Use address hint in mmap for search

PATCH New Andi Kleen 1 participant 1 message Jan 28, 2004

Subsystems (2)

Memory management Memory mapping

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Andi Kleen

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2004-01-28 16:34 UTC #0 lore raw

PATCH 1/1 Use address hint in mmap for search

When the user gave an address hint in mmap use it as starting point for the search for !MAP_FIXED. Currently it is only checked directly and when already used the free area cache is used as starting point. With this change you can use mmap(4096, ....) to e.g. get the lowest free address in your address space, which is sometimes useful. For example on x86-64 glibc wants to preferably allocate thread local data in the first 4GB but use higher addresses when this is not possible.

This can be a bit more costly in CPU time because it may have to skip over more VMAs, but gives better semantics for most cases. Most programs pass NULL as hint anyways so it won't make any difference for them.

I did it for the generic mmap and for x86-64 for now. Also minor white space fixes for x86-64.

-Andi

arch/x86_64/kernel/sys_x86_64.c renamed from arch/x86_64/kernel/sys_x86_64.c-o +3 -3

diff -u linux-2.6.2rc2-amd64/arch/x86_64/kernel/sys_x86_64.c-o linux-2.6.2rc2-amd64/arch/x86_64/kernel/sys_x86_64.c
@@ -105,13 +105,13 @@
 		return -ENOMEM;
 
 	if (addr) {
-	addr = PAGE_ALIGN(addr);
+		addr = PAGE_ALIGN(addr);
 		vma = find_vma(mm, addr);
 		if (end - len >= addr &&
 		    (!vma || addr + len <= vma->vm_start))
 			return addr;
-	}
-	addr = mm->free_area_cache;
+	} else
+		addr = mm->free_area_cache;
 	if (addr < begin) 
 		addr = begin; 
 	start_addr = addr;

mm/mmap.c renamed from mm/mmap.c-o +3 -2

diff -u linux-2.6.2rc2-amd64/mm/mmap.c-o linux-2.6.2rc2-amd64/mm/mmap.c
@@ -743,8 +743,9 @@
 		if (TASK_SIZE - len >= addr &&
 		    (!vma || addr + len <= vma->vm_start))
 			return addr;
-	}
-	start_addr = addr = mm->free_area_cache;
+	} else
+		addr = mm->free_area_cache;
+	start_addr = addr;
 
 full_search:
 	for (vma = find_vma(mm, addr); ; vma = vma->vm_next) {