Date:   Thu, 18 Aug 2022 10:04:26 -0700
From:   "Darrick J. Wong" <djwong@...nel.org>
To:     Shiyang Ruan <ruansy.fnst@...itsu.com>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-xfs@...r.kernel.org" <linux-xfs@...r.kernel.org>,
        "nvdimm@...ts.linux.dev" <nvdimm@...ts.linux.dev>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "david@...morbit.com" <david@...morbit.com>,
        "hch@...radead.org" <hch@...radead.org>,
        "jane.chu@...cle.com" <jane.chu@...cle.com>
Subject: Re: [RFC PATCH v6] mm, pmem, xfs: Introduce MF_MEM_REMOVE for unbind

On Thu, Aug 18, 2022 at 07:19:28PM +0800, Shiyang Ruan wrote:
> 
> 
> 在 2022/8/3 12:33, Darrick J. Wong 写道:
> > On Wed, Aug 03, 2022 at 02:43:20AM +0000, ruansy.fnst@...itsu.com wrote:
> > > 
> > > 在 2022/7/19 6:56, Dan Williams 写道:
> > > > Darrick J. Wong wrote:
> > > > > On Thu, Jul 14, 2022 at 11:21:44AM -0700, Dan Williams wrote:
> > > > > > ruansy.fnst@...itsu.com 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.
> > > > > > > 
> > > > > > > ==
> > > > > > > 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@dwillia2-desk3.amr.corp.intel.com/
> > > > > > > 
> > > > > > > Signed-off-by: Shiyang Ruan <ruansy.fnst@...itsu.com>
> > > > > > > ---
> > > > > > >    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);
> > > > > > 
> > > > > > Are all mappings invalidated after this point?
> > > > > 
> > > > > No; all this step does is pushes dirty filesystem [meta]data to pmem
> > > > > before we lose DAXDEV_ALIVE...
> > > > > 
> > > > > > The goal of the removal notification is to invalidate all DAX mappings
> > > > > > that are no pointing to pfns that do not exist anymore, so just syncing
> > > > > > does not seem like enough, and the shutdown is skipped above. What am I
> > > > > > missing?
> > > > > 
> > > > > ...however, the shutdown above only applies to filesystem metadata.  In
> > > > > effect, we avoid the fs shutdown in MF_MEM_PRE_REMOVE mode, which
> > > > > enables the mf_dax_kill_procs calls to proceed against mapped file data.
> > > > > I have a nagging suspicion that in non-PREREMOVE mode, we can end up
> > > > > shutting down the filesytem on an xattr block and the 'return
> > > > > -EFSCORRUPTED' actually prevents us from reaching all the remaining file
> > > > > data mappings.
> > > > > 
> > > > > IOWs, I think that clause above really ought to have returned zero so
> > > > > that we keep the filesystem up while we're tearing down mappings, and
> > > > > only call xfs_force_shutdown() after we've had a chance to let
> > > > > xfs_dax_notify_ddev_failure() tear down all the mappings.
> > > > > 
> > > > > I missed that subtlety in the initial ~30 rounds of review, but I figure
> > > > > at this point let's just land it in 5.20 and clean up that quirk for
> > > > > -rc1.
> > > > 
> > > > Sure, this is a good baseline to incrementally improve.
> > > 
> > > Hi Dan, Darrick
> > > 
> > > Do I need to fix somewhere on this patch?  I'm not sure if it is looked good...
> > 
> > Eh, wait for me to send the xfs pull request and then I'll clean things
> > up and send you a patch. :)
> 
> Hi, Darrick
> 
> How is your patch going on?  Forgive me for being so annoying.  I'm afraid
> of missing your patch, so I'm asking for confirmation.

<nod> I just sent you a patch.  Sorry, it took me a few days to unbusy
enough to start testing 6.0-rc1.  You're not annoying at all. :)

--D

> 
> --
> Thanks,
> Ruan.
> 
> > 
> > --D
> > 
> > > 
> > > --
> > > Thanks,
> > > Ruan.
> > > 
> > > > 
> > > > > 
> > > > > > Notice that kill_dev_dax() does unmap_mapping_range() after invalidating
> > > > > > the dax device and that ensures that all existing mappings are gone and
> > > > > > cannot be re-established. As far as I can see a process with an existing
> > > > > > dax mapping will still be able to use it after this runs, no?
> > > > > 
> > > > > I'm not sure where in akpm's tree I find kill_dev_dax()?  I'm cribbing
> > > > > off of:
> > > > > 
> > > > > https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git/tree/fs/xfs/xfs_notify_failure.c?h=mm-stable
> > > > 
> > > > https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git/tree/drivers/dax/bus.c?h=mm-stable#n381
> > > > 
> > > > Where the observation is that when device-dax is told that the device is
> > > > going away it invalidates all the active mappings to that single
> > > > character-device-inode. The hope being that in the fsdax case all the
> > > > dax-mapped filesystem inodes would experience the same irreversible
> > > > invalidation as the device is exiting.

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