On 12/13/18 9:21 PM, Dan Williams wrote:
> On Thu, Dec 13, 2018 at 7:53 PM John Hubbard <jhubbard@xxxxxxxxxx> 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
