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Subject[PATCH 3/3] mm/devmap: Remove pgmap accounting in the get_user_pages_fast() path
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Now that device-dax and filesystem-dax are guaranteed to unmap all user
mappings of devmap / DAX pages before tearing down the 'struct page'
array, get_user_pages_fast() can rely on its traditional synchronization
method "validate_pte(); get_page(); revalidate_pte()" to catch races with
device shutdown. Specifically the unmap guarantee ensures that gup-fast
either succeeds in taking a page reference (lock-less), or it detects a
need to fall back to the slow path where the device presence can be
revalidated with locks held.

Reported-by: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Shiyang Ruan <ruansy.fnst@fujitsu.com>
Cc: Vishal Verma <vishal.l.verma@intel.com>
Cc: Dave Jiang <dave.jiang@intel.com>
Cc: Ira Weiny <ira.weiny@intel.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
---
mm/gup.c | 38 ++++++++++++++++----------------------
1 file changed, 16 insertions(+), 22 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index e40579624f10..dfeb47e4e8d4 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1996,9 +1996,8 @@ static void __maybe_unused undo_dev_pagemap(int *nr, int nr_start,
static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
unsigned int flags, struct page **pages, int *nr)
{
- struct dev_pagemap *pgmap = NULL;
- int nr_start = *nr, ret = 0;
pte_t *ptep, *ptem;
+ int ret = 0;

ptem = ptep = pte_offset_map(&pmd, addr);
do {
@@ -2015,16 +2014,10 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
if (!pte_access_permitted(pte, flags & FOLL_WRITE))
goto pte_unmap;

- if (pte_devmap(pte)) {
- if (unlikely(flags & FOLL_LONGTERM))
- goto pte_unmap;
+ if (pte_devmap(pte) && (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;
- }
- } else if (pte_special(pte))
+ if (pte_special(pte))
goto pte_unmap;

VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
@@ -2063,8 +2056,6 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
ret = 1;

pte_unmap:
- if (pgmap)
- put_dev_pagemap(pgmap);
pte_unmap(ptem);
return ret;
}
@@ -2087,21 +2078,26 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
#endif /* CONFIG_ARCH_HAS_PTE_SPECIAL */

#if defined(CONFIG_ARCH_HAS_PTE_DEVMAP) && defined(CONFIG_TRANSPARENT_HUGEPAGE)
+
static int __gup_device_huge(unsigned long pfn, unsigned long addr,
unsigned long end, unsigned int flags,
struct page **pages, int *nr)
{
int nr_start = *nr;
- struct dev_pagemap *pgmap = NULL;

do {
- struct page *page = pfn_to_page(pfn);
+ struct page *page;
+
+ /*
+ * Typically pfn_to_page() on a devmap pfn is not safe
+ * without holding a live reference on the hosting
+ * pgmap. In the gup-fast path it is safe because any
+ * races will be resolved by either gup-fast taking a
+ * reference or the shutdown path unmapping the pte to
+ * trigger gup-fast to fall back to the slow path.
+ */
+ page = pfn_to_page(pfn);

- pgmap = get_dev_pagemap(pfn, pgmap);
- if (unlikely(!pgmap)) {
- undo_dev_pagemap(nr, nr_start, flags, pages);
- return 0;
- }
SetPageReferenced(page);
pages[*nr] = page;
if (unlikely(!try_grab_page(page, flags))) {
@@ -2112,8 +2108,6 @@ static int __gup_device_huge(unsigned long pfn, unsigned long addr,
pfn++;
} while (addr += PAGE_SIZE, addr != end);

- if (pgmap)
- put_dev_pagemap(pgmap);
return 1;
}

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