Re: [PATCH v3 1/8] mm: Remove special swap entry functions

[Alistair Popple <apopple@nvidia.com>]

On Saturday, 27 February 2021 2:59:09 AM AEDT Christoph Hellwig wrote:
> > -		struct page *page = migration_entry_to_page(entry);
> > +		struct page *page = pfn_to_page(swp_offset(entry));
> 
> I wonder if keeping a single special_entry_to_page() helper would still
> me a useful.  But I'm not entirely sure.  There are also two more open
> coded copies of this in the THP migration code.

I think it might be if only to clearly document where these entries are used. 
Will add it for the next version to see what it looks like.

> > -#define free_swap_and_cache(e) ({(is_migration_entry(e) || 
is_device_private_entry(e));})
> > -#define swapcache_prepare(e) ({(is_migration_entry(e) || 
is_device_private_entry(e));})
> > +#define free_swap_and_cache(e) is_special_entry(e)
> > +#define swapcache_prepare(e) is_special_entry(e)
> 
> Staring at this I'm really, really confused at what this is doing.
> 
> Looking a little closer these are the !CONFIG_SWAP stubs, but it could
> probably use a comment or two.

Will do, thanks.

 - Alistair
 
> >  	} else if (is_migration_entry(entry)) {
> > -		page = migration_entry_to_page(entry);
> > +		page = pfn_to_page(swp_offset(entry));
> >  
> >  		rss[mm_counter(page)]++;
> >  
> > @@ -737,7 +737,7 @@ copy_nonpresent_pte(struct mm_struct *dst_mm, struct 
mm_struct *src_mm,
> >  			set_pte_at(src_mm, addr, src_pte, pte);
> >  		}
> >  	} else if (is_device_private_entry(entry)) {
> > -		page = device_private_entry_to_page(entry);
> > +		page = pfn_to_page(swp_offset(entry));
> >  
> >  		/*
> >  		 * Update rss count even for unaddressable pages, as
> > @@ -1274,7 +1274,7 @@ static unsigned long zap_pte_range(struct mmu_gather 
*tlb,
> >  
> >  		entry = pte_to_swp_entry(ptent);
> >  		if (is_device_private_entry(entry)) {
> > -			struct page *page = device_private_entry_to_page(entry);
> > +			struct page *page = pfn_to_page(swp_offset(entry));
> >  
> >  			if (unlikely(details && details->check_mapping)) {
> >  				/*
> > @@ -1303,7 +1303,7 @@ static unsigned long zap_pte_range(struct mmu_gather 
*tlb,
> >  		else if (is_migration_entry(entry)) {
> >  			struct page *page;
> >  
> > -			page = migration_entry_to_page(entry);
> > +			page = pfn_to_page(swp_offset(entry));
> >  			rss[mm_counter(page)]--;
> >  		}
> >  		if (unlikely(!free_swap_and_cache(entry)))
> > @@ -3271,7 +3271,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >  			migration_entry_wait(vma->vm_mm, vmf->pmd,
> >  					     vmf->address);
> >  		} else if (is_device_private_entry(entry)) {
> > -			vmf->page = device_private_entry_to_page(entry);
> > +			vmf->page = pfn_to_page(swp_offset(entry));
> >  			ret = vmf->page->pgmap->ops->migrate_to_ram(vmf);
> >  		} else if (is_hwpoison_entry(entry)) {
> >  			ret = VM_FAULT_HWPOISON;
> > diff --git a/mm/migrate.c b/mm/migrate.c
> > index 20ca887ea769..72adcc3d8f5b 100644
> > --- a/mm/migrate.c
> > +++ b/mm/migrate.c
> > @@ -321,7 +321,7 @@ void __migration_entry_wait(struct mm_struct *mm, 
pte_t *ptep,
> >  	if (!is_migration_entry(entry))
> >  		goto out;
> >  
> > -	page = migration_entry_to_page(entry);
> > +	page = pfn_to_page(swp_offset(entry));
> >  
> >  	/*
> >  	 * Once page cache replacement of page migration started, page_count
> > @@ -361,7 +361,7 @@ void pmd_migration_entry_wait(struct mm_struct *mm, 
pmd_t *pmd)
> >  	ptl = pmd_lock(mm, pmd);
> >  	if (!is_pmd_migration_entry(*pmd))
> >  		goto unlock;
> > -	page = migration_entry_to_page(pmd_to_swp_entry(*pmd));
> > +	page = pfn_to_page(swp_offset(pmd_to_swp_entry(*pmd)));
> >  	if (!get_page_unless_zero(page))
> >  		goto unlock;
> >  	spin_unlock(ptl);
> > @@ -2437,7 +2437,7 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
> >  			if (!is_device_private_entry(entry))
> >  				goto next;
> >  
> > -			page = device_private_entry_to_page(entry);
> > +			page = pfn_to_page(swp_offset(entry));
> >  			if (!(migrate->flags &
> >  				MIGRATE_VMA_SELECT_DEVICE_PRIVATE) ||
> >  			    page->pgmap->owner != migrate->pgmap_owner)
> > diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
> > index 86e3a3688d59..34230d08556a 100644
> > --- a/mm/page_vma_mapped.c
> > +++ b/mm/page_vma_mapped.c
> > @@ -96,7 +96,7 @@ static bool check_pte(struct page_vma_mapped_walk *pvmw)
> >  		if (!is_migration_entry(entry))
> >  			return false;
> >  
> > -		pfn = migration_entry_to_pfn(entry);
> > +		pfn = swp_offset(entry);
> >  	} else if (is_swap_pte(*pvmw->pte)) {
> >  		swp_entry_t entry;
> >  
> > @@ -105,7 +105,7 @@ static bool check_pte(struct page_vma_mapped_walk 
*pvmw)
> >  		if (!is_device_private_entry(entry))
> >  			return false;
> >  
> > -		pfn = device_private_entry_to_pfn(entry);
> > +		pfn = swp_offset(entry);
> >  	} else {
> >  		if (!pte_present(*pvmw->pte))
> >  			return false;
> > @@ -200,7 +200,7 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk 
*pvmw)
> >  				if (is_migration_entry(pmd_to_swp_entry(*pvmw->pmd))) {
> >  					swp_entry_t entry = pmd_to_swp_entry(*pvmw->pmd);
> >  
> > -					if (migration_entry_to_page(entry) != page)
> > +					if (pfn_to_page(swp_offset(entry)) != page)
> >  						return not_found(pvmw);
> >  					return true;
> >  				}
> ---end quoted text---
>