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

ruansy.fnst
2022-08-03T02:43:36Z

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@fujitsu.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@fujitsu.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...

--
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.