Re: [PATCH 1/1] vfio/spapr_tce: convert get_user_pages() --> pin_user_pages()

From: John Hubbard
Date: Tue May 26 2020 - 15:45:24 EST

On 2020-05-26 12:28, Souptick Joarder wrote:
@@ -486,7 +486,7 @@ static int tce_iommu_use_page(unsigned long tce, unsigned long *hpa)
struct page *page = NULL;
enum dma_data_direction direction = iommu_tce_direction(tce);

- if (get_user_pages_fast(tce & PAGE_MASK, 1,
+ if (pin_user_pages_fast(tce & PAGE_MASK, 1,
direction != DMA_TO_DEVICE ? FOLL_WRITE : 0,
&page) != 1)
return -EFAULT;

There are few places where nr_pages is passed as 1 to get_user_pages_fast().
With similar conversion those will be changed to pin_user_pages_fast().

Does it make sense to add an inline like - pin_user_page_fast(), similar to
get_user_page_fast_only() ( now merged in linux-next) ?

Perhaps not *just* yet, IMHO. There are only two places so far: here, and
dax_lock_page(). And we don't expect that many places, either, because
pin_user_pages*(), unlike get_user_pages(), is more likely to operate on
a bunch of pages at once. Although, that could change if we look into the
remaining call sites and find more single-page cases that need a gup-to-pup

get_user_pages*() has a few more situations (Case 4, in
Documentation/core-api/pin_user_pages.rst: struct page manipulation) in
which it operates on single pages. Those will remain get_user_pages*()
calls, or perhaps change to get_user_page().

John Hubbard