Re: [v3 PATCH 1/2] mm: rmap: make try_to_unmap() void function

Minchan Kim
2021-05-25T16:46:33Z

On Tue, May 25, 2021 at 09:21:44AM -0700, Yang Shi wrote:
> Currently try_to_unmap() return bool value by checking page_mapcount(),
> however this may return false positive since page_mapcount() doesn't
> check all subpages of compound page. The total_mapcount() could be used
> instead, but its cost is higher since it traverses all subpages.
>
> Actually the most callers of try_to_unmap() don't care about the
> return value at all. So just need check if page is still mapped by
> page_mapped() when necessary. And page_mapped() does bail out early
> when it finds mapped subpage.
>
> Suggested-by: Hugh Dickins <hughd@google.com>
> Signed-off-by: Yang Shi <shy828301@gmail.com>
> ---
> include/linux/rmap.h | 2 +-
> mm/huge_memory.c | 4 +---
> mm/memory-failure.c | 13 ++++++-------
> mm/rmap.c | 6 +-----
> mm/vmscan.c | 3 ++-
> 5 files changed, 11 insertions(+), 17 deletions(-)
>
> diff --git a/include/linux/rmap.h b/include/linux/rmap.h
> index def5c62c93b3..116cb193110a 100644
> --- a/include/linux/rmap.h
> +++ b/include/linux/rmap.h
> @@ -194,7 +194,7 @@ static inline void page_dup_rmap(struct page *page, bool compound)
> int page_referenced(struct page *, int is_locked,
> struct mem_cgroup *memcg, unsigned long *vm_flags);
>
> -bool try_to_unmap(struct page *, enum ttu_flags flags);
> +void try_to_unmap(struct page *, enum ttu_flags flags);
>
> /* Avoid racy checks */
> #define PVMW_SYNC (1 << 0)
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 19195fca1aee..80fe642d742d 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2336,15 +2336,13 @@ static void unmap_page(struct page *page)
> {
> enum ttu_flags ttu_flags = TTU_IGNORE_MLOCK |
> TTU_RMAP_LOCKED | TTU_SPLIT_HUGE_PMD;
> - bool unmap_success;
>
> VM_BUG_ON_PAGE(!PageHead(page), page);
>
> if (PageAnon(page))
> ttu_flags |= TTU_SPLIT_FREEZE;
>
> - unmap_success = try_to_unmap(page, ttu_flags);
> - VM_BUG_ON_PAGE(!unmap_success, page);
> + try_to_unmap(page, ttu_flags);
> }
>
> static void remap_page(struct page *page, unsigned int nr)
> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> index 9dcc9bcea731..6dd53ff34825 100644
> --- a/mm/memory-failure.c
> +++ b/mm/memory-failure.c
> @@ -1126,7 +1126,7 @@ static bool hwpoison_user_mappings(struct page *p, unsigned long pfn,
> collect_procs(hpage, &tokill, flags & MF_ACTION_REQUIRED);
>
> if (!PageHuge(hpage)) {
> - unmap_success = try_to_unmap(hpage, ttu);
> + try_to_unmap(hpage, ttu);
> } else {
> if (!PageAnon(hpage)) {
> /*
> @@ -1138,17 +1138,16 @@ static bool hwpoison_user_mappings(struct page *p, unsigned long pfn,
> */
> mapping = hugetlb_page_mapping_lock_write(hpage);
> if (mapping) {
> - unmap_success = try_to_unmap(hpage,
> - ttu|TTU_RMAP_LOCKED);
> + try_to_unmap(hpage, ttu|TTU_RMAP_LOCKED);
> i_mmap_unlock_write(mapping);
> - } else {
> + } else
> pr_info("Memory failure: %#lx: could not lock mapping for mapped huge page\n", pfn);
> - unmap_success = false;
> - }
> } else {
> - unmap_success = try_to_unmap(hpage, ttu);
> + try_to_unmap(hpage, ttu);
> }
> }
> +
> + unmap_success = !page_mapped(hpage);
> if (!unmap_success)
> pr_err("Memory failure: %#lx: failed to unmap page (mapcount=%d)\n",
> pfn, page_mapcount(hpage));
> diff --git a/mm/rmap.c b/mm/rmap.c
> index a35cbbbded0d..728de421e43a 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1748,10 +1748,8 @@ static int page_not_mapped(struct page *page)
> *
> * Tries to remove all the page table entries which are mapping this
> * page, used in the pageout path. Caller must hold the page lock.
> - *
> - * If unmap is successful, return true. Otherwise, false.
> */
> -bool try_to_unmap(struct page *page, enum ttu_flags flags)
> +void try_to_unmap(struct page *page, enum ttu_flags flags)
> {
> struct rmap_walk_control rwc = {
> .rmap_one = try_to_unmap_one,
> @@ -1776,8 +1774,6 @@ bool try_to_unmap(struct page *page, enum ttu_flags flags)
> rmap_walk_locked(page, &rwc);
> else
> rmap_walk(page, &rwc);
> -
> - return !page_mapcount(page) ? true : false;

Couldn't we use page_mapped instead of page_mapcount here?
With boolean return of try sematic looks reasonable to me
rather than void.

> }
>
> /**
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index f96d62159720..fa5052ace415 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1499,7 +1499,8 @@ static unsigned int shrink_page_list(struct list_head *page_list,
> if (unlikely(PageTransHuge(page)))
> flags |= TTU_SPLIT_HUGE_PMD;
>
> - if (!try_to_unmap(page, flags)) {
> + try_to_unmap(page, flags);
> + if (page_mapped(page)) {
> stat->nr_unmap_fail += nr_pages;
> if (!was_swapbacked && PageSwapBacked(page))
> stat->nr_lazyfree_fail += nr_pages;
> --
> 2.26.2
>
>