On 2/20/21 1:18 AM, Dan Williams wrote:

On Tue, Dec 8, 2020 at 9:32 AM Joao Martins [off-list ref] wrote:

quoted

Patch 6 - 8: Optimize grabbing/release a page refcount changes given that we
are working with compound pages i.e. we do 1 increment/decrement to the head
page for a given set of N subpages compared as opposed to N individual writes.
{get,pin}_user_pages_fast() for zone_device with compound pagemap consequently
improves considerably, and unpin_user_pages() improves as well when passed a
set of consecutive pages:

                                           before          after
    (get_user_pages_fast 1G;2M page size) ~75k  us -> ~3.2k ; ~5.2k us
    (pin_user_pages_fast 1G;2M page size) ~125k us -> ~3.4k ; ~5.5k us

Compelling!

BTW is there any reason why we don't support pin_user_pages_fast() with FOLL_LONGTERM for
device-dax?

Looking at the history, I understand that fsdax can't support it atm, but I am not sure
that the same holds for device-dax. I have this small chunk (see below the scissors mark)
which relaxes this for a pgmap of type MEMORY_DEVICE_GENERIC, albeit not sure if there is
a fundamental issue for the other types that makes this an unwelcoming change.

	Joao

--------------------->8---------------------

Subject: [PATCH] mm/gup: allow FOLL_LONGTERM pin-fast for
 MEMORY_DEVICE_GENERIC

The downside would be one extra lookup in dev_pagemap tree
for other pgmap->types (P2P, FSDAX, PRIVATE). But just one
per gup-fast() call.

Signed-off-by: Joao Martins <redacted>
---
 include/linux/mm.h |  5 +++++
 mm/gup.c           | 24 +++++++++++++-----------
 2 files changed, 18 insertions(+), 11 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 32f0c3986d4f..c89a049bbd7a 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h

@@ -1171,6 +1171,11 @@ static inline bool is_pci_p2pdma_page(const struct page *page)
 		page->pgmap->type == MEMORY_DEVICE_PCI_P2PDMA;
 }

+static inline bool devmap_longterm_available(const struct dev_pagemap *pgmap)
+{
+	return pgmap->type == MEMORY_DEVICE_GENERIC;
+}
+
 /* 127: arbitrary random number, small enough to assemble well */
 #define page_ref_zero_or_close_to_overflow(page) \
 	((unsigned int) page_ref_count(page) + 127u <= 127u)

diff --git a/mm/gup.c b/mm/gup.c
index 222d1fdc5cfa..03e370d360e6 100644
--- a/mm/gup.c
+++ b/mm/gup.c

@@ -2092,14 +2092,18 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned

long end,
 			goto pte_unmap;

 		if (pte_devmap(pte)) {
-			if (unlikely(flags & FOLL_LONGTERM))
-				goto pte_unmap;
-
 			pgmap = get_dev_pagemap(pte_pfn(pte), pgmap);
 			if (unlikely(!pgmap)) {
 				undo_dev_pagemap(nr, nr_start, flags, pages);
 				goto pte_unmap;
 			}
+
+			if (unlikely(flags & FOLL_LONGTERM) &&
+			    !devmap_longterm_available(pgmap)) {
+				undo_dev_pagemap(nr, nr_start, flags, pages);
+				goto pte_unmap;
+			}
+
 		} else if (pte_special(pte))
 			goto pte_unmap;

@@ -2195,6 +2199,10 @@ static int __gup_device_huge(unsigned long pfn, unsigned long addr,
 			return 0;
 		}

+		if (unlikely(flags & FOLL_LONGTERM) &&
+		    !devmap_longterm_available(pgmap))
+			return 0;
+

@@ -2356,12 +2364,9 @@ static int gup_huge_pmd(pmd_t orig, pmd_t *pmdp, unsigned long addr,
 	if (!pmd_access_permitted(orig, flags & FOLL_WRITE))
 		return 0;

-	if (pmd_devmap(orig)) {
-		if (unlikely(flags & FOLL_LONGTERM))
-			return 0;
+	if (pmd_devmap(orig))
 		return __gup_device_huge_pmd(orig, pmdp, addr, end, flags,
 					     pages, nr);
-	}

 	page = pmd_page(orig) + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
 	refs = record_subpages(page, addr, end, pages + *nr);

@@ -2390,12 +2395,9 @@ static int gup_huge_pud(pud_t orig, pud_t *pudp, unsigned long addr,
 	if (!pud_access_permitted(orig, flags & FOLL_WRITE))
 		return 0;

-	if (pud_devmap(orig)) {
-		if (unlikely(flags & FOLL_LONGTERM))
-			return 0;
+	if (pud_devmap(orig))
 		return __gup_device_huge_pud(orig, pudp, addr, end, flags,
 					     pages, nr);
-	}

 	page = pud_page(orig) + ((addr & ~PUD_MASK) >> PAGE_SHIFT);
 	refs = record_subpages(page, addr, end, pages + *nr);

-- 
2.17.1