[RFC PATCH v2] mm, pmem, xfs: Introduce MF_MEM_REMOVE for unbind

[Shiyang Ruan <ruansy.fnst@xxxxxxxxxxx>]

This patch is inspired by Dan's "mm, dax, pmem: Introduce
dev_pagemap_failure()"[1].  With the help of dax_holder and
->notify_failure() mechanism, the pmem driver is able to ask filesystem
(or mapped device) on it to unmap all files in use and notify processes
who are using those files.

Call trace:
trigger unbind
 -> unbind_store()
  -> ... (skip)
   -> devres_release_all()   # was pmem driver ->remove() in v1
    -> kill_dax()
     -> dax_holder_notify_failure(dax_dev, 0, U64_MAX, MF_MEM_REMOVE)
      -> xfs_dax_notify_failure()

Introduce MF_MEM_REMOVE to let filesystem know this is a remove event.
So do not shutdown filesystem directly if something not supported, or if
failure range includes metadata area.  Make sure all files and processes
are handled correctly.

[1]: https://lore.kernel.org/linux-mm/161604050314.1463742.14151665140035795571.stgit@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
[2]: https://lore.kernel.org/linux-xfs/20220508143620.1775214-1-ruansy.fnst@xxxxxxxxxxx/

Signed-off-by: Shiyang Ruan <ruansy.fnst@xxxxxxxxxxx>

==
Changes since v1:
  1. Drop the needless change of moving {kill,put}_dax()
  2. Rebased on '[PATCHSETS] v14 fsdax-rmap + v11 fsdax-reflink'[2]

---
 drivers/dax/super.c         | 2 +-
 fs/xfs/xfs_notify_failure.c | 6 +++++-
 include/linux/mm.h          | 1 +
 3 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/drivers/dax/super.c b/drivers/dax/super.c
index 5ddb159c4653..44ca3b488e2a 100644
--- a/drivers/dax/super.c
+++ b/drivers/dax/super.c
@@ -313,7 +313,7 @@ void kill_dax(struct dax_device *dax_dev)
 		return;
 
 	if (dax_dev->holder_data != NULL)
-		dax_holder_notify_failure(dax_dev, 0, U64_MAX, 0);
+		dax_holder_notify_failure(dax_dev, 0, U64_MAX, MF_MEM_REMOVE);
 
 	clear_bit(DAXDEV_ALIVE, &dax_dev->flags);
 	synchronize_srcu(&dax_srcu);
diff --git a/fs/xfs/xfs_notify_failure.c b/fs/xfs/xfs_notify_failure.c
index aa8dc27c599c..91d3f05d4241 100644
--- a/fs/xfs/xfs_notify_failure.c
+++ b/fs/xfs/xfs_notify_failure.c
@@ -73,7 +73,9 @@ xfs_dax_failure_fn(
 	struct failure_info		*notify = data;
 	int				error = 0;
 
-	if (XFS_RMAP_NON_INODE_OWNER(rec->rm_owner) ||
+	/* Do not shutdown so early when device is to be removed */
+	if (!(notify->mf_flags & MF_MEM_REMOVE) ||
+	    XFS_RMAP_NON_INODE_OWNER(rec->rm_owner) ||
 	    (rec->rm_flags & (XFS_RMAP_ATTR_FORK | XFS_RMAP_BMBT_BLOCK))) {
 		xfs_force_shutdown(mp, SHUTDOWN_CORRUPT_ONDISK);
 		return -EFSCORRUPTED;
@@ -182,6 +184,8 @@ xfs_dax_notify_failure(
 
 	if (mp->m_logdev_targp && mp->m_logdev_targp->bt_daxdev == dax_dev &&
 	    mp->m_logdev_targp != mp->m_ddev_targp) {
+		if (mf_flags & MF_MEM_REMOVE)
+			return -EOPNOTSUPP;
 		xfs_err(mp, "ondisk log corrupt, shutting down fs!");
 		xfs_force_shutdown(mp, SHUTDOWN_CORRUPT_ONDISK);
 		return -EFSCORRUPTED;
diff --git a/include/linux/mm.h b/include/linux/mm.h
index e2c0f69f0660..ebcb5a7f3295 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -3226,6 +3226,7 @@ enum mf_flags {
 	MF_MUST_KILL = 1 << 2,
 	MF_SOFT_OFFLINE = 1 << 3,
 	MF_UNPOISON = 1 << 4,
+	MF_MEM_REMOVE = 1 << 5,
 };
 int mf_dax_kill_procs(struct address_space *mapping, pgoff_t index,
 		      unsigned long count, int mf_flags);
-- 
2.35.1