On Wed, Jun 24, 2020 at 04:21:24PM -0700, Dan Williams wrote: > On Wed, Jun 24, 2020 at 5:10 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > > On Tue, Jun 23, 2020 at 05:01:24PM -0700, Darrick J. Wong wrote: > > > Frankly, I've wondered why the filesystem shouldn't just be in charge of > > > all this-- > > > > > > 1. kernel receives machine check > > > 2. kernel tattles to xfs > > > 3. xfs looks up which file(s) own the pmem range > > > 4. xfs zeroes the region, clears the poison, and sets AS_EIO on the > > > files > > > > ... machine reboots, app restarts, gets no notification anything is wrong, > > treats zeroed region as good data, launches nuclear missiles. > > Isn't AS_EIO stored persistently in the file block allocation map? No. AS_EIO is in mapping->flags. Unless Darrick was using "sets AS_EIO" as shorthand for something else. > Even if it isn't today that is included in the proposal that the > filesystem maintains a list of poison that is coordinated with the > pmem driver. I'd like to see a concrete proposal here. > > > Apps shouldn't have to do this punch-and-reallocate dance, seeing as > > > they don't currently do that for SCSI disks and the like. > > > > The SCSI disk retains the error until the sector is rewritten. > > I'm not entirely sure whether you're trying to draw an analogy with > > error-in-page-cache or error-on-storage-medium. > > > > error-on-medium needs to persist until the app takes an affirmative step > > to clear it. I presume XFS does not write zeroes to sectors with > > errors on SCSI disks ... > > SCSI does not have an async mechanism to retrieve a list of poisoned > blocks from the hardware (that I know of), pmem does. I really think > we should not glom on pmem error handling semantics on top of the same > infrastructure that it has handling volatile / replaceable pages. When Erm ... commit 6100e34b2526 has your name on it. > the filesystem is enabled to get involved it should impose a different > model than generic memory error handling especially because generic > memory-error handling has no chance to solve the reflink problem. > > If an application wants to survive poison consumption, signals seem > only sufficient for interrupting an application that needs to take > immediate action because one of its instructions was prevented from > making forward progress. The interface for enumerating the extent of > errors for DAX goes beyond what signinfo can reasonably convey, that > piece is where the filesystem can be called to discover which file > extents are impacted by poison. > > I like Darrick's idea that the kernel stabilizes the storage by > default, and that the repair mechanism is just a write(2). I assume > "stabilize" means make sure that the file offset is permanently > recorded as poisoned until the next write(2), but read(2) and mmap(2) > return errors so no more machine checks are triggered. That seems like something we'd want to work into the iomap infrastructure, perhaps. Add an IOMAP_POISONED to indicate this range needs to be written before it can be read?
