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SubjectRe: [PATCH -V2] mm: Add PageLayzyFree() helper functions for MADV_FREE
Date
David Hildenbrand <david@redhat.com> writes:

> On 04.03.20 09:17, Huang, Ying wrote:
>> From: Huang Ying <ying.huang@intel.com>
>>
>> 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.
>>
>> 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>
>> ---
>>
>> Changelog:
>>
>> v2:
>>
>> - 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.
>>
>> - 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(-)
>>
>> 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 *page)
>> TESTPAGEFLAG_FALSE(Ksm)
>> #endif
>>
>> +/*
>> + * 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." ?

Thanks!

>> + */
>> +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);
>>
>> 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, struct vm_area_struct *vma,
>> }
>>
>> /* 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, struct vm_area_struct *vma,
>> * discarded. Remap the page to page table.
>> */
>> set_pte_at(mm, address, pvmw.pte, pteval);
>> - SetPageSwapBacked(page);
>> + ClearPageLazyFree(page);
>> ret = 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, struct 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 = PageActive(page);
>>
>> @@ -571,12 +571,7 @@ static void lru_lazyfree_fn(struct page *page, struct 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);
>>
>> __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 = &get_cpu_var(lru_lazyfree_pvecs);
>>
>> 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 page *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 = false;
>> *writeback = 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.

Personally, I still prefer the better named functions than the comments
here and there. But I can understand that people may have different
flavor.

Best Regards,
Huang, Ying

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