• Yosry Ahmed's avatar
    dma-mapping: use bit masking to check VM_DMA_COHERENT · fa3c109a
    Yosry Ahmed authored
    In dma_common_find_pages(), area->flags are compared directly with
    VM_DMA_COHERENT. This works because VM_DMA_COHERENT is the only set
    flag.
    
    During development of a new feature (ASI [1]), a new VM flag is
    introduced, and that flag can be injected into VM_DMA_COHERENT mappings
    (among others).  The presence of that flag caused
    dma_common_find_pages() to return NULL for VM_DMA_COHERENT addresses,
    leading to a lot of problems ending in crashing during boot. It took a
    bit of time to figure this problem out.
    
    It was a mistake to inject a VM flag to begin with, but it took a
    significant amount of debugging to figure out the problem. Most users of
    area->flags use bitmasking rather than equivalency to check for flags.
    Update dma_common_find_pages() and dma_common_free_remap() to do the
    same, which would have avoided the boot crashing. Instead, add a warning
    in dma_common_find_pages() if any extra VM flags are set to catch such
    problems more easily during development.
    
    No functional change intended.
    
    [1]https://lore.kernel.org/lkml/20240712-asi-rfc-24-v1-0-144b319a40d8@google.com/Signed-off-by: default avatarYosry Ahmed <yosryahmed@google.com>
    Signed-off-by: default avatarChristoph Hellwig <hch@lst.de>
    fa3c109a
remap.c 1.67 KB