Re: [RFC 1/3] lib: copy_{from,to}_user using gup & kmap_atomic()

[Russell King - ARM Linux admin <linux@armlinux.org.uk>]

On Sat, Jun 13, 2020 at 05:34:32PM +0530, afzal mohammed wrote:
> Hi,
> 
> On Fri, Jun 12, 2020 at 10:07:28PM +0200, Arnd Bergmann wrote:
> 
> > I think a lot
> > of usercopy calls are only for a few bytes, though this is of course
> > highly workload dependent and you might only care about the large
> > ones.
> 
> Observation is that max. pages reaching copy_{from,to}_user() is 2,
> observed maximum of n (number of bytes) being 1 page size. i think C
> library cuts any size read, write to page size (if it exceeds) &
> invokes the system call. Max. pages reaching 2, happens when 'n'
> crosses page boundary, this has been observed w/ small size request
> as well w/ ones of exact page size (but not page aligned).

You can't make that assumption about read(2).  stdio in the C library
may read a page size of data at a time, but programs are allowed to
call read(2) directly, and the C library will pass such a call straight
through to the kernel.  So, if userspace requests a 16k read via
read(2), then read(2) will be invoked covering 16k.

As an extreme case, for example:

$ strace -e read dd if=/dev/zero of=/dev/null bs=1048576 count=1
read(0, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 1048576) = 1048576

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