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

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Date:   Tue, 4 Dec 2018 17:57:29 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     Jerome Glisse <jglisse@...hat.com>,
        Matthew Wilcox <willy@...radead.org>
CC:     Dan Williams <dan.j.williams@...el.com>,
        John Hubbard <john.hubbard@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux MM <linux-mm@...ck.org>, Jan Kara <jack@...e.cz>,
        <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@...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 12/4/18 5:44 PM, Jerome Glisse wrote:
> On Tue, Dec 04, 2018 at 05:15:19PM -0800, Matthew Wilcox wrote:
>> On Tue, Dec 04, 2018 at 04:58:01PM -0800, John Hubbard wrote:
>>> On 12/4/18 3:03 PM, Dan Williams wrote:
>>>> Except the LRU fields are already in use for ZONE_DEVICE pages... how
>>>> does this proposal interact with those?
>>>
>>> Very badly: page->pgmap and page->hmm_data both get corrupted. Is there an entire
>>> use case I'm missing: calling get_user_pages() on ZONE_DEVICE pages? Said another
>>> way: is it reasonable to disallow calling get_user_pages() on ZONE_DEVICE pages?
>>>
>>> If we have to support get_user_pages() on ZONE_DEVICE pages, then the whole 
>>> LRU field approach is unusable.
>>
>> We just need to rearrange ZONE_DEVICE pages.  Please excuse the whitespace
>> damage:
>>
>> +++ b/include/linux/mm_types.h
>> @@ -151,10 +151,12 @@ struct page {
>>  #endif
>>                 };
>>                 struct {        /* ZONE_DEVICE pages */
>> +                       unsigned long _zd_pad_2;        /* LRU */
>> +                       unsigned long _zd_pad_3;        /* LRU */
>> +                       unsigned long _zd_pad_1;        /* uses mapping */
>>                         /** @pgmap: Points to the hosting device page map. */
>>                         struct dev_pagemap *pgmap;
>>                         unsigned long hmm_data;
>> -                       unsigned long _zd_pad_1;        /* uses mapping */
>>                 };
>>  
>>                 /** @rcu_head: You can use this to free a page by RCU. */
>>
>> You don't use page->private or page->index, do you Dan?
> 
> page->private and page->index are use by HMM DEVICE page.
> 

OK, so for the ZONE_DEVICE + HMM case, that leaves just one field remaining for 
dma-pinned information. Which might work. To recap, we need:

-- 1 bit for PageDmaPinned
-- 1 bit, if using LRU field(s), for PageDmaPinnedWasLru.
-- N bits for a reference count

Those *could* be packed into a single 64-bit field, if really necessary.


thanks,
-- 
John Hubbard
NVIDIA