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Subject[PATCH] [ARM] force dcache flush if dcache_dirty bit set
Date
On ARM, update_mmu_cache() does dcache flush for a page only if
it has a kernel mapping (page_mapping(page) != NULL). The correct
behavior would be to force the flush based on dcache_dirty bit only.

One of the cases where present logic would be a problem is when
a RAM based block device[1] is used as a swap disk. In this case,
we would have in-memory data corruption as shown in steps below:

do_swap_page()
{
- Allocate a new page (if not already in swap cache)
- Issue read from swap disk
- Block driver issues flush_dcache_page()
- flush_dcache_page() simply sets PG_dcache_dirty bit and does not
actually issue a flush since this page has no user space mapping yet.
- Now, if swap disk is almost full, this newly read page is removed
from swap cache and corrsponding swap slot is freed.
- Map this page anonymously in user space.
- update_mmu_cache()
- Since this page does not have kernel mapping (its not in page/swap
cache and is mapped anonymously), it does not issue dcache flush
even if dcache_dirty bit is set by flush_dcache_page() above.

<user now gets stale data since dcache was never flushed>
}

Same problem exists on mips too.

[1] example:
- brd (RAM based block device)
- ramzswap (RAM based compressed swap device)

Signed-off-by: Nitin Gupta <ngupta@vflare.org>
---
arch/arm/mm/fault-armv.c | 9 +++------
1 files changed, 3 insertions(+), 6 deletions(-)

diff --git a/arch/arm/mm/fault-armv.c b/arch/arm/mm/fault-armv.c
index bc0099d..d0d17b6 100644
--- a/arch/arm/mm/fault-armv.c
+++ b/arch/arm/mm/fault-armv.c
@@ -153,14 +153,11 @@ void update_mmu_cache(struct vm_area_struct *vma, unsigned long addr, pte_t pte)

page = pfn_to_page(pfn);
mapping = page_mapping(page);
- if (mapping) {
#ifndef CONFIG_SMP
- int dirty = test_and_clear_bit(PG_dcache_dirty, &page->flags);
-
- if (dirty)
- __flush_dcache_page(mapping, page);
+ if (test_and_clear_bit(PG_dcache_dirty, &page->flags))
+ __flush_dcache_page(mapping, page);
#endif
-
+ if (mapping) {
if (cache_is_vivt())
make_coherent(mapping, vma, addr, pfn);
else if (vma->vm_flags & VM_EXEC)
--
1.6.2.5


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