net: add support for flex_array_resize in flex_array

Without the support for the total_nr_elements's growing or shrinking dynamically, flex_array is not that 'flexible'. Like when users want to change the size, they have to redo flex_array_alloc and copy all the elements from the old to the new one. The worse thing is every element's memory gets changed.

To implement flex_array_resize based on current code, the difficult thing is to process the size border of FLEX_ARRAY_BASE_BYTES_LEFT, where the base data memory may change to an array for the 2nd level data memory for growing, likewise for shrinking.

To make this part easier, we separate the base data memory and define FLEX_ARRAY_BASE_SIZE as a same value of FLEX_ARRAY_PART_SIZE, as Neil suggested. When new size is crossing the border, the base memory is allocated as the array for the 2nd level data memory and its part[0] is pointed to the old base memory, and do the opposite for shrinking.

But it doesn't do any memory allocation or shrinking for elements in flex_array_resize, as which should be done by flex_array_prealloc or flex_array_shrink called by users. No memory leaks can be caused by that.

SCTP has benefited a lot from flex_array_resize() for managing its stream memory so far.

v1->v2:
Cc LKML and more developers.

Xin Long (3):
flex_array: make FLEX_ARRAY_BASE_SIZE the same value of
FLEX_ARRAY_PART_SIZE
flex_array: support flex_array_resize
sctp: fa_resize sctp stream instead of redo fa_alloc

 include/linux/flex_array.h | 40 ++++++++++-----------
 lib/flex_array.c           | 73 ++++++++++++++++++++++++++++++++++++--
 net/sctp/stream.c          | 87 +++++++++++++++++++++-------------------------
 3 files changed, 130 insertions(+), 70 deletions(-)

2.1.0