Re: [PATCH v2] rust: page: add byte-wise atomic memory copy methods

[Andreas Hindborg]

"Greg KH" <gregkh@xxxxxxxxxxxxxxxxxxx> writes:

> On Fri, Feb 13, 2026 at 10:55:57AM +0100, Peter Zijlstra wrote:
>> On Thu, Feb 12, 2026 at 03:51:24PM +0100, Andreas Hindborg wrote:
>> > When copying data from buffers that are mapped to user space, it is
>> > impossible to guarantee absence of concurrent memory operations on those
>> > buffers. Copying data to/from `Page` from/to these buffers would be
>> > undefined behavior if no special considerations are made.
>> >
>> > Add methods on `Page` to read and write the contents using byte-wise atomic
>> > operations.
>>
>> I'm completely failing to understand. What!?
>>
>> There is no such thing as an 'atomic' byte load, nor does it help one
>> whit against concurrent modification of the memory you're copying.
>
> I too am totally confused when reading this patch, and the previous
> ones.  Shouldn't the "normal" copy_from_user() stuff be used here
> correctly?  Why is anything new needed?

One use for this is copying data out of a `struct bio_vec`. As far as I
know, there is no way to know where the pages backing a bio_vec are
mapped. They could be mapped to user space.

Best regards,
Andreas Hindborg