Messages in this thread | | | Date | Fri, 8 Jan 2016 01:45:35 -0800 | From | Christoph Hellwig <> | Subject | Re: [PATCH] Add support for usbfs zerocopy. |
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On Wed, Jan 06, 2016 at 10:35:05AM -0500, Alan Stern wrote: > Indeed, the I/O operations we are using with mmap here are not reads or > writes; they are ioctls. As far as I know, the kernel doesn't have any > defined interface for zerocopy ioctls.
IF it was using mmap for I/O it would read in through the page fault handler an then mark the page dirty for writeback by the VM. Thats clearly not the case.
Instead it's using mmap on a file as a pecial purpose anonymous memory allocator, bypassing the VM and VM policies, including allowing to pin kernel memory that way.
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