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Subject[2.4.0-pre3] [prePATCH] mmap grows downward on i386

As suggested in the thread "Subtle MM bug (really 830MB barrier
question)", it would be beneficial on 32-bit hardware for mmap()
allocations without MAP_FIXED to grow downward from TASK_SIZE, rather than
upwards from TASK_UNMAPPED_BASE. This would allow both brk()-using and
mmap()-using programs to access almost 3GB (TASK_SIZE - some overhead).

So here is my first ever attempt at a kernel patch. It compiles and boots
and seems to work OK. Any comments would be very welcome, but please
excuse my ignorance of many things. :-)

A few comments:

(1) I have no idea how much room to allow for the stack on i386, so I
arbitrarily picked 128MB.

(2) I also have no idea what happens on architectures other than i386, so
I defined TASK_UNMAPPED_CEILING in include/asm-i386/processor.h. Then in
mm/mmap.c, if TASK_UNMAPPED_CEILING is undefined, get_unmapped_area()
behaves as before. I used #ifdef to do this, is that an OK style?

(3) The (original) search upwards version of get_unmapped_area() only
calls find_vma() once, and then it uses ->vm_next in the main loop to get
the next vm_area_struct. My search downwards version calls find_vma()
once every loop, as there is no ->vm_previous member in vm_area_struct.
Is the overhead of calling find_vma() every loop a problem, and if so, is
there a better solution than modifying vm_area_struct to make it a doubly
linked list?

(4) Lastly, there is the question of when get_unmapped_area() should just
fail. I notice that executables on i386 are loaded at 128MB, so I defined
TASK_UNMAPPED_FLOOR to be 128MB and I stop there. Is this the right place
to stop, and if so is there some other #define I should be referencing
instead of adding one?

Cheers,
Wayne


diff -ru linux-2.4.0-pre3/include/asm-i386/processor.h linux-2.4.0-pre3-hack/include/asm-i386/processor.h
--- linux-2.4.0-pre3/include/asm-i386/processor.h Thu Jan 11 12:56:05 2001
+++ linux-2.4.0-pre3-hack/include/asm-i386/processor.h Fri Jan 12 13:51:16 2001
@@ -260,10 +260,22 @@
*/
#define TASK_SIZE (PAGE_OFFSET)

-/* This decides where the kernel will search for a free chunk of vm
- * space during mmap's.
+/*
+ * When looking for a free chunk of vm space during mmap's, the kernel
+ * will search upwards from TASK_UNMAPPED_BASE (the usual algorithm),
+ * unless TASK_UNMAPPED_CEILING is defined, in which case it will
+ * search downwards from TASK_UNMAPPED_CEILING to TASK_UNMAPPED_FLOOR.
*/
#define TASK_UNMAPPED_BASE (TASK_SIZE / 3)
+
+/*
+ * We need to allow room for the stack to grow downard from TASK_SIZE,
+ * I really have no idea how large it can get, so I arbitrarily picked
+ * 128MB. Also, I'm not so sure where to stop searching and give up,
+ * so I pick 128MB, which seems to be where exectuables get loaded.
+ */
+#define TASK_UNMAPPED_CEILING (TASK_SIZE - 128 * 1024 * 1024)
+#define TASK_UNMAPPED_FLOOR (128 * 1024 * 1024)

/*
* Size of io_bitmap in longwords: 32 is ports 0-0x3ff.
diff -ru linux-2.4.0-pre3/mm/mmap.c linux-2.4.0-pre3-hack/mm/mmap.c
--- linux-2.4.0-pre3/mm/mmap.c Sat Dec 30 12:35:19 2000
+++ linux-2.4.0-pre3-hack/mm/mmap.c Fri Jan 12 13:54:02 2001
@@ -382,6 +382,22 @@

if (len > TASK_SIZE)
return 0;
+#ifdef TASK_UNMAPPED_CEILING
+ if (!addr)
+ addr = TASK_UNMAPPED_CEILING - len;
+
+ do {
+ /* align addr downards; PAGE_ALIGN aligns it upwards */
+ addr = addr&PAGE_MASK;
+ vmm = find_vma(current->mm,addr);
+ /* At this point: (!vmm || addr < vmm->vm_end). */
+ if (!vmm || addr + len <= vmm->vm_start)
+ return addr;
+ addr = vmm->vm_start - len;
+ } while (addr >= TASK_UNMAPPED_FLOOR);
+
+ return 0;
+#else
if (!addr)
addr = TASK_UNMAPPED_BASE;
addr = PAGE_ALIGN(addr);
@@ -394,6 +410,7 @@
return addr;
addr = vmm->vm_end;
}
+#endif
}
#endif

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