Why does pages allocation with order of 10 or 11 using __get_free_pages() usually fail?

Question

My system memory is plenty (a server with 24GB). In my system, the kernel space is allocated with 320MB and 120MB for crash kernel. The rest of the memory is used for other purp

Answer 1

If I remember correctly, __get_free_pages uses buddy allocation, which not only does scatter its allocations throughout physical memory, it does so in the worst possible pattern for subsequent attempts to allocate large contiguous blocks. If my calculations are correct, on your system with 24GB of physical RAM, even if no space whatsoever were occupied by anything but buddy allocations, it would take less than 8192 order-0 (4KB) allocations to render allocation of a 4MB chunk with __get_free_pages impossible.

There is a thing called the contiguous memory allocator, which is supposed to address the genuine need for large physically-contiguous allocations by device drivers; as of June 2011 it was not in the official kernel, but that was more than a year ago now. You should look into that.