From: Nitin Gupta <ngupta@vflare.org>
Date: Mon, 12 Oct 2009 08:50:23 +0000 (+0530)
Subject: ARM: force dcache flush if dcache_dirty bit set
X-Git-Tag: v2.6.32-rc5~36^2
X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=787b2faadc4356b6c2c71feb42fb944fece9a12f;p=pandora-kernel.git

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>
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