Re: [PATCH v2 fixed 08/16] util/mmap-alloc: Factor out calculation of pagesize to mmap_pagesize()

[David Hildenbrand <david@redhat.com>]

On 19.02.20 23:46, Peter Xu wrote:
> On Wed, Feb 12, 2020 at 02:42:46PM +0100, David Hildenbrand wrote:
>> Factor it out and add a comment.
>>
>> Reviewed-by: Igor Kotrasinski <i.kotrasinsk@partner.samsung.com>
>> Acked-by: Murilo Opsfelder Araujo <muriloo@linux.ibm.com>
>> Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
>> Cc: "Michael S. Tsirkin" <mst@redhat.com>
>> Cc: Murilo Opsfelder Araujo <muriloo@linux.ibm.com>
>> Cc: Greg Kurz <groug@kaod.org>
>> Cc: Eduardo Habkost <ehabkost@redhat.com>
>> Cc: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
>> Cc: Igor Mammedov <imammedo@redhat.com>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
>> ---
>>  util/mmap-alloc.c | 21 ++++++++++++---------
>>  1 file changed, 12 insertions(+), 9 deletions(-)
>>
>> diff --git a/util/mmap-alloc.c b/util/mmap-alloc.c
>> index 27dcccd8ec..82f02a2cec 100644
>> --- a/util/mmap-alloc.c
>> +++ b/util/mmap-alloc.c
>> @@ -82,17 +82,27 @@ size_t qemu_mempath_getpagesize(const char *mem_path)
>>      return qemu_real_host_page_size;
>>  }
>>  
>> +static inline size_t mmap_pagesize(int fd)
>> +{
>> +#if defined(__powerpc64__) && defined(__linux__)
>> +    /* Mappings in the same segment must share the same page size */
>> +    return qemu_fd_getpagesize(fd);
>> +#else
>> +    return qemu_real_host_page_size;
>> +#endif
>> +}
> 
> Pure question: This will return 4K even for huge pages on x86, is this
> what we want?

(was asking myself the same question) I *think* it's intended. It's
mainly only used to allocate one additional guard page. The callers of
qemu_ram_mmap() make sure that the size is properly aligned (e.g., to
huge pages).

Of course, a 4k guard page is sufficient - unless we can't use that
(special case for ppc64 here).

Thanks!

-- 
Thanks,

David / dhildenb