Hi all, Our current "advice" to people using persistent memory and FSDAX who wish to recover upon receipt of a media error (aka 'hwpoison') event from ACPI is to punch-hole that part of the file and then pwrite it, which will magically cause the pmem to be reinitialized and the poison to be cleared. Punching doesn't make any sense at all -- we don't allow userspace to allocate from specific parts of the storage, and another writer could grab the poisoned range in the meantime. In other words, the advice is seriously overfitted to incidental xfs and ext4 behavior and can completely fail. Worse yet, that concurrent writer now has to deal with the poison that it didn't know about, and someone else is trying to fix. AFAICT, the only reason why the "punch and write" dance works at all is that the XFS and ext4 currently call blkdev_issue_zeroout when allocating pmem as part of a pwrite call. A pwrite without the punch won't clear the poison, because pwrite on a DAX file calls dax_direct_access to access the memory directly, and dax_direct_access is only smart enough to bail out on poisoned pmem. It does not know how to clear it. Userspace could solve the problem by calling FIEMAP and issuing a BLKZEROOUT, but that requires rawio capabilities. The whole pmem poison recovery story is is wrong and needs to be corrected ASAP before everyone else starts doing this. Therefore, create a dax_zeroinit_range function that filesystems can call to reset the contents of the pmem to a known value and clear any state associated with the media error. Then, connect FALLOC_FL_ZERO_RANGE to this new function (for DAX files) so that unprivileged userspace has a safe way to reset the pmem and clear media errors. If you're going to start using this mess, you probably ought to just pull from my git trees, which are linked below. This is an extraordinary way to destroy everything. Enjoy! Comments and questions are, as always, welcome. --D kernel git tree: https://git.kernel.org/cgit/linux/kernel/git/djwong/xfs-linux.git/log/?h=dax-zeroinit-clear-poison-5.15 --- fs/dax.c | 72 +++++++++++++++++++++++++++++++++++++++++++++++++++ fs/ext4/extents.c | 19 +++++++++++++ fs/xfs/xfs_file.c | 20 ++++++++++++++ include/linux/dax.h | 7 +++++ 4 files changed, 118 insertions(+)
