[PATCH 5.4 183/348] dax: fix ENOMEM handling in grab_mapping_entry()

[Greg Kroah-Hartman]

From: Jan Kara <jack@xxxxxxx>

[ Upstream commit 1a14e3779dd58c16b30e56558146e5cc850ba8b0 ]

grab_mapping_entry() has a bug in handling of ENOMEM condition.  Suppose
we have a PMD entry at index i which we are downgrading to a PTE entry.
grab_mapping_entry() will set pmd_downgrade to true, lock the entry, clear
the entry in xarray, and decrement mapping->nrpages.  The it will call:

	entry = dax_make_entry(pfn_to_pfn_t(0), flags);
	dax_lock_entry(xas, entry);

which inserts new PTE entry into xarray.  However this may fail allocating
the new node.  We handle this by:

	if (xas_nomem(xas, mapping_gfp_mask(mapping) & ~__GFP_HIGHMEM))
		goto retry;

however pmd_downgrade stays set to true even though 'entry' returned from
get_unlocked_entry() will be NULL now.  And we will go again through the
downgrade branch.  This is mostly harmless except that mapping->nrpages is
decremented again and we temporarily have an invalid entry stored in
xarray.  Fix the problem by setting pmd_downgrade to false each time we
lookup the entry we work with so that it matches the entry we found.

Link: https://lkml.kernel.org/r/20210622160015.18004-1-jack@xxxxxxx
Fixes: b15cd800682f ("dax: Convert page fault handlers to XArray")
Signed-off-by: Jan Kara <jack@xxxxxxx>
Reviewed-by: Dan Williams <dan.j.williams@xxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@xxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 fs/dax.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/dax.c b/fs/dax.c
index 3b0e5da96d54..12953e892bb2 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -477,10 +477,11 @@ static void *grab_mapping_entry(struct xa_state *xas,
 		struct address_space *mapping, unsigned int order)
 {
 	unsigned long index = xas->xa_index;
-	bool pmd_downgrade = false; /* splitting PMD entry into PTE entries? */
+	bool pmd_downgrade;	/* splitting PMD entry into PTE entries? */
 	void *entry;
 
 retry:
+	pmd_downgrade = false;
 	xas_lock_irq(xas);
 	entry = get_unlocked_entry(xas, order);
 
-- 
2.30.2