Question

I am new to linux kernel. And recently, i’ve went through the sendfile syscall in kernel 2.6.33. The following is the sequence of my journey:

   do_sendfile()
=> do_splice_direct()
=> splice_direct_to_actor()
=> do_splice_to()
=> do_splice_from()
=> splice_read,splice_write

Throughout this sequence, I didn’t find the place where splice use the DMA copy. So where is the DMA copying taking place?

Answer 1

Splice doesn’t do any DMA copy. In fact the major usage of splice is to avoid copying at all – it tries to pass references to memory pages instead of copying the buffers.

The DMA mentioned in relation to splice will happen at the “leaf” – The origin of these pages that splice passes references to around will be created by, for example, a disk controller DMA into the buffer and will be sent by an Ethernet controller DMA of the content of the page as part of the packet – at least in a “perfect” zero copy sceanrio, which is difficult to achieve and rare.

Splice doesn’t do the DMA – it enables no copying between the first DMA to the last.