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
