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SubjectPatch: fix munmap on ARM
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Linus, lkml,

The following patch is required so that munmap(0x8000, *) does not cause
ARM kernels to hang. The problem is that the machine vectors are at
virtual address 0. Unfortunately, when free_pgtables() is called, it
clears first level page tables starting at 0 in this case, and takes
out the machine vectors. This then results in an unrecoverable hang.

We already have FIRST_USER_PGD_NR to define the first entry in the pgd
which may be cleared, so we use this to clamp "start_index"
appropriately. The following patch does this. Tested on ARM.

There is only one concern that I can see with this patch - GCC may warn
about comparison always zero on architectures that define
FIRST_USER_PGD_NR to be zero.

--- orig/mm/mmap.c Tue Sep 5 22:22:12 2000
+++ linux/mm/mmap.c Sat Sep 30 14:24:23 2000
@@ -620,6 +620,8 @@
* old method of shifting the VA >> by PGDIR_SHIFT doesn't work.
*/
start_index = pgd_index(first);
+ if (start_index < FIRST_USER_PGD_NR)
+ start_index = FIRST_USER_PGD_NR;
end_index = pgd_index(last);
if (end_index > start_index) {
clear_page_tables(mm, start_index, end_index - start_index);
_____
|_____| ------------------------------------------------- ---+---+-
| | Russell King rmk@arm.linux.org.uk --- ---
| | | | http://www.arm.linux.org.uk/personal/aboutme.html / / |
| +-+-+ --- -+-
/ | THE developer of ARM Linux |+| /|\
/ | | | --- |
+-+-+ ------------------------------------------------- /\\\ |
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