Re: [PATCH 1/1] infiniband/mm: convert put_page() to put_user_page*()

John Hubbard
2019-05-23T19:15:03Z

On 5/23/19 12:04 PM, Ira Weiny wrote:
> On Thu, May 23, 2019 at 10:46:38AM -0700, John Hubbard wrote:
>> On 5/23/19 10:32 AM, Jason Gunthorpe wrote:
>>> On Thu, May 23, 2019 at 10:28:52AM -0700, Ira Weiny wrote:
>>>>> @@ -686,8 +686,8 @@ int ib_umem_odp_map_dma_pages(struct ib_umem_odp *umem_odp, u64 user_virt,
>>>>> * ib_umem_odp_map_dma_single_page().
>>>>> */
>>>>> if (npages - (j + 1) > 0)
>>>>> - release_pages(&local_page_list[j+1],
>>>>> - npages - (j + 1));
>>>>> + put_user_pages(&local_page_list[j+1],
>>>>> + npages - (j + 1));
>>>>
>>>> I don't know if we discussed this before but it looks like the use of
>>>> release_pages() was not entirely correct (or at least not necessary) here. So
>>>> I think this is ok.
>>>
>>> Oh? John switched it from a put_pages loop to release_pages() here:
>>>
>>> commit 75a3e6a3c129cddcc683538d8702c6ef998ec589
>>> Author: John Hubbard <jhubbard@nvidia.com>
>>> Date: Mon Mar 4 11:46:45 2019 -0800
>>>
>>> RDMA/umem: minor bug fix in error handling path
>>> 1. Bug fix: fix an off by one error in the code that cleans up if it fails
>>> to dma-map a page, after having done a get_user_pages_remote() on a
>>> range of pages.
>>> 2. Refinement: for that same cleanup code, release_pages() is better than
>>> put_page() in a loop.
>>>
>>> And now we are going to back something called put_pages() that
>>> implements the same for loop the above removed?
>>>
>>> Seems like we are going in circles?? John?
>>>
>>
>> put_user_pages() is meant to be a drop-in replacement for release_pages(),
>> so I made the above change as an interim step in moving the callsite from
>> a loop, to a single call.
>>
>> And at some point, it may be possible to find a way to optimize put_user_pages()
>> in a similar way to the batching that release_pages() does, that was part
>> of the plan for this.
>>
>> But I do see what you mean: in the interim, maybe put_user_pages() should
>> just be calling release_pages(), how does that change sound?
>
> I'm certainly not the expert here but FWICT release_pages() was originally
> designed to work with the page cache.
>
> aabfb57296e3 mm: memcontrol: do not kill uncharge batching in free_pages_and_swap_cache
>
> But at some point it was changed to be more general?
>
> ea1754a08476 mm, fs: remove remaining PAGE_CACHE_* and page_cache_{get,release} usage
>
> ... and it is exported and used outside of the swapping code... and used at
> lease 1 place to directly "put" pages gotten from get_user_pages_fast()
> [arch/x86/kvm/svm.c]
>
> From that it seems like it is safe.
>
> But I don't see where release_page() actually calls put_page() anywhere? What
> am I missing?
>

For that question, I recall having to look closely at this function, as well:

void release_pages(struct page **pages, int nr)
{
int i;
LIST_HEAD(pages_to_free);
struct pglist_data *locked_pgdat = NULL;
struct lruvec *lruvec;
unsigned long uninitialized_var(flags);
unsigned int uninitialized_var(lock_batch);

for (i = 0; i < nr; i++) {
struct page *page = pages[i];

/*
* Make sure the IRQ-safe lock-holding time does not get
* excessive with a continuous string of pages from the
* same pgdat. The lock is held only if pgdat != NULL.
*/
if (locked_pgdat && ++lock_batch == SWAP_CLUSTER_MAX) {
spin_unlock_irqrestore(&locked_pgdat->lru_lock, flags);
locked_pgdat = NULL;
}

if (is_huge_zero_page(page))
continue;

/* Device public page can not be huge page */
if (is_device_public_page(page)) {
if (locked_pgdat) {
spin_unlock_irqrestore(&locked_pgdat->lru_lock,
flags);
locked_pgdat = NULL;
}
put_devmap_managed_page(page);
continue;
}

page = compound_head(page);
if (!put_page_testzero(page))

^here is where it does the put_page() call, is that what
you were looking for?

thanks,
--
John Hubbard
NVIDIA