Re: [RFC PATCH 0/3] Add mmap(MAP_CONTIG) support

[Anshuman Khandual]

On 10/12/2017 07:16 AM, Mike Kravetz wrote:
> The following is a 'possible' way to add such functionality.  I just
> did what was easy and pre-allocated contiguous pages which are used
> to populate the mapping.  I did not use any of the higher order
> allocators such as alloc_contig_range.  Therefore, it is limited to

Just tried with a small prototype with an implementation similar to that
of alloc_gigantic_page() where we scan the zones (applicable zonelist)
for contiguous valid PFN range and try allocating with alloc_contig_range.
Will share it soon.

> allocations of MAX_ORDER size.  Also, the allocations should probably

Just did a quick test and it worked till 1UL << (MAX_ORDER - 1) numbers
of pages on a POWER system with the current RFC patches. As the pages
are allocated during VMA creation time, comparison to normal page fault
speed while accessing the buffer wont be fair.

> be done outside mmap_sem but that was the easiest place to do it in
> this quick and easy POC.

Why it should be done outside the mmap_sem, because it can take some
time ? But then VMA can just go away while we are allocating the big
chunks of pages (if we dont hold mmap_sem).