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dma-mapping: use bit masking to check VM_DMA_COHERENT
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: Yosry Ahmed <yosryahmed@google.com> Signed-off-by: Christoph Hellwig <hch@lst.de>
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@ -10,8 +10,10 @@ struct page **dma_common_find_pages(void *cpu_addr)
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{
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struct vm_struct *area = find_vm_area(cpu_addr);
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if (!area || area->flags != VM_DMA_COHERENT)
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if (!area || !(area->flags & VM_DMA_COHERENT))
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return NULL;
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WARN(area->flags != VM_DMA_COHERENT,
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"unexpected flags in area: %p\n", cpu_addr);
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return area->pages;
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}
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@ -61,7 +63,7 @@ void dma_common_free_remap(void *cpu_addr, size_t size)
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{
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struct vm_struct *area = find_vm_area(cpu_addr);
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if (!area || area->flags != VM_DMA_COHERENT) {
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if (!area || !(area->flags & VM_DMA_COHERENT)) {
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WARN(1, "trying to free invalid coherent area: %p\n", cpu_addr);
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return;
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}
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