mm/mprotect: allow clean exclusive anon pages to be writable

Patch series "mm/autonuma: replace savedwrite infrastructure", v2.

As discussed in my talk at LPC, we can reuse the same mechanism for
deciding whether to map a pte writable when upgrading permissions via
mprotect() -- e.g., PROT_READ -> PROT_READ|PROT_WRITE -- to replace the
savedwrite infrastructure used for NUMA hinting faults (e.g., PROT_NONE ->
PROT_READ|PROT_WRITE).

Instead of maintaining previous write permissions for a pte/pmd, we
re-determine if the pte/pmd can be writable.  The big benefit is that we
have a common logic for deciding whether we can map a pte/pmd writable on
protection changes.

For private mappings, there should be no difference -- from what I
understand, that is what autonuma benchmarks care about.

I ran autonumabench for v1 on a system with 2 NUMA nodes, 96 GiB each via:
	perf stat --null --repeat 10
The numa01 benchmark is quite noisy in my environment and I failed to
reduce the noise so far.

numa01:
	mm-unstable:   146.88 +- 6.54 seconds time elapsed  ( +-  4.45% )
	mm-unstable++: 147.45 +- 13.39 seconds time elapsed  ( +-  9.08% )

numa02:
	mm-unstable:   16.0300 +- 0.0624 seconds time elapsed  ( +-  0.39% )
	mm-unstable++: 16.1281 +- 0.0945 seconds time elapsed  ( +-  0.59% )

It is worth noting that for shared writable mappings that require
writenotify, we will only avoid write faults if the pte/pmd is dirty
(inherited from the older mprotect logic).  If we ever care about
optimizing that further, we'd need a different mechanism to identify
whether the FS still needs to get notified on the next write access.

In any case, such an optimization will then not be autonuma-specific, but
mprotect() permission upgrades would similarly benefit from it.


This patch (of 7):

Anonymous pages might have the dirty bit clear, but this should not
prevent mprotect from making them writable if they are exclusive. 
Therefore, skip the test whether the page is dirty in this case.

Note that there are already other ways to get a writable PTE mapping an
anonymous page that is clean: for example, via MADV_FREE.  In an ideal
world, we'd have a different indication from the FS whether writenotify is
still required.

[[email protected]: return directly; update description]
Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Nadav Amit <[email protected]>
Signed-off-by: David Hildenbrand <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Dave Chinner <[email protected]>
Cc: Peter Xu <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Michael Ellerman <[email protected]>
Cc: Nicholas Piggin <[email protected]>
Cc: Mike Rapoport <[email protected]>
Cc: Anshuman Khandual <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

1 files changed, 3 insertions, 4 deletions

diff --git a/mm/mprotect.c b/mm/mprotect.c
index 8d770855b591..86a28c0e190f 100644
--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -46,7 +46,7 @@ static inline bool can_change_pte_writable(struct vm_area_struct *vma,
 
 	VM_BUG_ON(!(vma->vm_flags & VM_WRITE) || pte_write(pte));
 
-	if (pte_protnone(pte) || !pte_dirty(pte))
+	if (pte_protnone(pte))
 		return false;
 
 	/* Do we need write faults for softdirty tracking? */
@@ -65,11 +65,10 @@ static inline bool can_change_pte_writable(struct vm_area_struct *vma,
 		 * the PT lock.
 		 */
 		page = vm_normal_page(vma, addr, pte);
-		if (!page || !PageAnon(page) || !PageAnonExclusive(page))
-			return false;
+		return page && PageAnon(page) && PageAnonExclusive(page);
 	}
 
-	return true;
+	return pte_dirty(pte);
 }
 
 static unsigned long change_pte_range(struct mmu_gather *tlb,