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Date:   Mon, 31 Jan 2022 12:01:54 -0800
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Subject: Re: [PATCH 4/4] mm/gup: remove get_user_pages_locked()
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To:     Jason Gunthorpe <jgg@ziepe.ca>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Peter Xu <peterx@redhat.com>, Jan Kara <jack@suse.cz>,
        Claudio Imbrenda <imbrenda@linux.ibm.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Alex Williamson <alex.williamson@redhat.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= <jglisse@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>, linux-mm@kvack.org
References: <20220131051752.447699-1-jhubbard@nvidia.com>
 <20220131051752.447699-5-jhubbard@nvidia.com>
 <20220131133612.GW8034@ziepe.ca>
From:   John Hubbard <jhubbard@nvidia.com>
In-Reply-To: <20220131133612.GW8034@ziepe.ca>
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On 1/31/22 05:36, Jason Gunthorpe wrote:
...
>> @@ -968,15 +966,15 @@ static long do_get_mempolicy(int *policy, nodemask_t *nmask,
>>   	if (flags & MPOL_F_NODE) {
>>   		if (flags & MPOL_F_ADDR) {
>>   			/*
>> -			 * Take a refcount on the mpol, lookup_node()
>> -			 * will drop the mmap_lock, so after calling
>> -			 * lookup_node() only "pol" remains valid, "vma"
>> -			 * is stale.
>> +			 * Take a refcount on the mpol, because we are about to
>> +			 * drop the mmap_lock, after which only "pol" remains
>> +			 * valid, "vma" is stale.
>>   			 */
>>   			pol_refcount = pol;
>>   			vma = NULL;
>>   			mpol_get(pol);
>>   			err = lookup_node(mm, addr);
>> +			mmap_read_unlock(mm);
> 
> How about move the mmap_read_unlock up one line and then use
> get_user_pages_fast()
> 
> I'm guessing in most cases here the PTE will be present so that should
> be a net win?

Neat, I'll do that.


thanks,
-- 
John Hubbard
NVIDIA