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Date:   Fri, 15 May 2020 20:15:38 -0300
From:   Jason Gunthorpe <jgg@mellanox.com>
To:     Ralph Campbell <rcampbell@nvidia.com>
Cc:     linux-rdma@vger.kernel.org, linux-mm@kvack.org,
        linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org,
        Jerome Glisse <jglisse@redhat.com>,
        John Hubbard <jhubbard@nvidia.com>,
        Christoph Hellwig <hch@lst.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        Shuah Khan <shuah@kernel.org>
Subject: Re: [PATCH] mm/hmm/test: destroy xa_array instead of looping
Message-ID: <20200515231538.GD24561@mellanox.com>
References: <20200513214507.30592-1-rcampbell@nvidia.com>
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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