Re: [PATCH 0/3] dax: clear poison on the fly along pwrite

["Darrick J. Wong" <djwong@kernel.org>]

On Fri, Sep 17, 2021 at 01:21:25PM -0700, Dan Williams wrote:
> On Fri, Sep 17, 2021 at 8:27 AM Darrick J. Wong <djwong@kernel.org> wrote:
> >
> > On Fri, Sep 17, 2021 at 01:53:33PM +0100, Christoph Hellwig wrote:
> > > On Thu, Sep 16, 2021 at 11:40:28AM -0700, Dan Williams wrote:
> > > > > That was my gut feeling.  If everyone feels 100% comfortable with
> > > > > zeroingas the mechanism to clear poisoning I'll cave in.  The most
> > > > > important bit is that we do that through a dedicated DAX path instead
> > > > > of abusing the block layer even more.
> > > >
> > > > ...or just rename dax_zero_page_range() to dax_reset_page_range()?
> > > > Where reset == "zero + clear-poison"?
> > >
> > > I'd say that naming is more confusing than overloading zero.
> >
> > How about dax_zeroinit_range() ?
> 
> Works for me.
> 
> >
> > To go with its fallocate flag (yeah I've been too busy sorting out -rc1
> > regressions to repost this) FALLOC_FL_ZEROINIT_RANGE that will reset the
> > hardware (whatever that means) and set the contents to the known value
> > zero.
> >
> > Userspace usage model:
> >
> > void handle_media_error(int fd, loff_t pos, size_t len)
> > {
> >         /* yell about this for posterior's sake */
> >
> >         ret = fallocate(fd, FALLOC_FL_ZEROINIT_RANGE, pos, len);
> >
> >         /* yay our disk drive / pmem / stone table engraver is online */
> 
> The fallocate mode can still be error-aware though, right? When the FS
> has knowledge of the error locations the fallocate mode could be
> fallocate(fd, FALLOC_FL_OVERWRITE_ERRORS, pos, len) with the semantics
> of attempting to zero out any known poison extents in the given file
> range? At the risk of going overboard on new fallocate modes there
> could also (or instead of) be FALLOC_FL_PUNCH_ERRORS to skip trying to
> clear them and just ask the FS to throw error extents away.

It /could/ be, but for now I've stuck to what you see is what you get --
if you tell it to 'zero initialize' 1MB of pmem, it'll write zeroes and
clear the poison on all 1MB, regardless of the old contents.

IOWs, you can use it from a poison handler on just the range that it
told you about, or you could use it to bulk-clear a lot of space all at
once.

A dorky thing here is that the dax_zero_page_range function returns EIO
if you tell it to do more than one page...


> 
> > }
> >
> > > > > I'm really worried about both patartitions on DAX and DM passing through
> > > > > DAX because they deeply bind DAX to the block layer, which is just a bad
> > > > > idea.  I think we also need to sort that whole story out before removing
> > > > > the EXPERIMENTAL tags.
> > > >
> > > > I do think it was a mistake to allow for DAX on partitions of a pmemX
> > > > block-device.
> > > >
> > > > DAX-reflink support may be the opportunity to start deprecating that
> > > > support. Only enable DAX-reflink for direct mounting on /dev/pmemX
> > > > without partitions (later add dax-device direct mounting),
> > >
> > > I think we need to fully or almost fully sort this out.
> > >
> > > Here is my bold suggestions:
> > >
> > >  1) drop no drop the EXPERMINTAL on the current block layer overload
> > >     at all
> >
> > I don't understand this.
> >
> > >  2) add direct mounting of the nvdimm namespaces ASAP.  Because all
> > >     the filesystem currently also need the /dev/pmem0 device add a way
> > >     to open the block device by the dax_device instead of our current
> > >     way of doing the reverse
> > >  3) deprecate DAX support through block layer mounts with a say 2 year
> > >     deprecation period
> > >  4) add DAX remapping devices as needed
> >
> > What devices are needed?  linear for lvm, and maybe error so we can
> > actually test all this stuff?
> 
> The proposal would be zero lvm support. The nvdimm namespace
> definition would need to grow support for concatenation + striping.

Ah, ok.

> Soft error injection could be achieved by writing to the badblocks
> interface.

<nod>

I'll send out an RFC of what I have currently.

--D