Re: [PATCH v3 2/2] mm,thp: Add experimental config option RO_EXEC_FILEMAP_HUGE_FAULT_THP

[Kirill A. Shutemov]

On Wed, Jul 31, 2019 at 02:25:13AM -0600, William Kucharski wrote:
> Add filemap_huge_fault() to attempt to satisfy page
> faults on memory-mapped read-only text pages using THP when possible.
> 
> Signed-off-by: William Kucharski <william.kucharski@xxxxxxxxxx>
> ---
>  include/linux/huge_mm.h |  16 ++-
>  include/linux/mm.h      |   6 +
>  mm/Kconfig              |  15 ++
>  mm/filemap.c            | 300 +++++++++++++++++++++++++++++++++++++++-
>  mm/huge_memory.c        |   3 +
>  mm/mmap.c               |  36 ++++-
>  mm/rmap.c               |   8 ++
>  7 files changed, 374 insertions(+), 10 deletions(-)
> 
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index 45ede62aa85b..b1e5fd3179fd 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -79,13 +79,15 @@ extern struct kobj_attribute shmem_enabled_attr;
>  #define HPAGE_PMD_NR (1<<HPAGE_PMD_ORDER)
>  
>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> -#define HPAGE_PMD_SHIFT PMD_SHIFT
> -#define HPAGE_PMD_SIZE	((1UL) << HPAGE_PMD_SHIFT)
> -#define HPAGE_PMD_MASK	(~(HPAGE_PMD_SIZE - 1))
> -
> -#define HPAGE_PUD_SHIFT PUD_SHIFT
> -#define HPAGE_PUD_SIZE	((1UL) << HPAGE_PUD_SHIFT)
> -#define HPAGE_PUD_MASK	(~(HPAGE_PUD_SIZE - 1))
> +#define HPAGE_PMD_SHIFT		PMD_SHIFT
> +#define HPAGE_PMD_SIZE		((1UL) << HPAGE_PMD_SHIFT)
> +#define HPAGE_PMD_OFFSET	(HPAGE_PMD_SIZE - 1)
> +#define HPAGE_PMD_MASK		(~(HPAGE_PMD_OFFSET))
> +
> +#define HPAGE_PUD_SHIFT		PUD_SHIFT
> +#define HPAGE_PUD_SIZE		((1UL) << HPAGE_PUD_SHIFT)
> +#define HPAGE_PUD_OFFSET	(HPAGE_PUD_SIZE - 1)
> +#define HPAGE_PUD_MASK		(~(HPAGE_PUD_OFFSET))

OFFSET vs MASK semantics can be confusing without reading the definition.
We don't have anything similar for base page size, right (PAGE_OFFSET is
completely different thing :P)?

>  
>  extern bool is_vma_temporary_stack(struct vm_area_struct *vma);
>  
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 0334ca97c584..ba24b515468a 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2433,6 +2433,12 @@ extern void truncate_inode_pages_final(struct address_space *);
>  
>  /* generic vm_area_ops exported for stackable file systems */
>  extern vm_fault_t filemap_fault(struct vm_fault *vmf);
> +
> +#ifdef CONFIG_RO_EXEC_FILEMAP_HUGE_FAULT_THP
> +extern vm_fault_t filemap_huge_fault(struct vm_fault *vmf,
> +			enum page_entry_size pe_size);
> +#endif
> +

No need for #ifdef here.

>  extern void filemap_map_pages(struct vm_fault *vmf,
>  		pgoff_t start_pgoff, pgoff_t end_pgoff);
>  extern vm_fault_t filemap_page_mkwrite(struct vm_fault *vmf);
> diff --git a/mm/Kconfig b/mm/Kconfig
> index 56cec636a1fc..2debaded0e4d 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -736,4 +736,19 @@ config ARCH_HAS_PTE_SPECIAL
>  config ARCH_HAS_HUGEPD
>  	bool
>  
> +config RO_EXEC_FILEMAP_HUGE_FAULT_THP
> +	bool "read-only exec filemap_huge_fault THP support (EXPERIMENTAL)"
> +	depends on TRANSPARENT_HUGE_PAGECACHE && SHMEM
> +
> +	help
> +	    Introduce filemap_huge_fault() to automatically map executable
> +	    read-only pages of mapped files of suitable size and alignment
> +	    using THP if possible.
> +
> +	    This is marked experimental because it is a new feature and is
> +	    dependent upon filesystmes implementing readpages() in a way
> +	    that will recognize large THP pages and read file content to
> +	    them without polluting the pagecache with PAGESIZE pages due
> +	    to readahead.
> +
>  endmenu
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 38b46fc00855..db1d8df20367 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -199,6 +199,8 @@ static void unaccount_page_cache_page(struct address_space *mapping,
>  	nr = hpage_nr_pages(page);
>  
>  	__mod_node_page_state(page_pgdat(page), NR_FILE_PAGES, -nr);
> +
> +#ifndef	CONFIG_RO_EXEC_FILEMAP_HUGE_FAULT_THP
>  	if (PageSwapBacked(page)) {
>  		__mod_node_page_state(page_pgdat(page), NR_SHMEM, -nr);
>  		if (PageTransHuge(page))
> @@ -206,6 +208,13 @@ static void unaccount_page_cache_page(struct address_space *mapping,
>  	} else {
>  		VM_BUG_ON_PAGE(PageTransHuge(page), page);
>  	}
> +#else
> +	if (PageSwapBacked(page))
> +		__mod_node_page_state(page_pgdat(page), NR_SHMEM, -nr);
> +
> +	if (PageTransHuge(page))
> +		__dec_node_page_state(page, NR_SHMEM_THPS);
> +#endif

Again, no need for #ifdef: the new definition should be fine for
everybody.

>  	/*
>  	 * At this point page must be either written or cleaned by
> @@ -1663,7 +1672,8 @@ struct page *pagecache_get_page(struct address_space *mapping, pgoff_t offset,
>  no_page:
>  	if (!page && (fgp_flags & FGP_CREAT)) {
>  		int err;
> -		if ((fgp_flags & FGP_WRITE) && mapping_cap_account_dirty(mapping))
> +		if ((fgp_flags & FGP_WRITE) &&
> +			mapping_cap_account_dirty(mapping))
>  			gfp_mask |= __GFP_WRITE;
>  		if (fgp_flags & FGP_NOFS)
>  			gfp_mask &= ~__GFP_FS;
> @@ -2643,6 +2653,291 @@ vm_fault_t filemap_fault(struct vm_fault *vmf)
>  }
>  EXPORT_SYMBOL(filemap_fault);
>  
> +#ifdef CONFIG_RO_EXEC_FILEMAP_HUGE_FAULT_THP
> +/*
> + * Check for an entry in the page cache which would conflict with the address
> + * range we wish to map using a THP or is otherwise unusable to map a large
> + * cached page.
> + *
> + * The routine will return true if a usable page is found in the page cache
> + * (and *pagep will be set to the address of the cached page), or if no
> + * cached page is found (and *pagep will be set to NULL).
> + */
> +static bool
> +filemap_huge_check_pagecache_usable(struct xa_state *xas,
> +	struct page **pagep, pgoff_t hindex, pgoff_t hindex_max)
> +{
> +	struct page *page;
> +
> +	while (1) {
> +		page = xas_find(xas, hindex_max);
> +
> +		if (xas_retry(xas, page)) {
> +			xas_set(xas, hindex);
> +			continue;
> +		}
> +
> +		/*
> +		 * A found entry is unusable if:
> +		 *	+ the entry is an Xarray value, not a pointer
> +		 *	+ the entry is an internal Xarray node
> +		 *	+ the entry is not a Transparent Huge Page
> +		 *	+ the entry is not a compound page

PageCompound() and PageTransCompound() are the same thing if THP is
enabled at compile time.

PageHuge() check here is looking out of place. I don't thing we can ever
will see hugetlb pages here.

> +		 *	+ the entry is not the head of a compound page
> +		 *	+ the enbry is a page page with an order other than

Typo.

> +		 *	  HPAGE_PMD_ORDER

If you see unexpected page order in page cache, something went horribly
wrong, right?

> +		 *	+ the page's index is not what we expect it to be

Same here.

> +		 *	+ the page is not up-to-date
> +		 *	+ the page is unlocked

Confused here.

Do you expect caller to lock page before the check? If so, state it in the
comment for the function.

> +		 */
> +		if ((page) && (xa_is_value(page) || xa_is_internal(page) ||
> +			(!PageCompound(page)) || (PageHuge(page)) ||
> +			(!PageTransCompound(page)) ||
> +			page != compound_head(page) ||
> +			compound_order(page) != HPAGE_PMD_ORDER ||
> +			page->index != hindex || (!PageUptodate(page)) ||
> +			(!PageLocked(page))))
> +			return false;

Wow. That's unreadable. Can we rewrite it something like (commenting each
check):

		if (!page)
			break;

		if (xa_is_value(page) || xa_is_internal(page))
			return false;

		if (!PageCompound(page))
			return false;

		...

> +
> +		break;
> +	}
> +
> +	xas_set(xas, hindex);
> +	*pagep = page;
> +	return true;
> +}
> +
> +/**
> + * filemap_huge_fault - read in file data for page fault handling to THP
> + * @vmf:	struct vm_fault containing details of the fault
> + * @pe_size:	large page size to map, currently this must be PE_SIZE_PMD
> + *
> + * filemap_huge_fault() is invoked via the vma operations vector for a
> + * mapped memory region to read in file data to a transparent huge page during
> + * a page fault.
> + *
> + * If for any reason we can't allocate a THP, map it or add it to the page
> + * cache, VM_FAULT_FALLBACK will be returned which will cause the fault
> + * handler to try mapping the page using a PAGESIZE page, usually via
> + * filemap_fault() if so speicifed in the vma operations vector.
> + *
> + * Returns either VM_FAULT_FALLBACK or the result of calling allcc_set_pte()
> + * to map the new THP.
> + *
> + * NOTE: This routine depends upon the file system's readpage routine as
> + *       specified in the address space operations vector to recognize when it
> + *	 is being passed a large page and to read the approprate amount of data
> + *	 in full and without polluting the page cache for the large page itself
> + *	 with PAGESIZE pages to perform a buffered read or to pollute what
> + *	 would be the page cache space for any succeeding pages with PAGESIZE
> + *	 pages due to readahead.
> + *
> + *	 It is VITAL that this routine not be enabled without such filesystem
> + *	 support. As there is no way to determine how many bytes were read by
> + *	 the readpage() operation, if only a PAGESIZE page is read, this routine
> + *	 will map the THP containing only the first PAGESIZE bytes of file data
> + *	 to satisfy the fault, which is never the result desired.
> + */
> +vm_fault_t filemap_huge_fault(struct vm_fault *vmf,
> +		enum page_entry_size pe_size)
> +{
> +	struct file *filp = vmf->vma->vm_file;
> +	struct address_space *mapping = filp->f_mapping;
> +	struct vm_area_struct *vma = vmf->vma;
> +
> +	unsigned long haddr = vmf->address & HPAGE_PMD_MASK;
> +	pgoff_t hindex = round_down(vmf->pgoff, HPAGE_PMD_NR);
> +	pgoff_t hindex_max = hindex + HPAGE_PMD_NR;
> +
> +	struct page *cached_page, *hugepage;
> +	struct page *new_page = NULL;
> +
> +	vm_fault_t ret = VM_FAULT_FALLBACK;
> +	int error;
> +
> +	XA_STATE_ORDER(xas, &mapping->i_pages, hindex, HPAGE_PMD_ORDER);
> +
> +	/*
> +	 * Return VM_FAULT_FALLBACK if:
> +	 *
> +	 *	+ pe_size != PE_SIZE_PMD
> +	 *	+ FAULT_FLAG_WRITE is set in vmf->flags
> +	 *	+ vma isn't aligned to allow a PMD mapping
> +	 *	+ PMD would extend beyond the end of the vma
> +	 */
> +	if (pe_size != PE_SIZE_PMD || (vmf->flags & FAULT_FLAG_WRITE) ||
> +		(haddr < vma->vm_start ||
> +		(haddr + HPAGE_PMD_SIZE > vma->vm_end)))
> +		return ret;

You also need to check that VMA alignment is suitable for huge pages.
See transhuge_vma_suitable().

> +
> +	xas_lock_irq(&xas);
> +
> +retry_xas_locked:
> +	if (!filemap_huge_check_pagecache_usable(&xas, &cached_page, hindex,
> +		hindex_max)) {

I don't see how this check will ever succeed. Who locks the page here?

> +		/* found a conflicting entry in the page cache, so fallback */
> +		goto unlock;
> +	} else if (cached_page) {
> +		/* found a valid cached page, so map it */
> +		hugepage = cached_page;
> +		goto map_huge;
> +	}
> +
> +	xas_unlock_irq(&xas);
> +
> +	/* allocate huge THP page in VMA */
> +	new_page = __page_cache_alloc(vmf->gfp_mask | __GFP_COMP |
> +		__GFP_NOWARN | __GFP_NORETRY, HPAGE_PMD_ORDER);
> +
> +	if (unlikely(!new_page))
> +		return ret;
> +
> +	if (unlikely(!(PageCompound(new_page)))) {

How can it happen?

> +		put_page(new_page);
> +		return ret;
> +	}
> +
> +	prep_transhuge_page(new_page);
> +	new_page->index = hindex;
> +	new_page->mapping = mapping;
> +
> +	__SetPageLocked(new_page);
> +
> +	/*
> +	 * The readpage() operation below is expected to fill the large
> +	 * page with data without polluting the page cache with
> +	 * PAGESIZE entries due to a buffered read and/or readahead().
> +	 *
> +	 * A filesystem's vm_operations_struct huge_fault field should
> +	 * never point to this routine without such a capability, and
> +	 * without it a call to this routine would eventually just
> +	 * fall through to the normal fault op anyway.
> +	 */
> +	error = mapping->a_ops->readpage(vmf->vma->vm_file, new_page);
> +
> +	if (unlikely(error)) {
> +		put_page(new_page);
> +		return ret;
> +	}
> +
> +	/* XXX - use wait_on_page_locked_killable() instead? */
> +	wait_on_page_locked(new_page);
> +
> +	if (!PageUptodate(new_page)) {
> +		/* EIO */
> +		new_page->mapping = NULL;
> +		put_page(new_page);
> +		return ret;
> +	}
> +
> +	do {
> +		xas_lock_irq(&xas);
> +		xas_set(&xas, hindex);
> +		xas_create_range(&xas);
> +
> +		if (!(xas_error(&xas)))
> +			break;
> +
> +		if (!xas_nomem(&xas, GFP_KERNEL)) {
> +			if (new_page) {
> +				new_page->mapping = NULL;
> +				put_page(new_page);
> +			}
> +
> +			goto unlock;
> +		}
> +
> +		xas_unlock_irq(&xas);
> +	} while (1);
> +
> +	/*
> +	 * Double check that an entry did not sneak into the page cache while
> +	 * creating Xarray entries for the new page.
> +	 */
> +	if (!filemap_huge_check_pagecache_usable(&xas, &cached_page, hindex,
> +		hindex_max)) {
> +		/*
> +		 * An unusable entry was found, so delete the newly allocated
> +		 * page and fallback.
> +		 */
> +		new_page->mapping = NULL;
> +		put_page(new_page);
> +		goto unlock;
> +	} else if (cached_page) {
> +		/*
> +		 * A valid large page was found in the page cache, so free the
> +		 * newly allocated page and map the cached page instead.
> +		 */
> +		new_page->mapping = NULL;
> +		put_page(new_page);
> +		new_page = NULL;
> +		hugepage = cached_page;
> +		goto map_huge;
> +	}
> +
> +	__SetPageLocked(new_page);

Again?

> +
> +	/* did it get truncated? */
> +	if (unlikely(new_page->mapping != mapping)) {

Hm. IIRC this path only reachable for just allocated page that is not
exposed to anybody yet. How can it be truncated?

> +		unlock_page(new_page);
> +		put_page(new_page);
> +		goto retry_xas_locked;
> +	}
> +
> +	hugepage = new_page;
> +
> +map_huge:
> +	/* map hugepage at the PMD level */
> +	ret = alloc_set_pte(vmf, NULL, hugepage);

It has to be

	ret = alloc_set_pte(vmf, vmf->memcg, hugepage);

right?

> +
> +	VM_BUG_ON_PAGE((!(pmd_trans_huge(*vmf->pmd))), hugepage);
> +
> +	if (likely(!(ret & VM_FAULT_ERROR))) {
> +		/*
> +		 * The alloc_set_pte() succeeded without error, so
> +		 * add the page to the page cache if it is new, and
> +		 * increment page statistics accordingly.
> +		 */

It looks backwards to me. I believe the page must be in page cache
*before* it got mapped.

I expect all sorts of weird bug due to races when the page is mapped but
not visible via syscalls.

Hm?

> +		if (new_page) {
> +			unsigned long nr;
> +
> +			xas_set(&xas, hindex);
> +
> +			for (nr = 0; nr < HPAGE_PMD_NR; nr++) {
> +#ifndef	COMPOUND_PAGES_HEAD_ONLY
> +				xas_store(&xas, new_page + nr);
> +#else
> +				xas_store(&xas, new_page);
> +#endif
> +				xas_next(&xas);
> +			}
> +
> +			count_vm_event(THP_FILE_ALLOC);
> +			__inc_node_page_state(new_page, NR_SHMEM_THPS);
> +			__mod_node_page_state(page_pgdat(new_page),
> +				NR_FILE_PAGES, HPAGE_PMD_NR);
> +			__mod_node_page_state(page_pgdat(new_page),
> +				NR_SHMEM, HPAGE_PMD_NR);
> +		}
> +
> +		vmf->address = haddr;
> +		vmf->page = hugepage;
> +
> +		page_ref_add(hugepage, HPAGE_PMD_NR);
> +		count_vm_event(THP_FILE_MAPPED);
> +	} else if (new_page) {
> +		/* there was an error mapping the new page, so release it */
> +		new_page->mapping = NULL;
> +		put_page(new_page);
> +	}
> +
> +unlock:
> +	xas_unlock_irq(&xas);
> +	return ret;
> +}
> +EXPORT_SYMBOL(filemap_huge_fault);
> +#endif
> +
>  void filemap_map_pages(struct vm_fault *vmf,
>  		pgoff_t start_pgoff, pgoff_t end_pgoff)
>  {
> @@ -2925,7 +3220,8 @@ struct page *read_cache_page(struct address_space *mapping,
>  EXPORT_SYMBOL(read_cache_page);
>  
>  /**
> - * read_cache_page_gfp - read into page cache, using specified page allocation flags.
> + * read_cache_page_gfp - read into page cache, using specified page allocation
> + *			 flags.
>   * @mapping:	the page's address_space
>   * @index:	the page index
>   * @gfp:	the page allocator flags to use if allocating
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 1334ede667a8..26d74466d1f7 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -543,8 +543,11 @@ unsigned long thp_get_unmapped_area(struct file *filp, unsigned long addr,
>  
>  	if (addr)
>  		goto out;
> +
> +#ifndef CONFIG_RO_EXEC_FILEMAP_HUGE_FAULT_THP

IS_ENABLED()?

>  	if (!IS_DAX(filp->f_mapping->host) || !IS_ENABLED(CONFIG_FS_DAX_PMD))
>  		goto out;
> +#endif
>  
>  	addr = __thp_get_unmapped_area(filp, len, off, flags, PMD_SIZE);
>  	if (addr)
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 7e8c3e8ae75f..96ff80d2a8fb 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -1391,6 +1391,10 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
>  	struct mm_struct *mm = current->mm;
>  	int pkey = 0;
>  
> +#ifdef CONFIG_RO_EXEC_FILEMAP_HUGE_FAULT_THP
> +	unsigned long vm_maywrite = VM_MAYWRITE;
> +#endif
> +
>  	*populate = 0;
>  
>  	if (!len)
> @@ -1429,7 +1433,33 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
>  	/* Obtain the address to map to. we verify (or select) it and ensure
>  	 * that it represents a valid section of the address space.
>  	 */
> -	addr = get_unmapped_area(file, addr, len, pgoff, flags);
> +
> +#ifdef CONFIG_RO_EXEC_FILEMAP_HUGE_FAULT_THP
> +	/*
> +	 * If THP is enabled, it's a read-only executable that is
> +	 * MAP_PRIVATE mapped, the length is larger than a PMD page
> +	 * and either it's not a MAP_FIXED mapping or the passed address is
> +	 * properly aligned for a PMD page, attempt to get an appropriate
> +	 * address at which to map a PMD-sized THP page, otherwise call the
> +	 * normal routine.
> +	 */
> +	if ((prot & PROT_READ) && (prot & PROT_EXEC) &&
> +		(!(prot & PROT_WRITE)) && (flags & MAP_PRIVATE) &&

Why require PROT_EXEC && PROT_READ. You only must ask for !PROT_WRITE.

And how do you protect against mprotect() later? Should you ask for
ro-file instead?

> +		(!(flags & MAP_FIXED)) && len >= HPAGE_PMD_SIZE &&
> +		(!(addr & HPAGE_PMD_OFFSET))) {

All size considerations are already handled by thp_get_unmapped_area(). No
need to duplicate it here.

You might want to add thp_ro_get_unmapped_area() that would check file for
RO, before going for THP-suitable mapping.

> +		addr = thp_get_unmapped_area(file, addr, len, pgoff, flags);
> +
> +		if (addr && (!(addr & HPAGE_PMD_OFFSET)))
> +			vm_maywrite = 0;

Oh. That's way too hacky. Better to ask for RO file instead.

> +		else
> +			addr = get_unmapped_area(file, addr, len, pgoff, flags);
> +	} else {
> +#endif
> +		addr = get_unmapped_area(file, addr, len, pgoff, flags);
> +#ifdef CONFIG_RO_EXEC_FILEMAP_HUGE_FAULT_THP
> +	}
> +#endif
> +
>  	if (offset_in_page(addr))
>  		return addr;
>  
> @@ -1451,7 +1481,11 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
>  	 * of the memory object, so we don't do any here.
>  	 */
>  	vm_flags |= calc_vm_prot_bits(prot, pkey) | calc_vm_flag_bits(flags) |
> +#ifdef CONFIG_RO_EXEC_FILEMAP_HUGE_FAULT_THP
> +			mm->def_flags | VM_MAYREAD | vm_maywrite | VM_MAYEXEC;
> +#else
>  			mm->def_flags | VM_MAYREAD | VM_MAYWRITE | VM_MAYEXEC;
> +#endif
>  
>  	if (flags & MAP_LOCKED)
>  		if (!can_do_mlock())
> diff --git a/mm/rmap.c b/mm/rmap.c
> index e5dfe2ae6b0d..503612d3b52b 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1192,7 +1192,11 @@ void page_add_file_rmap(struct page *page, bool compound)
>  		}
>  		if (!atomic_inc_and_test(compound_mapcount_ptr(page)))
>  			goto out;
> +
> +#ifndef CONFIG_RO_EXEC_FILEMAP_HUGE_FAULT_THP
>  		VM_BUG_ON_PAGE(!PageSwapBacked(page), page);
> +#endif
> +

Just remove it. Don't add more #ifdefs.

>  		__inc_node_page_state(page, NR_SHMEM_PMDMAPPED);
>  	} else {
>  		if (PageTransCompound(page) && page_mapping(page)) {
> @@ -1232,7 +1236,11 @@ static void page_remove_file_rmap(struct page *page, bool compound)
>  		}
>  		if (!atomic_add_negative(-1, compound_mapcount_ptr(page)))
>  			goto out;
> +
> +#ifndef CONFIG_RO_EXEC_FILEMAP_HUGE_FAULT_THP
>  		VM_BUG_ON_PAGE(!PageSwapBacked(page), page);
> +#endif
> +

Ditto.

>  		__dec_node_page_state(page, NR_SHMEM_PMDMAPPED);
>  	} else {
>  		if (!atomic_add_negative(-1, &page->_mapcount))
> -- 
> 2.21.0
> 

-- 
 Kirill A. Shutemov