mm: thp_get_unmapped_area must honour topdown preference

The addition of commit efa7df3e ("mm: align larger anonymous
mappings on THP boundaries") caused the "virtual_address_range" mm
selftest to start failing on arm64. Let's fix that regression.

There were 2 visible problems when running the test; 1) it takes much
longer to execute, and 2) the test fails. Both are related:

The (first part of the) test allocates as many 1GB anonymous blocks as
it can in the low 256TB of address space, passing NULL as the addr hint
to mmap. Before the faulty patch, all allocations were abutted and
contained in a single, merged VMA. However, after this patch, each
allocation is in its own VMA, and there is a 2M gap between each VMA.
This causes the 2 problems in the test: 1) mmap becomes MUCH slower
because there are so many VMAs to check to find a new 1G gap. 2) mmap
fails once it hits the VMA limit (/proc/sys/vm/max_map_count). Hitting
this limit then causes a subsequent calloc() to fail, which causes the
test to fail.

The problem is that arm64 (unlike x86) selects
ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT. But __thp_get_unmapped_area()
allocates len+2M then always aligns to the bottom of the discovered gap.
That causes the 2M hole.

Fix this by detecting cases where we can still achive the alignment goal
when moved to the top of the allocated area, if configured to prefer
top-down allocation.

While we are at it, fix thp_get_unmapped_area's use of pgoff, which
should always be zero for anonymous mappings. Prior to the faulty
change, while it was possible for user space to pass in pgoff!=0, the
old mm->get_unmapped_area() handler would not use it.
thp_get_unmapped_area() does use it, so let's explicitly zero it before
calling the handler. This should also be the correct behavior for arches
that define their own get_unmapped_area() handler.

Fixes: efa7df3e ("mm: align larger anonymous mappings on THP boundaries")
Closes: https://lore.kernel.org/linux-mm/1e8f5ac7-54ce-433a-ae53-81522b2320e1@arm.com/


Cc: stable@vger.kernel.org
Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>