Re: [PATCH] iommu/iova: Retry from last rb tree node if iova search fails

[Vijayanand Jitta <vjitta@codeaurora.org>]

On 5/7/2020 6:54 PM, Robin Murphy wrote:
> On 2020-05-06 9:01 pm, vjitta@codeaurora.org wrote:
>> From: Vijayanand Jitta <vjitta@codeaurora.org>
>>
>> When ever a new iova alloc request comes iova is always searched
>> from the cached node and the nodes which are previous to cached
>> node. So, even if there is free iova space available in the nodes
>> which are next to the cached node iova allocation can still fail
>> because of this approach.
>>
>> Consider the following sequence of iova alloc and frees on
>> 1GB of iova space
>>
>> 1) alloc - 500MB
>> 2) alloc - 12MB
>> 3) alloc - 499MB
>> 4) free -  12MB which was allocated in step 2
>> 5) alloc - 13MB
>>
>> After the above sequence we will have 12MB of free iova space and
>> cached node will be pointing to the iova pfn of last alloc of 13MB
>> which will be the lowest iova pfn of that iova space. Now if we get an
>> alloc request of 2MB we just search from cached node and then look
>> for lower iova pfn's for free iova and as they aren't any, iova alloc
>> fails though there is 12MB of free iova space.
> 
> Yup, this could definitely do with improving. Unfortunately I think this
> particular implementation is slightly flawed...
> 
>> To avoid such iova search failures do a retry from the last rb tree node
>> when iova search fails, this will search the entire tree and get an iova
>> if its available
>>
>> Signed-off-by: Vijayanand Jitta <vjitta@codeaurora.org>
>> ---
>>   drivers/iommu/iova.c | 11 +++++++++++
>>   1 file changed, 11 insertions(+)
>>
>> diff --git a/drivers/iommu/iova.c b/drivers/iommu/iova.c
>> index 0e6a953..2985222 100644
>> --- a/drivers/iommu/iova.c
>> +++ b/drivers/iommu/iova.c
>> @@ -186,6 +186,7 @@ static int __alloc_and_insert_iova_range(struct
>> iova_domain *iovad,
>>       unsigned long flags;
>>       unsigned long new_pfn;
>>       unsigned long align_mask = ~0UL;
>> +    bool retry = false;
>>         if (size_aligned)
>>           align_mask <<= fls_long(size - 1);
>> @@ -198,6 +199,8 @@ static int __alloc_and_insert_iova_range(struct
>> iova_domain *iovad,
>>         curr = __get_cached_rbnode(iovad, limit_pfn);
>>       curr_iova = rb_entry(curr, struct iova, node);
>> +
>> +retry_search:
>>       do {
>>           limit_pfn = min(limit_pfn, curr_iova->pfn_lo);
>>           new_pfn = (limit_pfn - size) & align_mask;
>> @@ -207,6 +210,14 @@ static int __alloc_and_insert_iova_range(struct
>> iova_domain *iovad,
>>       } while (curr && new_pfn <= curr_iova->pfn_hi);
>>         if (limit_pfn < size || new_pfn < iovad->start_pfn) {
>> +        if (!retry) {
>> +            curr = rb_last(&iovad->rbroot);
> 
> Why walk when there's an anchor node there already? However...
> 
>> +            curr_iova = rb_entry(curr, struct iova, node);
>> +            limit_pfn = curr_iova->pfn_lo;
> 
> ...this doesn't look right, as by now we've lost the original limit_pfn
> supplied by the caller, so are highly likely to allocate beyond the
> range our caller asked for. In fact AFAICS we'd start allocating from
> directly directly below the anchor node, beyond the end of the entire
> address space.
> 
> The logic I was imagining we want here was something like the rapidly
> hacked up (and untested) diff below.
> 
> Thanks,
> Robin.
> 

Thanks for your comments ,I have gone through below logic and I see some
issue with retry check as there could be case where alloc_lo is set to
some pfn other than start_pfn in that case we don't retry and there can
still be iova available. I understand its a hacked up version, I can
work on this.

But how about we just store limit_pfn and get the node using that and
retry for once from that node, it would be similar to my patch just
correcting the curr node and limit_pfn update in retry check. do you see
any issue with this approach ?


Thanks,
Vijay.
> ----->8-----
> diff --git a/drivers/iommu/iova.c b/drivers/iommu/iova.c
> index 0e6a9536eca6..3574c19272d6 100644
> --- a/drivers/iommu/iova.c
> +++ b/drivers/iommu/iova.c
> @@ -186,6 +186,7 @@ static int __alloc_and_insert_iova_range(struct
> iova_domain *iovad,
>         unsigned long flags;
>         unsigned long new_pfn;
>         unsigned long align_mask = ~0UL;
> +       unsigned long alloc_hi, alloc_lo;
> 
>         if (size_aligned)
>                 align_mask <<= fls_long(size - 1);
> @@ -196,17 +197,27 @@ static int __alloc_and_insert_iova_range(struct
> iova_domain *iovad,
>                         size >= iovad->max32_alloc_size)
>                 goto iova32_full;
> 
> +       alloc_hi = IOVA_ANCHOR;
> +       alloc_lo = iovad->start_pfn;
> +retry:
>         curr = __get_cached_rbnode(iovad, limit_pfn);
>         curr_iova = rb_entry(curr, struct iova, node);
> +       if (alloc_hi < curr_iova->pfn_hi) {
> +               alloc_lo = curr_iova->pfn_hi;
> +               alloc_hi = limit_pfn;
> +       }
> +
>         do {
> -               limit_pfn = min(limit_pfn, curr_iova->pfn_lo);
> -               new_pfn = (limit_pfn - size) & align_mask;
> +               alloc_hi = min(alloc_hi, curr_iova->pfn_lo);
> +               new_pfn = (alloc_hi - size) & align_mask;
>                 prev = curr;
>                 curr = rb_prev(curr);
>                 curr_iova = rb_entry(curr, struct iova, node);
>         } while (curr && new_pfn <= curr_iova->pfn_hi);
> 
> -       if (limit_pfn < size || new_pfn < iovad->start_pfn) {
> +       if (limit_pfn < size || new_pfn < alloc_lo) {
> +               if (alloc_lo == iovad->start_pfn)
> +                       goto retry;
>                 iovad->max32_alloc_size = size;
>                 goto iova32_full;
>         }