[PATCH] filesystem-dax: Disable PMD support

[Dan Williams]

Ever since the conversion of DAX to the Xarray a RocksDB benchmark has
been encountering intermittent lockups. The backtraces always include
the filesystem-DAX PMD path, multi-order entries have been a source of
bugs in the past, and disabling the PMD path allows a test that fails in
minutes to run for an hour.

The regression has been bisected to "dax: Convert page fault handlers to
XArray", but little progress has been made on the root cause debug.
Unless / until root cause can be identified mark CONFIG_FS_DAX_PMD
broken to preclude intermittent lockups. Reverting the Xarray conversion
also works, but that change is too big to backport. The implementation
is committed to Xarray at this point.

Link: https://lore.kernel.org/linux-fsdevel/CAPcyv4hwHpX-MkUEqxwdTj7wCCZCN4RV-L4jsnuwLGyL_UEG4A@xxxxxxxxxxxxxx/
Fixes: b15cd800682f ("dax: Convert page fault handlers to XArray")
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: Jan Kara <jack@xxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Reported-by: Robert Barror <robert.barror@xxxxxxxxx>
Reported-by: Seema Pandit <seema.pandit@xxxxxxxxx>
Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
---
 fs/Kconfig |    3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/Kconfig b/fs/Kconfig
index f1046cf6ad85..85eecd0d4c5d 100644
--- a/fs/Kconfig
+++ b/fs/Kconfig
@@ -66,6 +66,9 @@ config FS_DAX_PMD
 	depends on FS_DAX
 	depends on ZONE_DEVICE
 	depends on TRANSPARENT_HUGEPAGE
+	# intermittent lockups since commit b15cd800682f "dax: Convert
+	# page fault handlers to XArray"
+	depends on BROKEN
 
 # Selected by DAX drivers that do not expect filesystem DAX to support
 # get_user_pages() of DAX mappings. I.e. "limited" indicates no support