s390/mm: implement software dirty bits

The s390 architecture is unique in respect to dirty page detection,
it uses the change bit in the per-page storage key to track page
modifications. All other architectures track dirty bits by means
of page table entries. This property of s390 has caused numerous
problems in the past, e.g. see git commit ef5d437f71afdf4a
"mm: fix XFS oops due to dirty pages without buffers on s390".

To avoid future issues in regard to per-page dirty bits convert
s390 to a fault based software dirty bit detection mechanism. All
user page table entries which are marked as clean will be hardware
read-only, even if the pte is supposed to be writable. A write by
the user process will trigger a protection fault which will cause
the user pte to be marked as dirty and the hardware read-only bit
is removed.

With this change the dirty bit in the storage key is irrelevant
for Linux as a host, but the storage key is still required for
KVM guests. The effect is that page_test_and_clear_dirty and the
related code can be removed. The referenced bit in the storage
key is still used by the page_test_and_clear_young primitive to
provide page age information.

For page cache pages of mappings with mapping_cap_account_dirty
there will not be any change in behavior as the dirty bit tracking
already uses read-only ptes to control the amount of dirty pages.
Only for swap cache pages and pages of mappings without
mapping_cap_account_dirty there can be additional protection faults.
To avoid an excessive number of additional faults the mk_pte
primitive checks for PageDirty if the pgprot value allows for writes
and pre-dirties the pte. That avoids all additional faults for
tmpfs and shmem pages until these pages are added to the swap cache.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

1 files changed, 0 insertions, 22 deletions

diff --git a/arch/s390/include/asm/page.h b/arch/s390/include/asm/page.h
index a86ad408407..75ce9b065f9 100644
--- a/arch/s390/include/asm/page.h
+++ b/arch/s390/include/asm/page.h
@@ -155,28 +155,6 @@ static inline int page_reset_referenced(unsigned long addr)
 #define _PAGE_ACC_BITS		0xf0	/* HW access control bits	*/
 
 /*
- * Test and clear dirty bit in storage key.
- * We can't clear the changed bit atomically. This is a potential
- * race against modification of the referenced bit. This function
- * should therefore only be called if it is not mapped in any
- * address space.
- *
- * Note that the bit gets set whenever page content is changed. That means
- * also when the page is modified by DMA or from inside the kernel.
- */
-#define __HAVE_ARCH_PAGE_TEST_AND_CLEAR_DIRTY
-static inline int page_test_and_clear_dirty(unsigned long pfn, int mapped)
-{
-	unsigned char skey;
-
-	skey = page_get_storage_key(pfn << PAGE_SHIFT);
-	if (!(skey & _PAGE_CHANGED))
-		return 0;
-	page_set_storage_key(pfn << PAGE_SHIFT, skey & ~_PAGE_CHANGED, mapped);
-	return 1;
-}
-
-/*
  * Test and clear referenced bit in storage key.
  */
 #define __HAVE_ARCH_PAGE_TEST_AND_CLEAR_YOUNG