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

From: Greg Kroah-Hartman
Date: Mon Jul 12 2021 - 02:24:47 EST

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;

+ pmd_downgrade = false;
entry = get_unlocked_entry(xas, order);