On Mon, Apr 25, 2016 at 10:14 AM, Verma, Vishal L <vishal.l.ve...@intel.com> wrote: > On Mon, 2016-04-25 at 01:31 -0700, h...@infradead.org 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 > > - 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 > > - 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 > > > I think if we want to keep allowing arbitrary alignments for the > dax_do_io path, we'd need: > 1. To represent badblocks at a finer granularity (likely cache lines) > 2. To allow the driver to do IO to a *block device* at sub-sector > granularity
3. Arrange for O_DIRECT to bypass dax_do_io(), and leave the optimization only for the dax "buffered I/O" case. 4. Skip dax_do_io() entirely in the presence of errors I think 3 is the most closely aligned with the typical block device model. In the typical case a buffered write may fail due to a badblock read when filling the page cache, but an O_DIRECT write would bypass the page cache and potentially clear the error / cause the block to be reallocated internally to the drive.
