The default max_cache_size_bytes for dm-bufio is meant to be the lesser of 25% of the size of the vmalloc area and 2% of the size of lowmem. However, on 32-bit systems the intermediate result in the expression

(VMALLOC_END - VMALLOC_START) * DM_BUFIO_VMALLOC_PERCENT / 100

overflows, causing the wrong result to be computed. For example, on a 32-bit system where the vmalloc area is 520093696 bytes, the result is 1174405 rather than the expected 130023424, which makes the maximum cache size much too small (far less than 2% of lowmem). This causes severe performance problems for dm-verity users on affected systems.

Fix this by using mult_frac() to correctly multiply by a percentage. Do this for all places in dm-bufio that multiply by a percentage. Also replace (VMALLOC_END - VMALLOC_START) with VMALLOC_TOTAL, which contrary to the comment is now defined in include/linux/vmalloc.h.

Fixes: 95d402f057f2 ("dm: add bufio") Cc: stable@vger.kernel.org # v3.2+ Signed-off-by: Eric Biggers ebiggers@google.com --- drivers/md/dm-bufio.c | 15 ++++++--------- 1 file changed, 6 insertions(+), 9 deletions(-)

diff --git a/drivers/md/dm-bufio.c b/drivers/md/dm-bufio.c index 33bb074d6941..b8ac591aaaa7 100644 --- a/drivers/md/dm-bufio.c +++ b/drivers/md/dm-bufio.c @@ -974,7 +974,8 @@ static void __get_memory_limit(struct dm_bufio_client *c, buffers = c->minimum_buffers;

*limit_buffers = buffers; - *threshold_buffers = buffers * DM_BUFIO_WRITEBACK_PERCENT / 100; + *threshold_buffers = mult_frac(buffers, + DM_BUFIO_WRITEBACK_PERCENT, 100); }

/* @@ -1910,19 +1911,15 @@ static int __init dm_bufio_init(void) memset(&dm_bufio_caches, 0, sizeof dm_bufio_caches); memset(&dm_bufio_cache_names, 0, sizeof dm_bufio_cache_names);

- mem = (__u64)((totalram_pages - totalhigh_pages) * - DM_BUFIO_MEMORY_PERCENT / 100) << PAGE_SHIFT; + mem = (__u64)mult_frac(totalram_pages - totalhigh_pages, + DM_BUFIO_MEMORY_PERCENT, 100) << PAGE_SHIFT;

if (mem > ULONG_MAX) mem = ULONG_MAX;

#ifdef CONFIG_MMU - /* - * Get the size of vmalloc space the same way as VMALLOC_TOTAL - * in fs/proc/internal.h - */ - if (mem > (VMALLOC_END - VMALLOC_START) * DM_BUFIO_VMALLOC_PERCENT / 100) - mem = (VMALLOC_END - VMALLOC_START) * DM_BUFIO_VMALLOC_PERCENT / 100; + if (mem > mult_frac(VMALLOC_TOTAL, DM_BUFIO_VMALLOC_PERCENT, 100)) + mem = mult_frac(VMALLOC_TOTAL, DM_BUFIO_VMALLOC_PERCENT, 100); #endif

dm_bufio_default_cache_size = mem;