linux-kernel - Re: [PATCH 1/2] mm: introduce put_user

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Message-ID: <01cf4e0c-b2d6-225a-3ee9-ef0f7e53684d@nvidia.com>
Date:   Thu, 13 Dec 2018 22:11:09 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     Dan Williams <dan.j.williams@...el.com>
CC:     david <david@...morbit.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Jan Kara <jack@...e.cz>, Matthew Wilcox <willy@...radead.org>,
        John Hubbard <john.hubbard@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux MM <linux-mm@...ck.org>, <tom@...pey.com>,
        Al Viro <viro@...iv.linux.org.uk>, <benve@...co.com>,
        Christoph Hellwig <hch@...radead.org>,
        Christopher Lameter <cl@...ux.com>,
        "Dalessandro, Dennis" <dennis.dalessandro@...el.com>,
        Doug Ledford <dledford@...hat.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Michal Hocko <mhocko@...nel.org>,
        Mike Marciniszyn <mike.marciniszyn@...el.com>,
        <rcampbell@...dia.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH 1/2] mm: introduce put_user_page*(), placeholder versions

On 12/13/18 9:21 PM, Dan Williams wrote:
> On Thu, Dec 13, 2018 at 7:53 PM John Hubbard <jhubbard@...dia.com> wrote:
>>
>> On 12/12/18 4:51 PM, Dave Chinner wrote:
>>> On Wed, Dec 12, 2018 at 04:59:31PM -0500, Jerome Glisse wrote:
>>>> On Thu, Dec 13, 2018 at 08:46:41AM +1100, Dave Chinner wrote:
>>>>> On Wed, Dec 12, 2018 at 10:03:20AM -0500, Jerome Glisse wrote:
>>>>>> On Mon, Dec 10, 2018 at 11:28:46AM +0100, Jan Kara wrote:
>>>>>>> On Fri 07-12-18 21:24:46, Jerome Glisse wrote:
>>>>>>> So this approach doesn't look like a win to me over using counter in struct
>>>>>>> page and I'd rather try looking into squeezing HMM public page usage of
>>>>>>> struct page so that we can fit that gup counter there as well. I know that
>>>>>>> it may be easier said than done...
>>>>>>
>>
>> Agreed. After all the discussion this week, I'm thinking that the original idea
>> of a per-struct-page counter is better. Fortunately, we can do the moral equivalent
>> of that, unless I'm overlooking something: Jerome had another proposal that he
>> described, off-list, for doing that counting, and his idea avoids the problem of
>> finding space in struct page. (And in fact, when I responded yesterday, I initially
>> thought that's where he was going with this.)
>>
>> So how about this hybrid solution:
>>
>> 1. Stay with the basic RFC approach of using a per-page counter, but actually
>> store the counter(s) in the mappings instead of the struct page. We can use
>> !PageAnon and page_mapping to look up all the mappings, stash the dma_pinned_count
>> there. So the total pinned count is scattered across mappings. Probably still need
>> a PageDmaPinned bit.
> 
> How do you safely look at page->mapping from the get_user_pages_fast()
> path? You'll be racing invalidation disconnecting the page from the
> mapping.
> 

I don't have an answer for that, so maybe the page->mapping idea is dead already. 

So in that case, there is still one more way to do all of this, which is to
combine ZONE_DEVICE, HMM, and gup/dma information in a per-page struct, and get
there via basically page->private, more or less like this:

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 5ed8f6292a53..13f651bb5cc1 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -67,6 +67,13 @@ struct hmm;
 #define _struct_page_alignment
 #endif
 
+struct page_aux {
+       struct dev_pagemap *pgmap;
+       unsigned long hmm_data;
+       unsigned long private;
+       atomic_t dma_pinned_count;
+};
+
 struct page {
        unsigned long flags;            /* Atomic flags, some possibly
                                         * updated asynchronously */
@@ -149,11 +156,13 @@ struct page {
                        spinlock_t ptl;
 #endif
                };
-               struct {        /* ZONE_DEVICE pages */
+               struct {        /* ZONE_DEVICE, HMM or get_user_pages() pages */
                        /** @pgmap: Points to the hosting device page map. */
-                       struct dev_pagemap *pgmap;
-                       unsigned long hmm_data;
-                       unsigned long _zd_pad_1;        /* uses mapping */
+                       unsigned long _zd_pad_1;        /* LRU */
+                       unsigned long _zd_pad_2;        /* LRU */
+                       unsigned long _zd_pad_3;        /* mapping */
+                       unsigned long _zd_pad_4;        /* index */
+                       struct page_aux *aux;           /* private */
                };
 
                /** @rcu_head: You can use this to free a page by RCU. */

...is there any appetite for that approach?

-- 
thanks,
John Hubbard
NVIDIA