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Subject: Re: [PATCH -V2] mm: Add PageLayzyFree() helper functions for
 MADV_FREE
To:     "Huang, Ying" <ying.huang@intel.com>,
        Andrew Morton <akpm@linux-foundation.org>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Mel Gorman <mgorman@suse.de>, Vlastimil Babka <vbabka@suse.cz>,
        Zi Yan <ziy@nvidia.com>, Michal Hocko <mhocko@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Minchan Kim <minchan@kernel.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Hugh Dickins <hughd@google.com>
References: <20200304081732.563536-1-ying.huang@intel.com>
From:   David Hildenbrand <david@redhat.com>
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Organization: Red Hat GmbH
Message-ID: <d7dcb472-76fa-9d8b-513a-793a7ab8580d@redhat.com>
Date:   Wed, 4 Mar 2020 09:28:37 +0100
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On 04.03.20 09:17, Huang, Ying wrote:
> From: Huang Ying <ying.huang@intel.com>
>=20
> Now PageSwapBacked() is used as the helper function to check whether
> pages have been freed lazily via MADV_FREE.  This isn't very obvious.
> So Dave suggested to add PageLazyFree() family helper functions to
> improve the code readability.
>=20
> Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
> Suggested-by: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: Mel Gorman <mgorman@suse.de>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Cc: Zi Yan <ziy@nvidia.com>
> Cc: Michal Hocko <mhocko@kernel.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Minchan Kim <minchan@kernel.org>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Hugh Dickins <hughd@google.com>
> ---
>=20
> Changelog:
>=20
> v2:
>=20
> - Avoid code bloat via removing VM_BUG_ON_PAGE(), which doesn't exist
>   in the original code.  Now there is no any text/data/bss size
>   change.
>=20
> - Fix one wrong replacement in try_to_unmap_one().
> ---
>  include/linux/page-flags.h | 25 +++++++++++++++++++++++++
>  mm/rmap.c                  |  4 ++--
>  mm/swap.c                  | 11 +++--------
>  mm/vmscan.c                |  7 +++----
>  4 files changed, 33 insertions(+), 14 deletions(-)
>=20
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index 49c2697046b9..192b593750d3 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -498,6 +498,31 @@ static __always_inline int PageKsm(struct page *pa=
ge)
>  TESTPAGEFLAG_FALSE(Ksm)
>  #endif
> =20
> +/*
> + * For pages freed lazily via MADV_FREE.  lazyfree pages are clean
> + * anonymous pages.  They have SwapBacked flag cleared to distinguish
> + * with normal anonymous pages

"They don't have PG_swapbacked set, to distinguish them from normal
anonymous pages." ?

> + */
> +static __always_inline int __PageLazyFree(struct page *page)
> +{
> +	return !PageSwapBacked(page);
> +}
> +
> +static __always_inline int PageLazyFree(struct page *page)
> +{
> +	return PageAnon(page) && __PageLazyFree(page);
> +}
> +
> +static __always_inline void SetPageLazyFree(struct page *page)
> +{
> +	ClearPageSwapBacked(page);
> +}
> +
> +static __always_inline void ClearPageLazyFree(struct page *page)
> +{
> +	SetPageSwapBacked(page);
> +}
> +
>  u64 stable_page_flags(struct page *page);
> =20
>  static inline int PageUptodate(struct page *page)
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 1c02adaa233e..6ec96c8e7826 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1609,7 +1609,7 @@ static bool try_to_unmap_one(struct page *page, s=
truct vm_area_struct *vma,
>  			}
> =20
>  			/* MADV_FREE page check */
> -			if (!PageSwapBacked(page)) {
> +			if (__PageLazyFree(page)) {
>  				if (!PageDirty(page)) {
>  					/* Invalidate as we cleared the pte */
>  					mmu_notifier_invalidate_range(mm,
> @@ -1623,7 +1623,7 @@ static bool try_to_unmap_one(struct page *page, s=
truct vm_area_struct *vma,
>  				 * discarded. Remap the page to page table.
>  				 */
>  				set_pte_at(mm, address, pvmw.pte, pteval);
> -				SetPageSwapBacked(page);
> +				ClearPageLazyFree(page);
>  				ret =3D false;
>  				page_vma_mapped_walk_done(&pvmw);
>  				break;
> diff --git a/mm/swap.c b/mm/swap.c
> index c1d3ca80ea10..d83f2cd4cdb8 100644
> --- a/mm/swap.c
> +++ b/mm/swap.c
> @@ -563,7 +563,7 @@ static void lru_deactivate_fn(struct page *page, st=
ruct lruvec *lruvec,
>  static void lru_lazyfree_fn(struct page *page, struct lruvec *lruvec,
>  			    void *arg)
>  {
> -	if (PageLRU(page) && PageAnon(page) && PageSwapBacked(page) &&
> +	if (PageLRU(page) && PageAnon(page) && !__PageLazyFree(page) &&

See my comment below

>  	    !PageSwapCache(page) && !PageUnevictable(page)) {
>  		bool active =3D PageActive(page);
> =20
> @@ -571,12 +571,7 @@ static void lru_lazyfree_fn(struct page *page, str=
uct lruvec *lruvec,
>  				       LRU_INACTIVE_ANON + active);
>  		ClearPageActive(page);
>  		ClearPageReferenced(page);
> -		/*
> -		 * lazyfree pages are clean anonymous pages. They have
> -		 * SwapBacked flag cleared to distinguish normal anonymous
> -		 * pages
> -		 */
> -		ClearPageSwapBacked(page);
> +		SetPageLazyFree(page);
>  		add_page_to_lru_list(page, lruvec, LRU_INACTIVE_FILE);
> =20
>  		__count_vm_events(PGLAZYFREE, hpage_nr_pages(page));
> @@ -678,7 +673,7 @@ void deactivate_page(struct page *page)
>   */
>  void mark_page_lazyfree(struct page *page)
>  {
> -	if (PageLRU(page) && PageAnon(page) && PageSwapBacked(page) &&
> +	if (PageLRU(page) && PageAnon(page) && !__PageLazyFree(page) &&

See my comment below.

>  	    !PageSwapCache(page) && !PageUnevictable(page)) {
>  		struct pagevec *pvec =3D &get_cpu_var(lru_lazyfree_pvecs);
> =20
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index eca49a1c2f68..40bb41ada2d2 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1043,8 +1043,7 @@ static void page_check_dirty_writeback(struct pag=
e *page,
>  	 * Anonymous pages are not handled by flushers and must be written
>  	 * from reclaim context. Do not stall reclaim based on them
>  	 */
> -	if (!page_is_file_cache(page) ||
> -	    (PageAnon(page) && !PageSwapBacked(page))) {
> +	if (!page_is_file_cache(page) || PageLazyFree(page)) {
>  		*dirty =3D false;
>  		*writeback =3D false;
>  		return;
> @@ -1235,7 +1234,7 @@ static unsigned long shrink_page_list(struct list=
_head *page_list,
>  		 * Try to allocate it some swap space here.
>  		 * Lazyfree page could be freed directly
>  		 */
> -		if (PageAnon(page) && PageSwapBacked(page)) {
> +		if (PageAnon(page) && !__PageLazyFree(page)) {

I don't think this chunk makes this code easier to understand. I'd just
keep this as is. IOW, I prefer PageSwapBacked() over !__PageLazyFree().


In general, I don't think this patch really improves the situation ...
it's only a handful of places where this change slightly makes the code
easier to understand. And there, only slightly ... I'd prefer better
comments instead (e.g., in PageAnon()), documenting what it means for a
anon page to either have PageSwapBacked() set or not.

--=20
Thanks,

David / dhildenb