linux-kernel - Re: [PATCH v2] RDMA/umem: minor bug fix and cleanup in error handling paths

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Date:   Sun, 3 Mar 2019 11:52:41 +0200
From:   Artemy Kovalyov <artemyko@...lanox.com>
To:     Ira Weiny <ira.weiny@...el.com>,
        "john.hubbard@...il.com" <john.hubbard@...il.com>
CC:     "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "John Hubbard" <jhubbard@...dia.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Doug Ledford <dledford@...hat.com>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>
Subject: Re: [PATCH v2] RDMA/umem: minor bug fix and cleanup in error handling
 paths



On 02/03/2019 21:44, Ira Weiny wrote:
> 
> On Sat, Mar 02, 2019 at 12:24:35PM -0800, john.hubbard@...il.com wrote:
>> From: John Hubbard <jhubbard@...dia.com>
>>
>> ...
>> 3. Dead code removal: the check for (user_virt & ~page_mask)
>> is checking for a condition that can never happen,
>> because earlier:
>>
>>      user_virt = user_virt & page_mask;
>>
>> ...so, remove that entire phrase.
>>
>>   
>>   		bcnt -= min_t(size_t, npages << PAGE_SHIFT, bcnt);
>>   		mutex_lock(&umem_odp->umem_mutex);
>>   		for (j = 0; j < npages; j++, user_virt += PAGE_SIZE) {
>> -			if (user_virt & ~page_mask) {
>> -				p += PAGE_SIZE;
>> -				if (page_to_phys(local_page_list[j]) != p) {
>> -					ret = -EFAULT;
>> -					break;
>> -				}
>> -				put_page(local_page_list[j]);
>> -				continue;
>> -			}
>> -
> 
> I think this is trying to account for compound pages. (ie page_mask could
> represent more than PAGE_SIZE which is what user_virt is being incrimented by.)
> But putting the page in that case seems to be the wrong thing to do?
> 
> Yes this was added by Artemy[1] now cc'ed.

Right, this is for huge pages, please keep it.
put_page() needed to decrement refcount of the head page.