On Tue, Sep 20, 2022 at 12:45:19PM +1000, Dave Chinner wrote:
> On Fri, Sep 02, 2022 at 10:36:01AM +, Shiyang Ruan wrote:
> > 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.
> >
> > [1]:
> > https://lore.kernel.org/linux-mm/161604050314.1463742.14151665140035795571.st...@dwillia2-desk3.amr.corp.intel.com/
> >
> > Signed-off-by: Shiyang Ruan
> > ---
> > drivers/dax/super.c | 3 ++-
> > fs/xfs/xfs_notify_failure.c | 23 +++
> > include/linux/mm.h | 1 +
> > 3 files changed, 26 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, _dev->flags);
> > synchronize_srcu(_srcu);
> > diff --git a/fs/xfs/xfs_notify_failure.c b/fs/xfs/xfs_notify_failure.c
> > index 3830f908e215..5e04ba7fa403 100644
> > --- a/fs/xfs/xfs_notify_failure.c
> > +++ b/fs/xfs/xfs_notify_failure.c
> > @@ -22,6 +22,7 @@
> >
> > #include
> > #include
> > +#include
> >
> > struct xfs_failure_info {
> > xfs_agblock_t startblock;
> > @@ -77,6 +78,9 @@ xfs_dax_failure_fn(
> >
> > if (XFS_RMAP_NON_INODE_OWNER(rec->rm_owner) ||
> > (rec->rm_flags & (XFS_RMAP_ATTR_FORK | XFS_RMAP_BMBT_BLOCK))) {
> > + /* The device is about to be removed. Not a really failure. */
> > + if (notify->mf_flags & MF_MEM_PRE_REMOVE)
> > + return 0;
> > notify->want_shutdown = true;
> > return 0;
> > }
> > @@ -182,12 +186,23 @@ xfs_dax_notify_failure(
> > struct xfs_mount*mp = dax_holder(dax_dev);
> > u64 ddev_start;
> > u64 ddev_end;
> > + int error;
> >
> > if (!(mp->m_super->s_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(>m_super->s_umount);
> > + error = sync_filesystem(mp->m_super);
> > + drop_pagecache_sb(mp->m_super, NULL);
> > + up_write(>m_super->s_umount);
> > + if (error)
> > + return error;
>
> If the device is about to go away unexpectedly, shouldn't this shut
> down the filesystem after syncing it here? If the filesystem has
> been shut down, then everything will fail before removal finally
> triggers, and the act of unmounting the filesystem post device
> removal will clean up the page cache and all the other caches.
IIRC they want to kill all the processes with MAP_SYNC mappings sooner
than whenever the admin gets around to unmounting the filesystem, which
is why PRE_REMOVE will then go walk the rmapbt to find processes to
shoot down. I'm not sure, though, if drop_pagecache_sb only touches
DRAM page cache or if it'll shoot down fsdax mappings too?
> IOWs, I don't understand why the page cache is considered special
> here (as opposed to, say, the inode or dentry caches), nor why we
> aren't shutting down the filesystem directly after syncing it to
> disk to ensure that we don't end up with applications losing data as
> a result of racing with the removal
But yeah, we might as well shut down the fs at the end of PRE_REMOVE
handling, if the rmap walk hasn't already done that.
--D
> Cheers,
>
> Dave.
> --
> Dave Chinner
> da...@fromorbit.com
