inline void swap_ra_clamp_pfn(struct vm_area_struct *vma,
>  		    PFN_DOWN((faddr & PMD_MASK) + PMD_SIZE));
>  }
>  
> -struct page *swap_readahead_detect(struct vm_fault *vmf,
> -				   struct vma_swap_readahead *swap_ra)
> +static void swap_ra_info(struct vm_fault *vmf,
> +			struct vma_swap_readahead *ra_info)
>  {
>  	struct vm_area_struct *vma = vmf->vma;
> -	unsigned long swap_ra_info;
> -	struct page *page;
> +	unsigned long ra_val;
>  	swp_entry_t entry;
>  	unsigned long faddr, pfn, fpfn;
>  	unsigned long start, end;
> -	pte_t *pte;
> +	pte_t *pte, *orig_pte;
>  	unsigned int max_win, hits, prev_win, win, left;
>  #ifndef CONFIG_64BIT
>  	pte_t *tpte;
> @@ -663,30 +668,32 @@ struct page *swap_readahead_detect(struct vm_fault *vmf,
>  	max_win = 1 << min_t(unsigned int, READ_ONCE(page_cluster),
>  			     SWAP_RA_ORDER_CEILING);
>  	if (max_win == 1) {
> -		swap_ra->win = 1;
> -		return NULL;
> +		ra_info->win = 1;
> +		return;
>  	}
>  
>  	faddr = vmf->address;
> -	entry = pte_to_swp_entry(vmf->orig_pte);
> -	if ((unlikely(non_swap_entry(entry))))
> -		return NULL;
> -	page = lookup_swap_cache(entry, vma, faddr);
> -	if (page)
> -		return page;
> +	orig_pte = pte = pte_offset_map(vmf->pmd, faddr);
> +	entry = pte_to_swp_entry(*pte);
> +	if ((unlikely(non_swap_entry(entry)))) {
> +		pte_unmap(orig_pte);
> +		return;
> +	}
>  
>  	fpfn = PFN_DOWN(faddr);
> -	swap_ra_info = GET_SWAP_RA_VAL(vma);
> -	pfn = PFN_DOWN(SWAP_RA_ADDR(swap_ra_info));
> -	prev_win = SWAP_RA_WIN(swap_ra_info);
> -	hits = SWAP_RA_HITS(swap_ra_info);
> -	swap_ra->win = win = __swapin_nr_pages(pfn, fpfn, hits,
> +	ra_val = GET_SWAP_RA_VAL(vma);
> +	pfn = PFN_DOWN(SWAP_RA_ADDR(ra_val));
> +	prev_win = SWAP_RA_WIN(ra_val);
> +	hits = SWAP_RA_HITS(ra_val);
> +	ra_info->win = win = __swapin_nr_pages(pfn, fpfn, hits,
>  					       max_win, prev_win);
>  	atomic_long_set(&vma->swap_readahead_info,
>  			SWAP_RA_VAL(faddr, win, 0));
>  
> -	if (win == 1)
> -		return NULL;
> +	if (win == 1) {
> +		pte_unmap(orig_pte);
> +		return;
> +	}
>  
>  	/* Copy the PTEs because the page table may be unmapped */
>  	if (fpfn == pfn + 1)
> @@ -699,23 +706,21 @@ struct page *swap_readahead_detect(struct vm_fault *vmf,
>  		swap_ra_clamp_pfn(vma, faddr, fpfn - left, fpfn + win - left,
>  				  &start, &end);
>  	}
> -	swap_ra->nr_pte = end - start;
> -	swap_ra->offset = fpfn - start;
> -	pte = vmf->pte - swap_ra->offset;
> +	ra_info->nr_pte = end - start;
> +	ra_info->offset = fpfn - start;
> +	pte -= ra_info->offset;
>  #ifdef CONFIG_64BIT
> -	swap_ra->ptes = pte;
> +	ra_info->ptes = pte;
>  #else
> -	tpte = swap_ra->ptes;
> +	tpte = ra_info->ptes;
>  	for (pfn = start; pfn != end; pfn++)
>  		*tpte++ = *pte++;
>  #endif
> -
> -	return NULL;
> +	pte_unmap(orig_pte);
>  }
>  
>  struct page *do_swap_page_readahead(swp_entry_t fentry, gfp_t gfp_mask,
> -				    struct vm_fault *vmf,
> -				    struct vma_swap_readahead *swap_ra)
> +				    struct vm_fault *vmf)
>  {
>  	struct blk_plug plug;
>  	struct vm_area_struct *vma = vmf->vma;
> @@ -724,12 +729,14 @@ struct page *do_swap_page_readahead(swp_entry_t fentry, gfp_t gfp_mask,
>  	swp_entry_t entry;
>  	unsigned int i;
>  	bool page_allocated;
> +	struct vma_swap_readahead ra_info = {0,};
>  
> -	if (swap_ra->win == 1)
> +	swap_ra_info(vmf, &ra_info);
> +	if (ra_info.win == 1)
>  		goto skip;
>  
>  	blk_start_plug(&plug);
> -	for (i = 0, pte = swap_ra->ptes; i < swap_ra->nr_pte;
> +	for (i = 0, pte = ra_info.ptes; i < ra_info.nr_pte;
>  	     i++, pte++) {
>  		pentry = *pte;
>  		if (pte_none(pentry))
> @@ -745,7 +752,7 @@ struct page *do_swap_page_readahead(swp_entry_t fentry, gfp_t gfp_mask,
>  			continue;
>  		if (page_allocated) {
>  			swap_readpage(page, false);
> -			if (i != swap_ra->offset &&
> +			if (i != ra_info.offset &&
>  			    likely(!PageTransCompound(page))) {
>  				SetPageReadahead(page);
>  				count_vm_event(SWAP_RA);
> @@ -757,7 +764,7 @@ struct page *do_swap_page_readahead(swp_entry_t fentry, gfp_t gfp_mask,
>  	lru_add_drain();
>  skip:
>  	return read_swap_cache_async(fentry, gfp_mask, vma, vmf->address,
> -				     swap_ra->win == 1);
> +				     ra_info.win == 1);
>  }
>  
>  #ifdef CONFIG_SYSFS