The following commit has been merged in the linux branch: commit 787b2faadc4356b6c2c71feb42fb944fece9a12f Author: Nitin Gupta ngupta@vflare.org Date: Mon Oct 12 14:20:23 2009 +0530
ARM: force dcache flush if dcache_dirty bit set
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 Signed-off-by: Russell King rmk+kernel@arm.linux.org.uk
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)