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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). Link: https://lkml.kernel.org/r/20240307010250.3847179-1-jthoughton@google.com Signed-off-by: James Houghton <jthoughton@google.com> Reviewed-by: Peter Xu <peterx@redhat.com> Cc: Axel Rasmussen <axelrasmussen@google.com> Cc: Muchun Song <songmuchun@bytedance.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.