On Mon, Apr 25, 2016 at 4:25 PM, Dave Chinner <david@xxxxxxxxxxxxx> wrote: > On Mon, Apr 25, 2016 at 05:14:36PM +0000, Verma, Vishal L wrote: >> On Mon, 2016-04-25 at 01:31 -0700, hch@xxxxxxxxxxxxx wrote: >> > On Sat, Apr 23, 2016 at 06:08:37PM +0000, Verma, Vishal L wrote: >> > > >> > > direct_IO might fail with -EINVAL due to misalignment, or -ENOMEM >> > > due >> > > to some allocation failing, and I thought we should return the >> > > original >> > > -EIO in such cases so that the application doesn't lose the >> > > information >> > > that the bad block is actually causing the error. >> > EINVAL is a concern here. Not due to the right error reported, but >> > because it means your current scheme is fundamentally broken - we >> > need to support I/O at any alignment for DAX I/O, and not fail due to >> > alignbment concernes for a highly specific degraded case. >> > >> > I think this whole series need to go back to the drawing board as I >> > don't think it can actually rely on using direct I/O as the EIO >> > fallback. >> > >> Agreed that DAX I/O can happen with any size/alignment, but how else do >> we send an IO through the driver without alignment restrictions? Also, >> the granularity at which we store badblocks is 512B sectors, so it >> seems natural that to clear such a sector, you'd expect to send a write >> to the whole sector. >> >> The expected usage flow is: >> >> - Application hits EIO doing dax_IO or load/store io >> >> - It checks badblocks and discovers it's files have lost data > > Lots of hand-waving here. How does the application map a bad > "sector" to a file without scanning the entire filesystem to find > the owner of the bad sector? > >> - It write()s those sectors (possibly converted to file offsets using >> fiemap) >> * This triggers the fallback path, but if the application is doing >> this level of recovery, it will know the sector is bad, and write the >> entire sector > > Where does the application find the data that was lost to be able to > rewrite it? > >> - Or it replaces the entire file from backup also using write() (not >> mmap+stores) >> * This just frees the fs block, and the next time the block is >> reallocated by the fs, it will likely be zeroed first, and that will be >> done through the driver and will clear errors > > There's an implicit assumption that applications will keep redundant > copies of their data at the /application layer/ and be able to > automatically repair it? And then there's the implicit assumption > that it will unlink and free the entire file before writing a new > copy, and that then assumes the the filesystem will zero blocks if > they get reused to clear errors on that LBA sector mapping before > they are accessible again to userspace.. > > It seems to me that there are a number of assumptions being made > across multiple layers here. Maybe I've missed something - can you > point me to the design/architecture description so I can see how > "app does data recovery itself" dance is supposed to work? > Maybe I missed something, but all these assumptions are already present for typical block devices, i.e. sectors may go bad and a write may make the sector usable again. This patch series is extending that out to the DAX-mmap case, but it's the same principle of "write to clear error" that we live with in the block-I/O path. What clarification are you looking for beyond that point? -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
