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 <jhubb...@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
