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* [patch 136/158] mm/thp: flush file for !is_shmem PageDirty() case in collapse_file()
@ 2019-12-01  1:57 akpm
  0 siblings, 0 replies; only message in thread
From: akpm @ 2019-12-01  1:57 UTC (permalink / raw)
  To: akpm, hannes, hughd, kirill.shutemov, linux-mm, mm-commits,
	songliubraving, torvalds, william.kucharski, willy

From: Song Liu <songliubraving@fb.com>
Subject: mm/thp: flush file for !is_shmem PageDirty() case in collapse_file()

For non-shmem file THPs, khugepaged only collapses read only .text mapping
(VM_DENYWRITE).  These pages should not be dirty except the case where the
file hasn't been flushed since first write.

Call filemap_flush() in collapse_file() to accelerate the write back in
such cases.

Link: http://lkml.kernel.org/r/20191106060930.2571389-3-songliubraving@fb.com
Signed-off-by: Song Liu <songliubraving@fb.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: William Kucharski <william.kucharski@oracle.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/khugepaged.c |   18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

--- a/mm/khugepaged.c~mm-thp-flush-file-for-is_shmem-pagedirty-case-in-collapse_file
+++ a/mm/khugepaged.c
@@ -1602,6 +1602,24 @@ static void collapse_file(struct mm_stru
 					result = SCAN_FAIL;
 					goto xa_unlocked;
 				}
+			} else if (PageDirty(page)) {
+				/*
+				 * khugepaged only works on read-only fd,
+				 * so this page is dirty because it hasn't
+				 * been flushed since first write. There
+				 * won't be new dirty pages.
+				 *
+				 * Trigger async flush here and hope the
+				 * writeback is done when khugepaged
+				 * revisits this page.
+				 *
+				 * This is a one-off situation. We are not
+				 * forcing writeback in loop.
+				 */
+				xas_unlock_irq(&xas);
+				filemap_flush(mapping);
+				result = SCAN_FAIL;
+				goto xa_unlocked;
 			} else if (trylock_page(page)) {
 				get_page(page);
 				xas_unlock_irq(&xas);
_


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2019-12-01  1:57 [patch 136/158] mm/thp: flush file for !is_shmem PageDirty() case in collapse_file() akpm

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