linux-kernel - Re: [PATCH 4/4] mm/gup: remove get_user_pages

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Date:   Mon, 31 Jan 2022 12:01:54 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Peter Xu <peterx@...hat.com>, Jan Kara <jack@...e.cz>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org
Subject: Re: [PATCH 4/4] mm/gup: remove get_user_pages_locked()

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