Re: [PATCH v1 08/13] util/mmap-alloc: Prepare for resizable mmaps

[David Hildenbrand <david@redhat.com>]

On 06.02.20 00:00, Murilo Opsfelder Araújo wrote:
> Hello, David.
> 
> On Monday, February 3, 2020 3:31:20 PM -03 David Hildenbrand wrote:
>> When shrinking a mmap we want to re-reserve the already populated area.
>> When growing a memory region, we want to populate starting with a given
>> fd_offset. Prepare by allowing to pass these parameters.
>>
>> Also, let's make sure we always process full pages, to avoid
>> unmapping/remapping pages that are already in use when
>> growing/shrinking. (existing callers seem to guarantee this, but that's
>> not obvious)
>>
>> Cc: "Michael S. Tsirkin" <mst@redhat.com>
>> Cc: Greg Kurz <groug@kaod.org>
>> Cc: Murilo Opsfelder Araujo <muriloo@linux.ibm.com>
>> Cc: Eduardo Habkost <ehabkost@redhat.com>
>> Cc: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
>> ---
>>  util/mmap-alloc.c | 32 +++++++++++++++++++++-----------
>>  1 file changed, 21 insertions(+), 11 deletions(-)
>>
>> diff --git a/util/mmap-alloc.c b/util/mmap-alloc.c
>> index f043ccb0ab..63ad6893b7 100644
>> --- a/util/mmap-alloc.c
>> +++ b/util/mmap-alloc.c
>> @@ -83,12 +83,12 @@ size_t qemu_mempath_getpagesize(const char *mem_path)
>>  }
>>
>>  /*
>> - * Reserve a new memory region of the requested size to be used for mapping
>> - * from the given fd (if any).
>> + * Reserve a new memory region of the requested size or re-reserve parts
>> + * of an existing region to be used for mapping from the given fd (if any).
>> */
>> -static void *mmap_reserve(size_t size, int fd)
>> +static void *mmap_reserve(void *ptr, size_t size, int fd)
>>  {
>> -    int flags = MAP_PRIVATE;
>> +    int flags = MAP_PRIVATE | (ptr ? MAP_FIXED : 0);
>>
>>  #if defined(__powerpc64__) && defined(__linux__)
>>      /*
>> @@ -111,19 +111,23 @@ static void *mmap_reserve(size_t size, int fd)
>>      flags |= MAP_ANONYMOUS;
>>  #endif
>>
>> -    return mmap(0, size, PROT_NONE, flags, fd, 0);
>> +    return mmap(ptr, size, PROT_NONE, flags, fd, 0);
>>  }
>>
>>  /*
>>   * Populate memory in a reserved region from the given fd (if any).
>>   */
>> -static void *mmap_populate(void *ptr, size_t size, int fd, bool shared,
>> -                           bool is_pmem)
>> +static void *mmap_populate(void *ptr, size_t size, int fd, size_t
>> fd_offset, +                           bool shared, bool is_pmem)
>>  {
>>      int map_sync_flags = 0;
>>      int flags = MAP_FIXED;
>>      void *new_ptr;
>>
>> +    if (fd == -1) {
>> +        fd_offset = 0;
>> +    }
>> +
>>      flags |= fd == -1 ? MAP_ANONYMOUS : 0;
>>      flags |= shared ? MAP_SHARED : MAP_PRIVATE;
>>      if (shared && is_pmem) {
>> @@ -131,7 +135,7 @@ static void *mmap_populate(void *ptr, size_t size, int
>> fd, bool shared, }
>>
>>      new_ptr = mmap(ptr, size, PROT_READ | PROT_WRITE, flags |
>> map_sync_flags, -                   fd, 0);
>> +                   fd, fd_offset);
>>      if (new_ptr == MAP_FAILED && map_sync_flags) {
>>          if (errno == ENOTSUP) {
>>              char *proc_link = g_strdup_printf("/proc/self/fd/%d", fd);
>> @@ -153,7 +157,7 @@ static void *mmap_populate(void *ptr, size_t size, int
>> fd, bool shared, * If mmap failed with MAP_SHARED_VALIDATE | MAP_SYNC, we
>> will try * again without these flags to handle backwards compatibility. */
>> -        new_ptr = mmap(ptr, size, PROT_READ | PROT_WRITE, flags, fd, 0);
>> +        new_ptr = mmap(ptr, size, PROT_READ | PROT_WRITE, flags, fd,
>> fd_offset); }
>>      return new_ptr;
>>  }
>> @@ -178,13 +182,16 @@ void *qemu_ram_mmap(int fd,
>>      size_t offset, total;
>>      void *ptr, *guardptr;
>>
>> +    /* we can only map whole pages */
>> +    size = QEMU_ALIGN_UP(size, pagesize);
>> +
> 
> Caller already rounds up size to block->page_size.
> 
> Why this QEMU_ALIGN_UP is necessary?

Thanks for having a look

I guess you read the patch description, right? :)

"(existing callers seem to guarantee this, but that's
  not obvious)"

Do you prefer a g_assert(IS_ALIGNED()) instead?

Thanks!

-- 
Thanks,

David / dhildenb