Re: [PATCH v15] mm, pmem, xfs: Introduce MF_MEM_PRE_REMOVE for unbind

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On Fri, Oct 20, 2023 at 08:40:09 AM -0700, Darrick J. Wong wrote:
> On Fri, Oct 20, 2023 at 03:26:32PM +0530, Chandan Babu R wrote:
>> On Thu, Sep 28, 2023 at 06:32:27 PM +0800, Shiyang Ruan wrote:
>> > ====
>> > Changes since v14:
>> >  1. added/fixed code comments per Dan's comments
>> > ====
>> >
>> > 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
>> >
>> > 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@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
>> > [2]: https://lore.kernel.org/linux-xfs/169116275623.3187159.16862410128731457358.stg-ugh@frogsfrogsfrogs/
>> >
>> > Signed-off-by: Shiyang Ruan <ruansy.fnst@xxxxxxxxxxx>
>> > Reviewed-by: Darrick J. Wong <djwong@xxxxxxxxxx>
>> > Acked-by: Dan Williams <dan.j.williams@xxxxxxxxx>
>> 
>> Hi Andrew,
>> 
>> Shiyang had indicated that this patch has been added to
>> akpm/mm-hotfixes-unstable branch. However, I don't see the patch listed in
>> that branch.
>> 
>> I am about to start collecting XFS patches for v6.7 cycle. Please let me know
>> if you have any objections with me taking this patch via the XFS tree.
>
> V15 was dropped from his tree on 28 Sept., you might as well pull it
> into your own tree for 6.7.  It's been testing fine on my trees for the
> past 3 weeks.
>
> https://lore.kernel.org/mm-commits/20230928172815.EE6AFC433C8@xxxxxxxxxxxxxxx/

Shiyang, this patch does not apply cleanly on v6.6-rc7. Can you please rebase
the patch on v6.6-rc7 and send it to the mailing list?

-- 
Chandan



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