linux-kernel - [PATCH 12/13] mm/thp: collapse_file() do try_to_unmap(TTU_BATCH

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Date:   Sun, 6 Feb 2022 13:51:45 -0800 (PST)
From:   Hugh Dickins <hughd@...gle.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
cc:     Michal Hocko <mhocko@...e.com>, Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Matthew Wilcox <willy@...radead.org>,
        David Hildenbrand <david@...hat.com>,
        Alistair Popple <apopple@...dia.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Rik van Riel <riel@...riel.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Yu Zhao <yuzhao@...gle.com>, Greg Thelen <gthelen@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: [PATCH 12/13] mm/thp: collapse_file() do
 try_to_unmap(TTU_BATCH_FLUSH)

collapse_file() is using unmap_mapping_pages(1) on each small page found
mapped, unlike others (reclaim, migration, splitting, memory-failure) who
use try_to_unmap().  There are four advantages to try_to_unmap(): first,
its TTU_IGNORE_MLOCK option now avoids leaving mlocked page in pagevec;
second, its vma lookup uses i_mmap_lock_read() not i_mmap_lock_write();
third, it breaks out early if page is not mapped everywhere it might be;
fourth, its TTU_BATCH_FLUSH option can be used, as in page reclaim, to
save up all the TLB flushing until all of the pages have been unmapped.

Wild guess: perhaps it was originally written to use try_to_unmap(),
but hit the VM_BUG_ON_PAGE(page_mapped) after unmapping, because without
TTU_SYNC it may skip page table locks; but unmap_mapping_pages() never
skips them, so fixed the issue.  I did once hit that VM_BUG_ON_PAGE()
since making this change: we could pass TTU_SYNC here, but I think just
delete the check - the race is very rare, this is an ordinary small page
so we don't need to be so paranoid about mapcount surprises, and the
page_ref_freeze() just below already handles the case adequately.

Signed-off-by: Hugh Dickins <hughd@...gle.com>
---
 mm/khugepaged.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index d5e387c58bde..e0883a33efd6 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1829,13 +1829,12 @@ static void collapse_file(struct mm_struct *mm,
 		}

 		if (page_mapped(page))
-			unmap_mapping_pages(mapping, index, 1, false);
+			try_to_unmap(page, TTU_IGNORE_MLOCK | TTU_BATCH_FLUSH);

 		xas_lock_irq(&xas);
 		xas_set(&xas, index);

 		VM_BUG_ON_PAGE(page != xas_load(&xas), page);
-		VM_BUG_ON_PAGE(page_mapped(page), page);

 		/*
 		 * The page is expected to have page_count() == 3:
@@ -1899,6 +1898,13 @@ static void collapse_file(struct mm_struct *mm,
 	xas_unlock_irq(&xas);
 xa_unlocked:

+	/*
+	 * If collapse is successful, flush must be done now before copying.
+	 * If collapse is unsuccessful, does flush actually need to be done?
+	 * Do it anyway, to clear the state.
+	 */
+	try_to_unmap_flush();
+
 	if (result == SCAN_SUCCEED) {
 		struct page *page, *tmp;

-- 
2.34.1