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

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Minchan Kim <minchan@xxxxxxxxxx> writes:

> Hi Huang,
>
> On Wed, Nov 01, 2017 at 01:41:00PM +0800, Huang, Ying wrote:
>> 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()?
>
> That's why I calls pte_offset_map in swap_ra_info before the access.

Oh, I found it!  Thanks for explanation!

Best Regards,
Huang, Ying

>> 
>> 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
>> 
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