Messages in this thread | | | Subject | Re: copy_{to,from}_user | From | Peter Zijlstra <> | Date | Fri, 09 Jan 2009 22:54:21 +0100 |
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On Fri, 2009-01-09 at 12:52 -0500, Brad Parker wrote: > I have a question about copy_{to,from}_user. > > Most implementations I've seen do in-order copies and notice when an > exception occurs and report back the progress. This is straight > forward. > > (but to be honest, I have suspicions about how just how accurate those > reports are i.e. +/- 1-3 bytes on some architectures) > > On some cpu's it is advantageous to do an out-of-order copy to take > advantage of various cache fill mechanisms. > > The problem is that the out-of-order copy makes it impossible to know > where the exception occurred (in terms of progress). > > Would it be permissible to have a version of copy_{to,from}_user which > does an out-of-order copy and when an exception occurs, restarts the > copy from the beginning using a simple in-order copy, to make it > possible to identify where the exception occurs? > > The idea is that exceptions are rare and so the performance hit of doing > the "recopy" would be minimal and would provide the required accuracy.
x86_64 already does some unrolling and is inaccurate as to where exactly it happens. The only thing that is very important is that you _never_ say you copied more than you actually did.
That was the source of a data corruption bug a while ago, the code did something like sequences: read 8 words, write 8 words. And reported the number of bytes read, instead of bytes written, which is an over-estimation.
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