Re: [PATCH v14] mm, pmem, xfs: Introduce MF_MEM_PRE_REMOVE for unbind

[Shiyang Ruan <ruansy.fnst@fujitsu.com>]

在 2023/9/27 16:17, Dan Williams 写道:
> Shiyang Ruan wrote:
>> ====
>> Changes since v13:
>>   1. don't return error if _freeze(FREEZE_HOLDER_KERNEL) got other error
>> ====
>>
>> Now, if we suddenly remove a PMEM device(by calling unbind) which
>> contains FSDAX while programs are still accessing data in this device,
>> e.g.:
>> ```
>>   $FSSTRESS_PROG -d $SCRATCH_MNT -n 99999 -p 4 &
>>   # $FSX_PROG -N 1000000 -o 8192 -l 500000 $SCRATCH_MNT/t001 &
>>   echo "pfn1.1" > /sys/bus/nd/drivers/nd_pmem/unbind
>> ```
>> it could come into an unacceptable state:
>>    1. device has gone but mount point still exists, and umount will fail
>>         with "target is busy"
>>    2. programs will hang and cannot be killed
>>    3. may crash with NULL pointer dereference
> 
> Thanks, this addresses my main concern that this new capability is needed
> otherwise DAX regresses the survivability of the kernel when removing a
> device from underneath the mounted filesystem compared to removing a
> non-DAX capable block device.
> 
>>
>> To fix this, we introduce a MF_MEM_PRE_REMOVE flag to let it know that we
>> are going to remove the whole device, and make sure all related processes
>> could be notified so that they could end up gracefully.
>>
>> 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
>> 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()
>>      -> kill_dax()
>>       -> dax_holder_notify_failure(dax_dev, 0, U64_MAX, MF_MEM_PRE_REMOVE)
>>        -> xfs_dax_notify_failure()
>>        `-> freeze_super()             // freeze (kernel call)
>>        `-> do xfs rmap
>>        ` -> mf_dax_kill_procs()
>>        `  -> collect_procs_fsdax()    // all associated processes
>>        `  -> unmap_and_kill()
>>        ` -> invalidate_inode_pages2_range() // drop file's cache
>>        `-> thaw_super()               // thaw (both kernel & user call)
>>
>> Introduce MF_MEM_PRE_REMOVE to let filesystem know this is a remove
>> event.  Use the exclusive freeze/thaw[2] to lock the filesystem to prevent
>> new dax mapping from being created.  Do not shutdown filesystem directly
>> if configuration is not supported, or if failure range includes metadata
>> area.  Make sure all files and processes(not only the current progress)
>> are handled correctly.  Also drop the cache of associated files before
>> pmem is removed.
>>
>> [1]: https://lore.kernel.org/linux-mm/161604050314.1463742.14151665140035795571.stgit@dwillia2-desk3.amr.corp.intel.com/
>> [2]: https://lore.kernel.org/linux-xfs/169116275623.3187159.16862410128731457358.stg-ugh@frogsfrogsfrogs/
> 
> I only have some questions and comment suggestions below, but otherwise
> consider this:
> 
> Acked-by: Dan Williams <dan.j.williams@intel.com>
> 
>>
>> Signed-off-by: Shiyang Ruan <ruansy.fnst@fujitsu.com>
>> ---
>>   drivers/dax/super.c         |  3 +-
>>   fs/xfs/xfs_notify_failure.c | 99 ++++++++++++++++++++++++++++++++++---
>>   include/linux/mm.h          |  1 +
>>   mm/memory-failure.c         | 17 +++++--
>>   4 files changed, 109 insertions(+), 11 deletions(-)
>>
>> diff --git a/drivers/dax/super.c b/drivers/dax/super.c
>> index 0da9232ea175..f4b635526345 100644
>> --- a/drivers/dax/super.c
>> +++ b/drivers/dax/super.c
>> @@ -326,7 +326,8 @@ 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 4a9bbd3fe120..79586abc75bf 100644
>> --- a/fs/xfs/xfs_notify_failure.c
>> +++ b/fs/xfs/xfs_notify_failure.c
>> @@ -22,6 +22,7 @@
>>   
>>   #include <linux/mm.h>
>>   #include <linux/dax.h>
>> +#include <linux/fs.h>
>>   
>>   struct xfs_failure_info {
>>   	xfs_agblock_t		startblock;
>> @@ -73,10 +74,16 @@ xfs_dax_failure_fn(
>>   	struct xfs_mount		*mp = cur->bc_mp;
>>   	struct xfs_inode		*ip;
>>   	struct xfs_failure_info		*notify = data;
>> +	struct address_space		*mapping;
>> +	pgoff_t				pgoff;
>> +	unsigned long			pgcnt;
>>   	int				error = 0;
>>   
>>   	if (XFS_RMAP_NON_INODE_OWNER(rec->rm_owner) ||
>>   	    (rec->rm_flags & (XFS_RMAP_ATTR_FORK | XFS_RMAP_BMBT_BLOCK))) {
>> +		/* Continue the query because this isn't a failure. */
>> +		if (notify->mf_flags & MF_MEM_PRE_REMOVE)
>> +			return 0;
>>   		notify->want_shutdown = true;
>>   		return 0;
>>   	}
>> @@ -92,14 +99,60 @@ xfs_dax_failure_fn(
>>   		return 0;
>>   	}
>>   
>> -	error = mf_dax_kill_procs(VFS_I(ip)->i_mapping,
>> -				  xfs_failure_pgoff(mp, rec, notify),
>> -				  xfs_failure_pgcnt(mp, rec, notify),
>> -				  notify->mf_flags);
>> +	mapping = VFS_I(ip)->i_mapping;
>> +	pgoff = xfs_failure_pgoff(mp, rec, notify);
>> +	pgcnt = xfs_failure_pgcnt(mp, rec, notify);
>> +
>> +	/* Continue the rmap query if the inode isn't a dax file. */
>> +	if (dax_mapping(mapping))
>> +		error = mf_dax_kill_procs(mapping, pgoff, pgcnt,
>> +					  notify->mf_flags);
>> +
>> +	/* Invalidate the cache in dax pages. */
>> +	if (notify->mf_flags & MF_MEM_PRE_REMOVE)
>> +		invalidate_inode_pages2_range(mapping, pgoff,
>> +					      pgoff + pgcnt - 1);
>> +
>>   	xfs_irele(ip);
>>   	return error;
>>   }
>>   
>> +static int
>> +xfs_dax_notify_failure_freeze(
>> +	struct xfs_mount	*mp)
>> +{
>> +	struct super_block	*sb = mp->m_super;
>> +	int			error;
>> +
>> +	error = freeze_super(sb, FREEZE_HOLDER_KERNEL);
>> +	if (error)
>> +		xfs_emerg(mp, "already frozen by kernel, err=%d", error);
>> +
>> +	return error;
>> +}
>> +
>> +static void
>> +xfs_dax_notify_failure_thaw(
>> +	struct xfs_mount	*mp,
>> +	bool			kernel_frozen)
>> +{
>> +	struct super_block	*sb = mp->m_super;
>> +	int			error;
>> +
>> +	if (kernel_frozen) {
>> +		error = thaw_super(sb, FREEZE_HOLDER_KERNEL);
>> +		if (error)
>> +			xfs_emerg(mp, "still frozen after notify failure, err=%d",
>> +				error);
>> +	}
>> +
>> +	/*
>> +	 * Also thaw userspace call anyway because the device is about to be
>> +	 * removed immediately.
>> +	 */
>> +	thaw_super(sb, FREEZE_HOLDER_USERSPACE);
> 
> I don't understand why this is not paired with a freeze in
> xfs_dax_notify_failure_freeze()?

What we want to do is freezing the filesystem, so acutally 
freeze_super(sb, FREEZE_HOLDER_KERNEL) is enough.  But adding 
thaw_super(sb, FREEZE_HOLDER_USERSPACE) here is to make sure the mount 
point could be umounted after unbind, while other userspace program is 
holding the freeze state of this filesystem.  Otherwize, after unbind, 
the mount point still exists and `umount /mnt/scratch` fails with 
"target is busy." `xfs_freeze -u /mnt/scratch` doesn't work too.

> 
>> +}
>> +
>>   static int
>>   xfs_dax_notify_ddev_failure(
>>   	struct xfs_mount	*mp,
>> @@ -112,15 +165,29 @@ xfs_dax_notify_ddev_failure(
>>   	struct xfs_btree_cur	*cur = NULL;
>>   	struct xfs_buf		*agf_bp = NULL;
>>   	int			error = 0;
>> +	bool			kernel_frozen = false;
>>   	xfs_fsblock_t		fsbno = XFS_DADDR_TO_FSB(mp, daddr);
>>   	xfs_agnumber_t		agno = XFS_FSB_TO_AGNO(mp, fsbno);
>>   	xfs_fsblock_t		end_fsbno = XFS_DADDR_TO_FSB(mp,
>>   							     daddr + bblen - 1);
>>   	xfs_agnumber_t		end_agno = XFS_FSB_TO_AGNO(mp, end_fsbno);
>>   
>> +	if (mf_flags & MF_MEM_PRE_REMOVE) {
>> +		xfs_info(mp, "Device is about to be removed!");
>> +		/*
>> +		 * Freeze fs to prevent new mappings from being created.
>> +		 * - Keep going on if others already hold the kernel forzen.
>> +		 * - Keep going on if other errors too because this device is
>> +		 *   starting to fail.
>> +		 * - If kernel frozen state is hold successfully here, thaw it
>> +		 *   here as well at the end.
>> +		 */
>> +		kernel_frozen = xfs_dax_notify_failure_freeze(mp) == 0;
>> +	}
>> +
>>   	error = xfs_trans_alloc_empty(mp, &tp);
>>   	if (error)
>> -		return error;
>> +		goto out;
>>   
>>   	for (; agno <= end_agno; agno++) {
>>   		struct xfs_rmap_irec	ri_low = { };
>> @@ -165,11 +232,23 @@ xfs_dax_notify_ddev_failure(
>>   	}
>>   
>>   	xfs_trans_cancel(tp);
>> +
>> +	/*
>> +	 * Determine how to shutdown the filesystem according to the
>> +	 * error code and flags.
>> +	 */
> 
> This comment is not adding any value. It would be better if it clarified
> why why want_shutdown will be false in the pre-remove case?
> 
>>   	if (error || notify.want_shutdown) {
>>   		xfs_force_shutdown(mp, SHUTDOWN_CORRUPT_ONDISK);
>>   		if (!error)
>>   			error = -EFSCORRUPTED;
>> -	}
>> +	} else if (mf_flags & MF_MEM_PRE_REMOVE)
>> +		xfs_force_shutdown(mp, SHUTDOWN_FORCE_UMOUNT);
>> +
>> +out:
>> +	/* Thaw the fs if it is frozen before. */
>> +	if (mf_flags & MF_MEM_PRE_REMOVE)
>> +		xfs_dax_notify_failure_thaw(mp, kernel_frozen);
>> +
>>   	return error;
>>   }
>>   
>> @@ -197,6 +276,8 @@ 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) {
> 
> Maybe a comment:
> 
> /*
>   * In the pre-remove case the failure notification is attempting to
>   * trigger a force unmount, the expectation is that the device is still
>   * present, but its removal is in progress and can not be cancelled,
>   * proceed with accessing the log device.
>   */
> 
>> +		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;
>> @@ -210,6 +291,12 @@ xfs_dax_notify_failure(
>>   	ddev_start = mp->m_ddev_targp->bt_dax_part_off;
>>   	ddev_end = ddev_start + bdev_nr_bytes(mp->m_ddev_targp->bt_bdev) - 1;
>>   
>> +	/* Notify failure on the whole device. */
>> +	if (offset == 0 && len == U64_MAX) {
>> +		offset = ddev_start;
>> +		len = bdev_nr_bytes(mp->m_ddev_targp->bt_bdev);
>> +	}
>> +
>>   	/* Ignore the range out of filesystem area */
>>   	if (offset + len - 1 < ddev_start)
>>   		return -ENXIO;
>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>> index 2dd73e4f3d8e..a10c75bebd6d 100644
>> --- a/include/linux/mm.h
>> +++ b/include/linux/mm.h
>> @@ -3665,6 +3665,7 @@ enum mf_flags {
>>   	MF_UNPOISON = 1 << 4,
>>   	MF_SW_SIMULATED = 1 << 5,
>>   	MF_NO_RETRY = 1 << 6,
>> +	MF_MEM_PRE_REMOVE = 1 << 7,
>>   };
>>   int mf_dax_kill_procs(struct address_space *mapping, pgoff_t index,
>>   		      unsigned long count, int mf_flags);
>> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
>> index e245191e6b04..e71616ccc643 100644
>> --- a/mm/memory-failure.c
>> +++ b/mm/memory-failure.c
>> @@ -683,7 +683,7 @@ static void add_to_kill_fsdax(struct task_struct *tsk, struct page *p,
>>    */
>>   static void collect_procs_fsdax(struct page *page,
>>   		struct address_space *mapping, pgoff_t pgoff,
>> -		struct list_head *to_kill)
>> +		struct list_head *to_kill, bool pre_remove)
>>   {
>>   	struct vm_area_struct *vma;
>>   	struct task_struct *tsk;
>> @@ -691,8 +691,15 @@ static void collect_procs_fsdax(struct page *page,
>>   	i_mmap_lock_read(mapping);
>>   	read_lock(&tasklist_lock);
>>   	for_each_process(tsk) {
>> -		struct task_struct *t = task_early_kill(tsk, true);
>> +		struct task_struct *t = tsk;
>>   
>> +		/*
>> +		 * Search for all tasks while MF_MEM_PRE_REMOVE is set, because
>> +		 * the current may not be the one accessing the fsdax page.
>> +		 * Otherwise, search for the current task.
>> +		 */
>> +		if (!pre_remove)
>> +			t = task_early_kill(tsk, true);
>>   		if (!t)
>>   			continue;
>>   		vma_interval_tree_foreach(vma, &mapping->i_mmap, pgoff, pgoff) {
>> @@ -1788,6 +1795,7 @@ int mf_dax_kill_procs(struct address_space *mapping, pgoff_t index,
>>   	dax_entry_t cookie;
>>   	struct page *page;
>>   	size_t end = index + count;
>> +	bool pre_remove = mf_flags & MF_MEM_PRE_REMOVE;
>>   
>>   	mf_flags |= MF_ACTION_REQUIRED | MF_MUST_KILL;
>>   
>> @@ -1799,9 +1807,10 @@ int mf_dax_kill_procs(struct address_space *mapping, pgoff_t index,
>>   		if (!page)
>>   			goto unlock;
>>   
>> -		SetPageHWPoison(page);
>> +		if (!pre_remove)
>> +			SetPageHWPoison(page);
> 
> This problably wants a comment like:
> 
> /*
>   * The pre_remove case is revoking access, the memory is still good and
>   * could theoretically be put back into service
>   */
> 
>>   
>> -		collect_procs_fsdax(page, mapping, index, &to_kill);
>> +		collect_procs_fsdax(page, mapping, index, &to_kill, pre_remove);
>>   		unmap_and_kill(&to_kill, page_to_pfn(page), mapping,
>>   				index, mf_flags);
>>   unlock:

I'll add/modify these comments as you suggested.  Thanks!


--
Ruan.

>> -- 
>> 2.41.0
>>