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Date:   Fri, 15 May 2020 17:56:10 -0700
From:   Ralph Campbell <rcampbell@...dia.com>
To:     Jason Gunthorpe <jgg@...lanox.com>
CC:     <linux-rdma@...r.kernel.org>, <linux-mm@...ck.org>,
        <linux-kselftest@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        "Jerome Glisse" <jglisse@...hat.com>,
        John Hubbard <jhubbard@...dia.com>,
        "Christoph Hellwig" <hch@....de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Shuah Khan <shuah@...nel.org>
Subject: Re: [PATCH] mm/hmm/test: destroy xa_array instead of looping


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@...dia.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
> 

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

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