On 02.07.2014 21:38, Alan Stern wrote:
On Wed, 2 Jul 2014, Oliver Neukum wrote:
Stefan Klug
Software Developer
Basler AG
An der Strusbek 60-62
22926 Ahrensburg
Germany
Tel. +49 4102 463 582
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On Wed, 2014-07-02 at 17:53 +0200, Stefan Klug wrote:
Implementation details:
The patch only touches drivers/usb/core/devio.c.
In procy_do_submiturb(), it is checked if zerocopy is allowed. This is
currently a rough
check which compares the number of required pages to
ps->dev->bus->sg_tablesize.
It seems to me that the check is per call, so using
multiple calls one could still pin unlimited amounts
of memory.
usbfs keeps track of the total amount of pinned memory and enforces an
overall limit. It will be necessary to add the size of the transfer
buffer to that total.
Leaving the zerocopy transfers out of this limit was intentional. I
thought this is user-memory so we shouldn't add it to the overall limit
as it is not allocated by usbfs.
But I didn't think of the pinning problem. So yes, I can add it to the
overall limit.
I don't know if there is more to check there.
Then the user memory provided inside the usbdevfs_urb structure is
pinned to
physical memory using get_user_pages_fast().
All the user pages are added to the scatter-gather list and the logic
continues as before.
How do you enforce the cache coherency rules?
There is no way to do this. If the user program accesses memory when
it shouldn't, the transfer might not work right.
So this one is fine, right?
Also you don't have a fall back if get_user_pages_fast()
returns less than requested. It seems to me that than you
ought to fall back buffered IO.
Agreed.
Good point. I'll add the fallback to the next iteration.
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