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    SubjectRe: [PATCH v2 5/5] dax: handle media errors in dax_do_io
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    On Mon, Apr 25, 2016 at 10:14 AM, Verma, Vishal L
    <vishal.l.verma@intel.com> wrote:
    > On Mon, 2016-04-25 at 01:31 -0700, hch@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.

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