linux-kernel - Re: [PATCH v2] mm,thp: recheck each page before collapsing file THP

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Message-ID: <20191018165844.GB179426@cmpxchg.org>
Date:   Fri, 18 Oct 2019 12:58:44 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Song Liu <songliubraving@...com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        akpm@...ux-foundation.org, matthew.wilcox@...cle.com,
        kernel-team@...com, william.kucharski@...cle.com,
        kirill.shutemov@...ux.intel.com, Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH v2] mm,thp: recheck each page before collapsing file THP

On Fri, Oct 18, 2019 at 12:49:46PM -0400, Johannes Weiner wrote:
> On Fri, Oct 18, 2019 at 09:37:54AM -0700, Song Liu wrote:
> > In collapse_file(), after locking the page, it is necessary to recheck
> > that the page is up-to-date. Add PageUptodate() check for both shmem THP
> > and file THP.
> > 
> > Current khugepaged should not try to collapse dirty file THP, because it
> > is limited to read only text. Add a PageDirty check and warning for file
> > THP. This is added after page_mapping() check, because if the page is
> > truncated, it might be dirty.
> > 
> > Fixes: 99cb0dbd47a1 ("mm,thp: add read-only THP support for (non-shmem) FS")
> > Cc: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> > Cc: Johannes Weiner <hannes@...xchg.org>
> > Cc: Hugh Dickins <hughd@...gle.com>
> > Cc: William Kucharski <william.kucharski@...cle.com>
> > Cc: Andrew Morton <akpm@...ux-foundation.org>
> > Signed-off-by: Song Liu <songliubraving@...com>
> 
> Acked-by: Johannes Weiner <hannes@...xchg.org>

We should also be able to remove the unlocked tests for those two
conditions, right?

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 0a1b4b484ac5..a3ef6ce86bfa 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1601,17 +1601,6 @@ static void collapse_file(struct mm_struct *mm,
 					result = SCAN_FAIL;
 					goto xa_unlocked;
 				}
-			} else if (!PageUptodate(page)) {
-				xas_unlock_irq(&xas);
-				wait_on_page_locked(page);
-				if (!trylock_page(page)) {
-					result = SCAN_PAGE_LOCK;
-					goto xa_unlocked;
-				}
-				get_page(page);
-			} else if (PageDirty(page)) {
-				result = SCAN_FAIL;
-				goto xa_locked;
 			} else if (trylock_page(page)) {
 				get_page(page);
 				xas_unlock_irq(&xas);