On Fri, Nov 12, 2021 at 9:58 AM Jane Chu <[email protected]> wrote:
>
> On 11/11/2021 4:51 PM, Dan Williams wrote:
> > On Thu, Nov 11, 2021 at 4:30 PM Jane Chu <[email protected]> wrote:
> >>
> >> Just a quick update -
> >>
> >> I managed to test the 'NP' and 'UC' effect on a pmem dax file.
> >> The result is, as expected, both setting 'NP' and 'UC' works
> >> well in preventing the prefetcher from accessing the poisoned
> >> pmem page.
> >>
> >> I injected back-to-back poisons to the 3rd block(512B) of
> >> the 3rd page in my dax file. With 'NP', the 'mc_safe read'
> >> stops after reading the 1st and 2nd pages, with 'UC',
> >> the 'mc_safe read' was able to read [2 pages + 2 blocks] on
> >> my test machine.
> >
> > My expectation is that dax_direct_access() / dax_recovery_read() has
> > installed a temporary UC alias for the pfn, or has temporarily flipped
> > NP to UC. Outside of dax_recovery_read() the page will always be NP.
> >
>
> Okay. Could we only flip the memtype within dax_recovery_read, and
> not within dax_direct_access? dax_direct_access does not need to
> access the page.
True, dax_direct_access() does not need to do the page permission
change, it just needs to indicate if dax_recovery_{read,write}() may
be attempted. I was thinking that the DAX pages only float between NP
and WB depending on whether poison is present in the page. If
dax_recovery_read() wants to do UC reads around the poison it can use
ioremap() or vmap() to create a temporary UC alias. The temporary UC
alias is only possible if there might be non-clobbered data remaining
in the page. I.e. the current "whole_page()" determination in
uc_decode_notifier() needs to be plumbed into the PMEM driver so that
it can cooperate with a virtualized environment that injects virtual
#MC at page granularity. I.e. nfit_handle_mce() is broken in that it
only assumes a single cacheline around the failure address is
poisoned, it needs that same whole_page() logic.
