Re: [PATCH v1 06/13] util/mmap-alloc: Factor out reserving of a memory region to mmap_reserve()

[David Hildenbrand]

On 06.02.20 12:55, Richard Henderson wrote:
> On 2/3/20 6:31 PM, David Hildenbrand wrote:
>> We want to reserve a memory region without actually populating memory.
>> Let's factor that out.
>>
>> Cc: "Michael S. Tsirkin" <address@hidden>
>> Cc: Greg Kurz <address@hidden>
>> Cc: Murilo Opsfelder Araujo <address@hidden>
>> Cc: Eduardo Habkost <address@hidden>
>> Cc: "Dr. David Alan Gilbert" <address@hidden>
>> Signed-off-by: David Hildenbrand <address@hidden>
>> ---
>>  util/mmap-alloc.c | 58 +++++++++++++++++++++++++++--------------------
>>  1 file changed, 33 insertions(+), 25 deletions(-)
>>
>> diff --git a/util/mmap-alloc.c b/util/mmap-alloc.c
>> index 82f02a2cec..43a26f38a8 100644
>> --- a/util/mmap-alloc.c
>> +++ b/util/mmap-alloc.c
>> @@ -82,6 +82,38 @@ size_t qemu_mempath_getpagesize(const char *mem_path)
>>      return qemu_real_host_page_size;
>>  }
>>  
>> +/*
>> + * Reserve a new memory region of the requested size to be used for mapping
>> + * from the given fd (if any).
>> + */
>> +static void *mmap_reserve(size_t size, int fd)
>> +{
>> +    int flags = MAP_PRIVATE;
>> +
>> +#if defined(__powerpc64__) && defined(__linux__)
>> +    /*
>> +     * On ppc64 mappings in the same segment (aka slice) must share the same
>> +     * page size. Since we will be re-allocating part of this segment
>> +     * from the supplied fd, we should make sure to use the same page size, 
>> to
>> +     * this end we mmap the supplied fd.  In this case, set MAP_NORESERVE to
>> +     * avoid allocating backing store memory.
>> +     * We do this unless we are using the system page size, in which case
>> +     * anonymous memory is OK.
>> +     */
>> +    if (fd == -1 || qemu_fd_getpagesize(fd) == qemu_real_host_page_size) {
>> +        fd = -1;
>> +        flags |= MAP_ANONYMOUS;
>> +    } else {
>> +        flags |= MAP_NORESERVE;
>> +    }
>> +#else
>> +    fd = -1;
>> +    flags |= MAP_ANONYMOUS;
>> +#endif
> 
> Because this is just code movement,
> Reviewed-by: Richard Henderson <address@hidden>
> 
> But is there a reason not to add MAP_NORESERVE all of the time?
> It seems to match intent, in that we're reserving vma but not planning to use
> the memory, anonymous or not.

AFAIK, if you mmap something PROT_NONE, it's an implicit MAP_NORESERVE.
I keep setting in conditionally, because I am not sure if any POSIX
system (or older kernel) might choke on MAP_NORESERVE. e.g.,

"In kernels before 2.6, this flag had effect only for private writable
mappings." sounds like it would get ignored. But also sounds like it's
somewhat Linux specific.

-- 
Thanks,

David / dhildenb