Hussam reports:

I was poking around and for no real reason, I did cat /dev/mem and strings /dev/mem. Then I saw the following warning in dmesg. I saved it and rebooted immediately.

memremap attempted on mixed range 0x000000000009c000 size: 0x1000 ------------[ cut here ]------------ WARNING: CPU: 0 PID: 11810 at kernel/memremap.c:98 memremap+0x104/0x170 [..] Call Trace: xlate_dev_mem_ptr+0x25/0x40 read_mem+0x89/0x1a0 __vfs_read+0x36/0x170

The memremap() implementation checks for attempts to remap System RAM with MEMREMAP_WB and instead redirects those mapping attempts to the linear map. However, that only works if the physical address range being remapped is page aligned. In low memory we have situations like the following:

00000000-00000fff : Reserved 00001000-0009fbff : System RAM 0009fc00-0009ffff : Reserved

...where System RAM intersects Reserved ranges on a sub-page page granularity.

Given that devmem_is_allowed() special cases any attempt to map System RAM in the first 1MB of memory, replace page_is_ram() with the more precise region_intersects() to trap attempts to map disallowed ranges.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=199999 Fixes: 92281dee825f ("arch: introduce memremap()") Cc: stable@vger.kernel.org Cc: Christoph Hellwig hch@lst.de Reported-by: Hussam Al-Tayeb me@hussam.eu.org Signed-off-by: Dan Williams dan.j.williams@intel.com --- arch/x86/mm/init.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c index fec82b577c18..cee58a972cb2 100644 --- a/arch/x86/mm/init.c +++ b/arch/x86/mm/init.c @@ -706,7 +706,9 @@ void __init init_mem_mapping(void) */ int devmem_is_allowed(unsigned long pagenr) { - if (page_is_ram(pagenr)) { + if (region_intersects(PFN_PHYS(pagenr), PAGE_SIZE, + IORESOURCE_SYSTEM_RAM, IORES_DESC_NONE) + != REGION_DISJOINT) { /* * For disallowed memory regions in the low 1MB range, * request that the page be shown as all zeros.