Re: [PATCH] mm/hmm/test: destroy xa_array instead of looping

[Ralph Campbell]

On 5/15/20 4:15 PM, Jason Gunthorpe wrote:
> On Wed, May 13, 2020 at 02:45:07PM -0700, Ralph Campbell wrote:
> > The test driver uses an xa_array to store virtual to physical address
> > translations for a simulated hardware device. The MMU notifier
> > invalidation callback is used to keep the table consistent with the CPU
> > page table and is frequently called only for a page or two. However, if
> > the test process exits unexpectedly or is killed, the range can be
> > [0..ULONG_MAX] in which case calling xa_erase() for every possible PFN
> > results in CPU timeouts. Munmap() can result in a large range being
> > invalidated but in that case, the xa_array is likely to contain entries
> > that need to be invalidated.
> > Check for [0..ULONG_MAX] explicitly and just destroy the whole table.
> >
> > Signed-off-by: Ralph Campbell <rcampbell@xxxxxxxxxx>
> >
> > This patch is based on Jason Gunthorpe's hmm tree and should be folded
> > into the ("mm/hmm/test: add selftest driver for HMM") patch once this
> > patch is reviewed, etc.
> >
> >   lib/test_hmm.c | 6 ++++++
> >   1 file changed, 6 insertions(+)
> >
> > diff --git a/lib/test_hmm.c b/lib/test_hmm.c
> > index 8b36c26b717b..b89852ec3c29 100644
> > +++ b/lib/test_hmm.c
> > @@ -201,7 +201,13 @@ static void dmirror_do_update(struct dmirror *dmirror, unsigned long start,
> >   	 * The XArray doesn't hold references to pages since it relies on
> >   	 * the mmu notifier to clear page pointers when they become stale.
> >   	 * Therefore, it is OK to just clear the entry.
> > +	 * However, if the entire address space is being invalidated, it
> > +	 * takes too long to clear them one at a time so destroy the array.
> >   	 */
> > +	if (start == 0 && end == ULONG_MAX) {
> > +		xa_destroy(&dmirror->pt);
> > +		return;
> > +	}
> >   	for (pfn = start >> PAGE_SHIFT; pfn < (end >> PAGE_SHIFT); pfn++)
> >   		xa_erase(&dmirror->pt, pfn);
> >   }
>
> Just use xa_for_each_range() instead of the naive loop, it already
> optimizes against membership and avoids the need for the xa_destroy
> hack
>
> Jason

For some reason I had looked at that and rejected it but of course, it works
fine. :-)
Thanks!