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

[David Hildenbrand <david@redhat.com>]

On 06.02.20 13:31, Murilo Opsfelder Araújo wrote:
> Hello, David.
> 
> On Thursday, February 6, 2020 5:52:26 AM -03 David Hildenbrand wrote:
>> 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?
> 
> I guess you mean QEMU_IS_ALIGNED().  But yes, I think we could just check
> alignments here, so callers do the alignments first.

Yeh, you got the idea :)

I'll convert these to asserts for now. Thanks!

-- 
Thanks,

David / dhildenb