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Subject: Re: [PATCH v3 hmm 02/11] mm/mmu_notifiers: do not speculatively
 allocate a mmu_notifier_mm
To:     Jason Gunthorpe <jgg@ziepe.ca>, <linux-mm@kvack.org>
CC:     Andrea Arcangeli <aarcange@redhat.com>,
        Christoph Hellwig <hch@lst.de>,
        John Hubbard <jhubbard@nvidia.com>,
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References: <20190806231548.25242-1-jgg@ziepe.ca>
 <20190806231548.25242-3-jgg@ziepe.ca>
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From:   Ralph Campbell <rcampbell@nvidia.com>
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Date:   Wed, 14 Aug 2019 13:32:13 -0700
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On 8/6/19 4:15 PM, Jason Gunthorpe wrote:
> From: Jason Gunthorpe <jgg@mellanox.com>
> 
> A prior commit e0f3c3f78da2 ("mm/mmu_notifier: init notifier if necessary")
> made an attempt at doing this, but had to be reverted as calling
> the GFP_KERNEL allocator under the i_mmap_mutex causes deadlock, see
> commit 35cfa2b0b491 ("mm/mmu_notifier: allocate mmu_notifier in advance").
> 
> However, we can avoid that problem by doing the allocation only under
> the mmap_sem, which is already happening.
> 
> Since all writers to mm->mmu_notifier_mm hold the write side of the
> mmap_sem reading it under that sem is deterministic and we can use that to
> decide if the allocation path is required, without speculation.
> 
> The actual update to mmu_notifier_mm must still be done under the
> mm_take_all_locks() to ensure read-side coherency.
> 
> Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

Looks good to me.
Reviewed-by: Ralph Campbell <rcampbell@nvidia.com>