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    SubjectRe: [PATCH] mm/hmm/test: destroy xa_array instead of looping
    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@nvidia.com>
    >
    > 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

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