[PATCH] mm/huge_memory: fix a folio_split() race condition with folio_try_get()

[Zi Yan]

During a pagecache folio split, the values in the related xarray should not
be changed from the original folio at xarray split time until all
after-split folios are well formed and stored in the xarray. Current use
of xas_try_split() in __split_unmapped_folio() lets some after-split folios
show up at wrong indices in the xarray. When these misplaced after-split
folios are unfrozen, before correct folios are stored via __xa_store(), and
grabbed by folio_try_get(), they are returned to userspace at wrong file
indices, causing data corruption.

Fix it by using the original folio in xas_try_split() calls, so that
folio_try_get() can get the right after-split folios after the original
folio is unfrozen.

Uniform split, split_huge_page*(), is not affected, since it uses
xas_split_alloc() and xas_split() only once and stores the original folio
in the xarray.

Fixes below points to the commit introduces the code, but folio_split() is
used in a later commit 7460b470a131f ("mm/truncate: use folio_split() in
truncate operation").

Fixes: 00527733d0dc8 ("mm/huge_memory: add two new (not yet used) functions for folio_split()")
Reported-by: Bas van Dijk <bas@xxxxxxxxxxx>
Closes: https://lore.kernel.org/all/CAKNNEtw5_kZomhkugedKMPOG-sxs5Q5OLumWJdiWXv+C9Yct0w@xxxxxxxxxxxxxx/
Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
---
 mm/huge_memory.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 56db54fa48181..e4ed0404e8b55 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3647,6 +3647,7 @@ static int __split_unmapped_folio(struct folio *folio, int new_order,
 	const bool is_anon = folio_test_anon(folio);
 	int old_order = folio_order(folio);
 	int start_order = split_type == SPLIT_TYPE_UNIFORM ? new_order : old_order - 1;
+	struct folio *origin_folio = folio;
 	int split_order;
 
 	/*
@@ -3672,7 +3673,13 @@ static int __split_unmapped_folio(struct folio *folio, int new_order,
 				xas_split(xas, folio, old_order);
 			else {
 				xas_set_order(xas, folio->index, split_order);
-				xas_try_split(xas, folio, old_order);
+				/*
+				 * use the original folio, so that a parallel
+				 * folio_try_get() waits on it until xarray is
+				 * updated with after-split folios and
+				 * the original one is unfrozen.
+				 */
+				xas_try_split(xas, origin_folio, old_order);
 				if (xas_error(xas))
 					return xas_error(xas);
 			}
-- 
2.51.0