Hi,

I've been observing a high rate of failures with CMA allocations on my ARM system. I've set up a test case set up with a 56MB CMA region that essentially does the following:

total_failures = 0; loop forever: loop_failure = 0; for (i = 0; i < 56; i++) chunk[i] = dma_allocate(&cma_dev, 1MB) if (!chunk[i]) loop_failure = 0

if (loop_failure) total_failures++ loop_failure = 0

for (i = 0; i < 56; i++) dma_free(&cma_dev, chunk[i], 1MB)

In the background, I also have a process doing some amount of filesystem activity (adb push/pull since this is an android system). During the course of my investigations I generally get ~8500 loops total and ~450 total failures (i.e. one or more buffers could not be allocated). This is unacceptably high for our use cases.

In every case the allocation failure was ultimately due to a migration failure; the pages contained buffers which could not be dropped because the buffers were busy (move_to_new_page -> fallback_migrate_page -> try_to_release_page -> try_to_free_buffers -> drop_buffers -> buffer_busy). In every case, the b_count on the buffer head was always 1.

The problem arises because of the LRU lists for buffer heads:

__getblk __getblk_slow grow_buffers grow_dev_page find_or_create_page -- create a possibly movable page __find_get_block __find_get_block_slow find_get_page -- return the movable page bh_lru_install get_bh -- buffer head now has a reference

The reference taken in bh_lru_install won't be dropped until the bh is evicted from the lru. This means the page cannot be migrated as long as the buffer exists on an LRU list. The real issue is that unless the buffer gets evicted quickly the page can remain non-migratible for long periods of time. This makes CMA regions unusable for long periods of time given that we generally don't want to size CMA regions any larger than necessary ergo any failure will cause a problem.

My quick and dirty workaround for testing is to remove the GFP_MOVABLE flag from find_or_create_page but this seems significantly less than optimal. Ideally, it seems like the buffers should be evicted from the LRU when trying to drop (expand on invalid_bh_lru?) but I'm not familiar enough with the code path to know if this is a good approach.

Any suggestions/feedback is appreciated. Thanks.

Laura