On Sat, Feb 27, 2021 at 2:36 PM Dave Chinner <david@xxxxxxxxxxxxx> wrote: > > On Fri, Feb 26, 2021 at 02:41:34PM -0800, Dan Williams wrote: > > On Fri, Feb 26, 2021 at 1:28 PM Dave Chinner <david@xxxxxxxxxxxxx> wrote: > > > On Fri, Feb 26, 2021 at 12:59:53PM -0800, Dan Williams wrote: > > > > On Fri, Feb 26, 2021 at 12:51 PM Dave Chinner <david@xxxxxxxxxxxxx> wrote: > > > > > > My immediate concern is the issue Jason recently highlighted [1] with > > > > > > respect to invalidating all dax mappings when / if the device is > > > > > > ripped out from underneath the fs. I don't think that will collide > > > > > > with Ruan's implementation, but it does need new communication from > > > > > > driver to fs about removal events. > > > > > > > > > > > > [1]: http://lore.kernel.org/r/CAPcyv4i+PZhYZiePf2PaH0dT5jDfkmkDX-3usQy1fAhf6LPyfw@xxxxxxxxxxxxxx > > > > > > > > > > Oh, yay. > > > > > > > > > > The XFS shutdown code is centred around preventing new IO from being > > > > > issued - we don't actually do anything about DAX mappings because, > > > > > well, I don't think anyone on the filesystem side thought they had > > > > > to do anything special if pmem went away from under it. > > > > > > > > > > My understanding -was- that the pmem removal invalidates > > > > > all the ptes currently mapped into CPU page tables that point at > > > > > the dax device across the system. THe vmas that manage these > > > > > mappings are not really something the filesystem really manages, > > > > > but a function of the mm subsystem. What the filesystem cares about > > > > > is that it gets page faults triggered when a change of state occurs > > > > > so that it can remap the page to it's backing store correctly. > > > > > > > > > > IOWs, all the mm subsystem needs to when pmem goes away is clear the > > > > > CPU ptes, because then when then when userspace tries to access the > > > > > mapped DAX pages we get a new page fault. In processing the fault, the > > > > > filesystem will try to get direct access to the pmem from the block > > > > > device. This will get an ENODEV error from the block device because > > > > > because the backing store (pmem) has been unplugged and is no longer > > > > > there... > > > > > > > > > > AFAICT, as long as pmem removal invalidates all the active ptes that > > > > > point at the pmem being removed, the filesystem doesn't need to > > > > > care about device removal at all, DAX or no DAX... > > > > > > > > How would the pmem removal do that without walking all the active > > > > inodes in the fs at the time of shutdown and call > > > > unmap_mapping_range(inode->i_mapping, 0, 0, 1)? > > > > > > Which then immediately ends up back at the vmas that manage the ptes > > > to unmap them. > > > > > > Isn't finding the vma(s) that map a specific memory range exactly > > > what the rmap code in the mm subsystem is supposed to address? > > > > rmap can lookup only vmas from a virt address relative to a given > > mm_struct. The driver has neither the list of mm_struct objects nor > > virt addresses to do a lookup. All it knows is that someone might have > > mapped pages through the fsdax interface. > > So there's no physical addr to vma translation in the mm subsystem > at all? > > That doesn't make sense. We do exactly this for hwpoison for DAX > mappings. While we don't look at ptes, we get a pfn, True hwpoison does get a known failing pfn and then uses page->mapping to get the 'struct address_space' to do the unmap. I discounted that approach from the outset because it would mean walking every pfn in a multi-terabyte device just in case one is mapped at device shutdown. > it points to, check if it points to the PMEM that is being removed, > grab the page it points to, map that to the relevant struct page, > run collect_procs() on that page, then kill the user processes that > map that page. > > So why can't we walk the ptescheck the physical pages that they > map to and if they map to a pmem page we go poison that > page and that kills any user process that maps it. > > i.e. I can't see how unexpected pmem device unplug is any different > to an MCE delivering a hwpoison event to a DAX mapped page. I guess the tradeoff is walking a long list of inodes vs walking a large array of pages. There's likely always more pages than inodes, but perhaps it's more efficient to walk the 'struct page' array than sb->s_inodes? > Both > indicate a physical address range now contains invalid data and the > filesystem has to take the same action... > > IOWs, we could just call ->corrupted_range(0, EOD) here to tell the > filesystem the entire device went away. Then the filesystem deal > with this however it needs to. However, it would be more efficient > from an invalidation POV to just call it on the pages that have > currently active ptes because once the block device is dead > new page faults on DAX mappings will get a SIGBUS naturally. There is no efficient way to lookup "currently active ptes" relative to a physical pfn range. SIGBUS will happen naturally either way. I don't think the hwpoison signal with the extra BUS_MCEERR_* info is appropriate given that indicates data loss vs data offline of a device being unplugged. > > > To me this looks like a notifier that fires from memunmap_pages() > > after dev_pagemap_kill() to notify any block_device associated with > > that dev_pagemap() to say that any dax mappings arranged through this > > block_device are now invalid. The reason to do this after > > dev_pagemap_kill() is so that any new mapping attempts that are racing > > the removal will be blocked. > > I don't see why this needs a unique notifier. At the filesystem > level, we want a single interface that tells us "something bad > happened to the block device", not a proliferation of similar but > subtly different "bad thing X happened to block device" interfaces > that are unique to specific physical device drivers... > > > The receiver of that notification needs to go from a block_device to a > > superblock that has mapped inodes and walk ->sb_inodes triggering the > > unmap/invalidation. > > Not necessarily. > > What if the filesystem is managing mirrored data across multiple > devices and this device is only one leg of the mirror? I can see DAX mapping for read access to one leg of the mirror. The unplug would fire zap_pte for all the inodes with DAX mappings for that fs. Filesystem is still free at that point to wait for the next user access, take a refault, and re-establish the mapping to another leg of the mirror. > Or that the > pmem was used by the RT device in XFS and the data/log devices are > still fine? I was assuming that the callback would only be triggered for a dax device as the data device. So xfs_open_devices() would register mp->m_super for dax_rtdev. > What if the pmem is just being used as a cache tier, and > no data was actually lost? That's fine the cache mapping is zapped and re-fault figures out what to do. If anything these questions are a reason not to use ->corrupted_range() for this because recovery can happen at refault vs taking permanent action on a data loss event. > > IOWs, what needs to happen at this point is very filesystem > specific. Assuming that "device unplug == filesystem dead" is not > correct, nor is specifying a generic action that assumes the > filesystem is dead because a device it is using went away. Ok, I think I set this discussion in the wrong direction implying any mapping of this action to a "filesystem dead" event. It's just a "zap all ptes" event and upper layers recover from there.