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 08:55:51 -0800
From:   Ira Weiny <ira.weiny@...el.com>
To:     Artemy Kovalyov <artemyko@...lanox.com>
Cc:     "john.hubbard@...il.com" <john.hubbard@...il.com>,
        "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 Sun, Mar 03, 2019 at 11:52:41AM +0200, 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.

You mean decrement the refcount of the _non_-head pages?

Ira

>