[PATCH] khugepaged: hold invalidate_lock across collapse_file() readahead

[Nguyen Ngoc Thang <ngocthang2710.1999@gmail.com>]

collapse_file() calls page_cache_sync_readahead() to fault in missing
pages before collapsing them into a THP. That helper takes
mapping->invalidate_lock itself for the duration of the call, then
drops it -- but truncate (e.g. ext4_setattr() -> truncate_pagecache())
takes invalidate_lock and then waits on each page's folio lock while
holding it. If collapse_file() has already locked one of those folios
by the time truncate reaches it, and then tries to acquire
invalidate_lock again (e.g. on the next iteration, or via a nested
readahead call), the two paths can deadlock/hang on each other's lock:
truncate blocked on the folio lock collapse holds, and collapse
blocked waiting for invalidate_lock that truncate holds.

Reproducing this over ~150,000 collapse iterations with truncate
racing concurrently reliably hits hung_task: blocked tasks within
about 20 seconds on an unpatched kernel.

Fix it by taking invalidate_lock_shared once for the whole scan, before
locking any folio, and using page_cache_ra_unbounded() directly in the
readahead call site instead of page_cache_sync_readahead(), since the
latter would try to retake the lock we already hold.
page_cache_ra_unbounded() does not clamp to EOF like the helper it
replaces, so clamp the requested range explicitly.

Reported-by: syzbot+16bf7cd0ebeb1de93aa5@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=16bf7cd0ebeb1de93aa5
Signed-off-by: Nguyen Ngoc Thang <ngocthang2710.1999@gmail.com>
---
 mm/khugepaged.c | 25 ++++++++++++++++++++++---
 1 file changed, 22 insertions(+), 3 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 11ff98d55c76..690ccbcdf593 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -2267,6 +2267,13 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr,
 	VM_WARN_ON_ONCE(!is_shmem && !mapping_pmd_folio_support(mapping));
 	VM_WARN_ON_ONCE(start & (HPAGE_PMD_NR - 1));
 
+	/*
+	 * Take invalidate_lock before any folio lock: the readahead below
+	 * needs it, and truncate holds it while waiting on folio locks.
+	 */
+	if (!is_shmem)
+		filemap_invalidate_lock_shared(mapping);
+
 	result = alloc_charge_folio(&new_folio, mm, cc, HPAGE_PMD_ORDER);
 	if (result != SCAN_SUCCEED)
 		goto out;
@@ -2337,10 +2344,20 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr,
 			}
 		} else {	/* !is_shmem */
 			if (!folio || xa_is_value(folio)) {
+				DEFINE_READAHEAD(ractl, file, &file->f_ra,
+						  mapping, index);
+				pgoff_t eof = DIV_ROUND_UP(i_size_read(mapping->host),
+							    PAGE_SIZE);
+
 				xas_unlock_irq(&xas);
-				page_cache_sync_readahead(mapping, &file->f_ra,
-							  file, index,
-							  end - index);
+				/*
+				 * invalidate_lock held above; don't retake it.
+				 * page_cache_ra_unbounded(), unlike the readahead
+				 * helper this replaces, does not clamp to EOF.
+				 */
+				if (index < eof)
+					page_cache_ra_unbounded(&ractl,
+						min(end, eof) - index, 0);
 				/* drain lru cache to help folio_isolate_lru() */
 				lru_add_drain();
 				folio = filemap_lock_folio(mapping, index);
@@ -2672,6 +2689,8 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr,
 	folio_unlock(new_folio);
 	folio_put(new_folio);
 out:
+	if (!is_shmem)
+		filemap_invalidate_unlock_shared(mapping);
 	VM_BUG_ON(!list_empty(&pagelist));
 	trace_mm_khugepaged_collapse_file(mm, new_folio, index, addr, is_shmem, file, HPAGE_PMD_NR, result);
 	return result;
-- 
2.43.0