Re: [PATCH] dax: Fix Xarray conversion of dax_unlock_mapping_entry()

From: Dan Williams
Date: Fri Nov 30 2018 - 10:55:04 EST

On Fri, Nov 30, 2018 at 7:49 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> On Thu, Nov 29, 2018 at 04:13:46PM -0800, Dan Williams wrote:
> > Internal to dax_unlock_mapping_entry(), dax_unlock_entry() is used to
> > store a replacement entry in the Xarray at the given xas-index with the
> > DAX_LOCKED bit clear. When called, dax_unlock_entry() expects the unlocked
> > value of the entry relative to the current Xarray state to be specified.
> >
> > In most contexts dax_unlock_entry() is operating in the same scope as
> > the matched dax_lock_entry(). However, in the dax_unlock_mapping_entry()
> > case the implementation needs to recall the original entry. In the case
> > where the original entry is a 'pmd' entry it is possible that the pfn
> > performed to do the lookup is misaligned to the value retrieved in the
> > Xarray.
> So far, dax_unlock_mapping_entry only has the one caller. I'd rather we
> returned the 'entry' to the caller, then had them pass it back to the
> unlock function. That matches the flow in the rest of DAX and doesn't
> pose an undue burden to the caller.
> I plan to reclaim the DAX_LOCK bit (and the DAX_EMPTY bit for that
> matter), instead using a special DAX_LOCK value. DAX is almost free of
> assumptions about the other bits in a locked entry, and this will remove
> the assuption that there's a PMD bit in the entry.
> How does this look?

Looks good to me, although can we make that cookie an actual type? I
think it's mostly ok to pass around (void *) for 'entry' inside of
fs/dax.c, but once an entry leaves that file I'd like it to have an
explicit type to catch people that might accidentally pass a (struct
page *) to the unlock routine.