[PATCH] mm/readahead: limit page cache size in page_cache_ra_order()

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In page_cache_ra_order(), the maximal order of the page cache to be
allocated shouldn't be larger than MAX_PAGECACHE_ORDER.  Otherwise, it's
possible the large page cache can't be supported by xarray when the
corresponding xarray entry is split.

For example, HPAGE_PMD_ORDER is 13 on ARM64 when the base page size is
64KB.  The PMD-sized page cache can't be supported by xarray.

Link: https://lkml.kernel.org/r/20240627003953.1262512-3-gshan@xxxxxxxxxx
Fixes: 793917d997df ("mm/readahead: Add large folio readahead")
Signed-off-by: Gavin Shan <gshan@xxxxxxxxxx>
Acked-by: David Hildenbrand <david@xxxxxxxxxx>
Cc: Darrick J. Wong <djwong@xxxxxxxxxx>
Cc: Don Dutile <ddutile@xxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
Cc: Ryan Roberts <ryan.roberts@xxxxxxx>
Cc: William Kucharski <william.kucharski@xxxxxxxxxx>
Cc: Zhenyu Zhang <zhenyzha@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>	[5.18+]
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>

diff --git a/mm/readahead.c b/mm/readahead.c
index c1b23989d9ca..817b2a352d78 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -503,11 +503,11 @@ void page_cache_ra_order(struct readahead_control *ractl,
 
 	limit = min(limit, index + ra->size - 1);
 
-	if (new_order < MAX_PAGECACHE_ORDER) {
+	if (new_order < MAX_PAGECACHE_ORDER)
 		new_order += 2;
-		new_order = min_t(unsigned int, MAX_PAGECACHE_ORDER, new_order);
-		new_order = min_t(unsigned int, new_order, ilog2(ra->size));
-	}
+
+	new_order = min_t(unsigned int, MAX_PAGECACHE_ORDER, new_order);
+	new_order = min_t(unsigned int, new_order, ilog2(ra->size));
 
 	/* See comment in page_cache_ra_unbounded() */
 	nofs = memalloc_nofs_save();





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