[PATCH v5 13/14] mm/khugepaged: enable clean pagecache folio collapse for writable files

[Zi Yan <ziy@nvidia.com>]

collapse_file() is capable of collapsing pagecache folios from writable
files to PMD folios. Now enable clean pagecache folio collapse in addition
to read-only pagecache folio collapse by removing the
inode_is_open_for_write() from file_thp_enabled() and only performing
filemap_flush() if the file is read-only.

This means userspace needs to explicitly flush the content of pagecache
folios before khugepaged can collapse the folios, or use
madvise(MADV_COLLAPSE), which does the flush in the retry. The reason is
that blindly enabling dirty pagecache folio from writable files collapse
makes khugepaged flush these folios all the time. It is undesirable to
cause system level pagecache flushes.

To properly support dirty pagecache folio collapse, filemap_flush() needs
to be avoided. Potentially, merging associated buffer instead of dropping
it with filemap_release_folio() might be needed.

NOTE: this breaks khugepaged selftests for writable file pagecache
collapse, which is set to fail all the time. The next commit fix it.

Signed-off-by: Zi Yan <ziy@nvidia.com>
---
 mm/huge_memory.c | 2 +-
 mm/khugepaged.c  | 9 ++++++++-
 2 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 9b3abb98a7e51..e1e9d59db6e70 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -97,7 +97,7 @@ static inline bool file_thp_enabled(struct vm_area_struct *vma)
 	if (!mapping_pmd_folio_support(vma->vm_file->f_mapping))
 		return false;
 
-	return !inode_is_open_for_write(inode) && S_ISREG(inode->i_mode);
+	return S_ISREG(inode->i_mode);
 }
 
 /* If returns true, we are unable to access the VMA's folios. */
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 1ee15b48962a3..fb7ff643973cc 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -2345,7 +2345,14 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr,
 				 * forcing writeback in loop.
 				 */
 				xas_unlock_irq(&xas);
-				filemap_flush(mapping);
+				/*
+				 * Only flush for read-only files. Writable
+				 * files can have their folios dirty at any
+				 * time; blindly flushing them would cause
+				 * undesirable system-wide writeback.
+				 */
+				if (!inode_is_open_for_write(mapping->host))
+					filemap_flush(mapping);
 				result = SCAN_PAGE_DIRTY_OR_WRITEBACK;
 				goto xa_unlocked;
 			} else if (folio_test_writeback(folio)) {
-- 
2.53.0