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 14:37:33 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     Artemy Kovalyov <artemyko@...lanox.com>,
        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>,
        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 3/3/19 1:52 AM, Artemy Kovalyov wrote:
> 
> 
> 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.
> 

OK, thanks for explaining! Artemy, while you're here, any thoughts about the
release_pages, and the change of the starting point, from the other part of the 
patch:

@@ -684,9 +677,11 @@ int ib_umem_odp_map_dma_pages(struct ib_umem_odp *umem_odp, 
u64 user_virt,
	mutex_unlock(&umem_odp->umem_mutex);

  		if (ret < 0) {
-			/* Release left over pages when handling errors. */
-			for (++j; j < npages; ++j)
-				put_page(local_page_list[j]);
+			/*
+			 * Release pages, starting at the the first page
+			 * that experienced an error.
+			 */
+			release_pages(&local_page_list[j], npages - j);
  			break;
  		}
  	}

?

thanks,
-- 
John Hubbard
NVIDIA