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Shutemov" , =?UTF-8?B?SE9SSUdVQ0hJIE5BT1lBKOWggOWPoyDnm7TkuZ8p?= , Wang Yugui , Andrew Morton , Linux MM , Linux Kernel Mailing List Content-Type: text/plain; charset="UTF-8" X-Rspamd-Queue-Id: AD5512C6 Authentication-Results: imf12.hostedemail.com; dkim=pass header.d=gmail.com header.s=20161025 header.b=Lr4oQetM; dmarc=pass (policy=none) header.from=gmail.com; spf=pass (imf12.hostedemail.com: domain of shy828301@gmail.com designates 209.85.208.50 as permitted sender) smtp.mailfrom=shy828301@gmail.com X-Rspamd-Server: rspam03 X-Stat-Signature: pq6b3tfqq379rkp6w9iye4yjw4mtki1h X-HE-Tag: 1621976662-699618 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Tue, May 25, 2021 at 10:34 AM Minchan Kim wrote: > > On Tue, May 25, 2021 at 10:07:05AM -0700, Yang Shi wrote: > > On Tue, May 25, 2021 at 9:46 AM Minchan Kim wrote: > > > > > > 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 > > > > Signed-off-by: Yang Shi > > > > --- > > > > 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? > > > > Yes, of course. Actually this has been discussed in v2 review. Most > > (or half) callers actually don't check the return value of > > try_to_unmap() except hwpoison, vmscan and THP split. It sounds > > suboptimal to have everyone pay the cost. So I thought Hugh's > > suggestion made sense to me. > > Not sure most callers ignore the ret. I am seeing only do_migrate_range > ignores it. Other than that, they checked the success with page_mapped > in the end. I'd think this falls into the "ignore" category as well since the code doesn't check the return value of try_to_unmap() :-). The patch does convert the return value check to page_mapped() check right after try_to_unmap(). > > With returning void, I feel like it's not try sematic function > any longer. If you still want to go with it, I suggest adding > some comment how to check the function's successness in the > comment place you removed above. Thanks for the suggestion. Will add some notes about how to do the check. > > > > > Quoted the discussion below: > > > > > @@ -1777,7 +1779,7 @@ bool try_to_unmap(struct page *page, enum ttu_flags flags) > > > else > > > rmap_walk(page, &rwc); > > > > > > - return !page_mapcount(page) ? true : false; > > > + return !total_mapcount(page) ? true : false; > > > > That always made me wince: "return !total_mapcount(page);" surely. > > > > Or slightly better, "return !page_mapped(page);", since at least that > > one breaks out as soon as it sees a mapcount. Though I guess I'm > > being silly there, since that case should never occur, so both > > total_mapcount() and page_mapped() scan through all pages. > > > > Or better, change try_to_unmap() to void: most callers ignore its > > return value anyway, and make their own decisions; the remaining > > few could be changed to do the same. Though again, I may be > > being silly, since the expensive THP case is not the common case. > > > > > > > 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 > > > > > > > >