Date:   Thu, 18 Aug 2022 19:19:28 +0800
From:   Shiyang Ruan <ruansy.fnst@...itsu.com>
To:     "Darrick J. Wong" <djwong@...nel.org>
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



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.


--
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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