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    Subject[PATCH 3/3] iommu/iova: Try harder to allocate from rcache magazine
    Date
    When devices with different DMA masks are using the same domain, or for
    PCI devices where we usually try a speculative 32-bit allocation first,
    there is a fair possibility that the top PFN of the rcache stack at any
    given time may be unsuitable for the lower limit, prompting a fallback
    to allocating anew from the rbtree. Consequently, we may end up
    artifically increasing pressure on the 32-bit IOVA space as unused IOVAs
    accumulate lower down in the rcache stacks, while callers with 32-bit
    masks also impose unnecessary rbtree overhead.

    In such cases, let's try a bit harder to satisfy the allocation locally
    first - scanning the whole stack should still be relatively inexpensive,
    and even rotating an entry up from the very bottom probably has less
    overall impact than going to the rbtree.

    Signed-off-by: Robin Murphy <robin.murphy@arm.com>
    ---
    drivers/iommu/iova.c | 19 ++++++++++++++++---
    1 file changed, 16 insertions(+), 3 deletions(-)

    diff --git a/drivers/iommu/iova.c b/drivers/iommu/iova.c
    index 8f8b436afd81..a7af8273fa98 100644
    --- a/drivers/iommu/iova.c
    +++ b/drivers/iommu/iova.c
    @@ -826,12 +826,25 @@ static bool iova_magazine_empty(struct iova_magazine *mag)
    static unsigned long iova_magazine_pop(struct iova_magazine *mag,
    unsigned long limit_pfn)
    {
    + int i;
    + unsigned long pfn;
    +
    BUG_ON(iova_magazine_empty(mag));

    - if (mag->pfns[mag->size - 1] > limit_pfn)
    - return 0;
    + /*
    + * If we can pull a suitable pfn from anywhere in the stack, that's
    + * still probably preferable to falling back to the rbtree.
    + */
    + for (i = mag->size - 1; mag->pfns[i] > limit_pfn; i--)
    + if (i == 0)
    + return 0;

    - return mag->pfns[--mag->size];
    + pfn = mag->pfns[i];
    + mag->size--;
    + for (; i < mag->size; i++)
    + mag->pfns[i] = mag->pfns[i + 1];
    +
    + return pfn;
    }

    static void iova_magazine_push(struct iova_magazine *mag, unsigned long pfn)
    --
    2.13.4.dirty
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