[RFC PATCH v6] mm, pmem, xfs: Introduce MF_MEM_REMOVE for unbind

[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_PRE_REMOVE)
      -> xfs_dax_notify_failure()

Introduce MF_MEM_PRE_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.

==
Changes since v5:
  1. Renamed MF_MEM_REMOVE to MF_MEM_PRE_REMOVE
  2. hold s_umount before sync_filesystem()
  3. move sync_filesystem() after SB_BORN check
  4. Rebased on next-20220714

Changes since v4:
  1. sync_filesystem() at the beginning when MF_MEM_REMOVE
  2. Rebased on next-20220706

[1]: https://lore.kernel.org/linux-mm/161604050314.1463742.14151665140035795571.stgit@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/

Signed-off-by: Shiyang Ruan <ruansy.fnst@xxxxxxxxxxx>
---
 drivers/dax/super.c         |  3 ++-
 fs/xfs/xfs_notify_failure.c | 15 +++++++++++++++
 include/linux/mm.h          |  1 +
 3 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/drivers/dax/super.c b/drivers/dax/super.c
index 9b5e2a5eb0ae..cf9a64563fbe 100644
--- a/drivers/dax/super.c
+++ b/drivers/dax/super.c
@@ -323,7 +323,8 @@ 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_PRE_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 69d9c83ea4b2..6da6747435eb 100644
--- a/fs/xfs/xfs_notify_failure.c
+++ b/fs/xfs/xfs_notify_failure.c
@@ -76,6 +76,9 @@ xfs_dax_failure_fn(
 
 	if (XFS_RMAP_NON_INODE_OWNER(rec->rm_owner) ||
 	    (rec->rm_flags & (XFS_RMAP_ATTR_FORK | XFS_RMAP_BMBT_BLOCK))) {
+		/* Do not shutdown so early when device is to be removed */
+		if (notify->mf_flags & MF_MEM_PRE_REMOVE)
+			return 0;
 		xfs_force_shutdown(mp, SHUTDOWN_CORRUPT_ONDISK);
 		return -EFSCORRUPTED;
 	}
@@ -174,12 +177,22 @@ xfs_dax_notify_failure(
 	struct xfs_mount	*mp = dax_holder(dax_dev);
 	u64			ddev_start;
 	u64			ddev_end;
+	int			error;
 
 	if (!(mp->m_sb.sb_flags & SB_BORN)) {
 		xfs_warn(mp, "filesystem is not ready for notify_failure()!");
 		return -EIO;
 	}
 
+	if (mf_flags & MF_MEM_PRE_REMOVE) {
+		xfs_info(mp, "device is about to be removed!");
+		down_write(&mp->m_super->s_umount);
+		error = sync_filesystem(mp->m_super);
+		up_write(&mp->m_super->s_umount);
+		if (error)
+			return error;
+	}
+
 	if (mp->m_rtdev_targp && mp->m_rtdev_targp->bt_daxdev == dax_dev) {
 		xfs_warn(mp,
 			 "notify_failure() not supported on realtime device!");
@@ -188,6 +201,8 @@ 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_PRE_REMOVE)
+			return 0;
 		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 4287bec50c28..2ddfb76c8a83 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -3188,6 +3188,7 @@ enum mf_flags {
 	MF_SOFT_OFFLINE = 1 << 3,
 	MF_UNPOISON = 1 << 4,
 	MF_SW_SIMULATED = 1 << 5,
+	MF_MEM_PRE_REMOVE = 1 << 6,
 };
 int mf_dax_kill_procs(struct address_space *mapping, pgoff_t index,
 		      unsigned long count, int mf_flags);
-- 
2.37.0