[PATCH] [RFC]hugetlbfs: Get unmapped area below TASK_UNMAPPED_BASE for hugetlbfs

[Shijie Hu <hushijie3@huawei.com>]

In 32-bit programs, the address space is limited. When the normal mmap
consumes the space above TASK_UNMAPPED_BASE on legacy mode, it can still
successfully obtain unmapped area below TASK_UNMAPPED_BASE, but mmap or
shmat for huge pages will fail. This seems "not fair".

When the request for huge pages fails, fall back to reuse mmap_min_addr
~ TASK_UNMAPPED_BASE for hugetlbfs.

Signed-off-by: Shijie Hu <hushijie3@huawei.com>
---
 fs/hugetlbfs/inode.c | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
index aff8642f0c2e..0f5997394aaa 100644
--- a/fs/hugetlbfs/inode.c
+++ b/fs/hugetlbfs/inode.c
@@ -224,7 +224,21 @@ hugetlb_get_unmapped_area(struct file *file, unsigned long addr,
 	info.high_limit = TASK_SIZE;
 	info.align_mask = PAGE_MASK & ~huge_page_mask(h);
 	info.align_offset = 0;
-	return vm_unmapped_area(&info);
+	addr = vm_unmapped_area(&info);
+
+	/*
+	 * A failed request for huge pages very likely causes application
+	 * failure, so fall back to the top-down function here.
+	 */
+	if (unlikely(offset_in_page(addr))) {
+		VM_BUG_ON(addr != -ENOMEM);
+		info.flags = VM_UNMAPPED_AREA_TOPDOWN;
+		info.low_limit = max(PAGE_SIZE, mmap_min_addr);
+		info.high_limit = TASK_UNMAPPED_BASE;
+		addr = vm_unmapped_area(&info);
+	}
+
+	return addr;
 }
 #endif
 
-- 
2.12.3