On Thu, Mar 07, 2024 at 01:02:50AM +0000, James Houghton wrote: > Users of UFFDIO_CONTINUE may reasonably assume that a write memory > barrier is included as part of UFFDIO_CONTINUE. That is, a user may > believe that all writes it has done to a page that it is now > UFFDIO_CONTINUE'ing are guaranteed to be visible to anyone subsequently > reading the page through the newly mapped virtual memory region. > > Today, such a user happens to be correct. mmget_not_zero(), for example, > is called as part of UFFDIO_CONTINUE (and comes before any PTE updates), > and it implicitly gives us a write barrier. > > To be resilient against future changes, include an explicit smp_wmb(). > While we're at it, optimize the smp_wmb() that is already incidentally > present for the HugeTLB case. > > Merely making a syscall does not generally imply the memory ordering > constraints that we need (including on x86). > > Signed-off-by: James Houghton <jthoughton@xxxxxxxxxx> Reviewed-by: Peter Xu <peterx@xxxxxxxxxx> -- Peter Xu
