Re: [PATCH 2/3] mm, dax, pmem: Introduce dev_pagemap_failure()

> Now I'm trying to reconcile the fact that platform > poison handling will hit memory_failure() first and may not > immediately reach the driver, if ever (see the perennially awkward > firmware-first-mode error handling: ghes_handle_memory_failure()) . So > even if the ->memory_failure(dev...) up call exists there is no > guarantee it can get called for all poison before the memory_failure() > down call happens. Which means regardless of whether > ->memory_failure(dev...) exists memory_failure() needs to be able to > do the right thing. I don't see how a poor implementation of memory_failure in a driver or hardware is even remotely relevant to the interface used to notify the filesystem of a media or device failure. It sounds like you are trying to use memory_failure() for something it was never intended to support and that there's a bunch of driver and infrastructure work needed to make it work how you want it to work. And even then it may not work the way we want it to work.... > Combine that with the fact that new buses like CXL might be configured > in "poison on decode error" mode which means that a memory_failure() > storm can happen regardless of whether the driver initiates it > programatically. Straw man argument. "We can't make this interface a ranged notification because the hardware might only be able to do page-by-page notification."

You can do page-by-page notification with a range based interface. We are talking about how to efficiently and reliably inform the filesystem that a range of a device is no longer accessible and so it needs to revoke all mappings over that range of it's address space. That does not need to be a single page at a time interface. If your hardware is configured to do stupid things, then that is not the fault of the software interface used to communicate faults up the stack, nor is it something that the notfication interface should try to fix or mitigate..... > How about a mechanism to optionally let a filesystem take over memory > failure handling for a range of pfns that the memory_failure() can > consult to fail ranges at a time rather than one by one? So a new > 'struct dax_operations' op (void) (*memory_failure_register(struct > dax_device *, void *data). Where any agent that claims a dax_dev can > register to take over memory_failure() handling for any event that > happens in that range. This would be routed through device-mapper like > any other 'struct dax_operations' op. I think that meets your > requirement to get notifications of all the events you want to handle, > but still allows memory_failure() to be the last resort for everything > that has not opted into this error handling. Which is basically the same as the proposed ->corrupted_range stack, except it doesn't map the pfns back to LBA addresses the filesystem needs to make sense of the failure. fs-dax filesystems have no clue what pfns are, or how to translate them to LBAs in their block device address space that the map everything to. The fs-dax infrastructure asks the filesystem for bdev/sector based mappings, and internally converts them to pfns by a combination of bdev and daxdev callouts. Hence fs-dax filesystems never see nor interpret pfns at all. Nor do they have the capability to convert a PFN to a LBA address. And neither the underlying block device nor the associated DAX device provide a method for doing this reverse mapping translation.

So if we have an interface that hands a {daxdev,PFN,len} tuple to the filesystem, exactly what is the filesystem supposed to do with it? How do we turn that back into a {bdev,sector,len} tuple so we can do reverse mapping lookups to find the filesystem objects that allocated within the notified range? I'll point out again that these address space translations were something that the ->corrupted_range callbacks handled directly - no layer in the stack was handed a range that it didn't know how to map to it's own internal structures. By the time it got to the filesystem, it was a {bdev,sector,len} tuple, and the filesystem could feed that directly to it's reverse mapping lookups.... Maybe I'm missing something magical about ->memory_failure that does all this translation for us, but I don't see it in this patchset. I just don't see how this proposed interface is a usable at the filesystem level as it stands.