Re: find_get_entries_tag regression bisected

[Dan Williams]

On Sat, Feb 16, 2019 at 1:11 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>
> On Sat, Feb 16, 2019 at 09:29:48AM -0800, Matthew Wilcox wrote:
> > On Sat, Feb 16, 2019 at 07:35:11AM -0800, Matthew Wilcox wrote:
> > > Another way to fix this would be to mask the address in dax_entry_mkclean(),
> > > but I think this is cleaner.
> >
> > That's clearly rubbish, dax_entry_mkclean() can't possibly mask the
> > address.  It might be mis-aligned in another process.  But ... if it's
> > misaligned in another process, dax_entry_mkclean() will only clean the first
> > PTE associated with the PMD; it won't clean the whole thing.  I think we need
> > something like this:
>
> Nope, this isn't enough.  It's _necessary_ to find the processes that
> have part of this PMD page mapped, but not the start of it.  But it's
> not _sufficient_ because it'll still only mkclean the first PTE.  So we
> need a loop.  I'm feeling a bit over my head here.  I may have a go at
> a fuller fix, but if someone else wants to have a go at it, be my guest!

Nothing comes to mind outside of pseudo-reverting this conversion by
introducing a way to get back to the old semantics. If you don't see a
path forward, us mere Xarray-mortals stand no chance.