On Wed, Jul 05, 2017 at 05:41:14PM -0700, prakash.sangappa wrote:

On 07/04/2017 11:28 AM, Mike Rapoport wrote:

quoted

On Tue, Jun 27, 2017 at 09:08:40AM -0700, Prakash Sangappa wrote:

quoted

Applications like the database use hugetlbfs for performance reason.
Files on hugetlbfs filesystem are created and huge pages allocated
using fallocate() API. Pages are deallocated/freed using fallocate() hole
punching support. These files are mmap'ed and accessed by many
single threaded processes as shared memory.  The database keeps
track of which offsets in the hugetlbfs file have pages allocated.

[ ... ]

quoted

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+     *
+     * UFFD_FEATURE_SIGBUS feature means no page-fault
+     * (UFFD_EVENT_PAGEFAULT) event will be delivered, instead
+     * a SIGBUS signal will be sent to the faulting process.
+     * The application process can enable this behavior by adding
+     * it to uffdio_api.features.

I think that it maybe worth making UFFD_FEATURE_SIGBUS mutually exclusive
with the non-cooperative events. There is no point of having monitor if the
page fault handler will anyway just kill the faulting process.

Will this not be too restrictive?. The non-cooperative events could
still be useful if an application wants to track changes
to VA ranges that are registered even though it expects
a signal on page fault.

I wouldn't say that we must make UFFD_FEATURE_SIGBUS mutually exclusive
with other events, but, IMHO, it's something we should at least think
about.

In my view, if you anyway have uffd monitor, you may process page faults
there as well and then there is no actual need in UFFD_FEATURE_SIGBUS.

-- 
Sincerely yours,
Mike.