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[Qemu-devel] [patch] fix mmap 0.9.1 mmap regression in linux-user

Subject: [Qemu-devel] [patch] fix mmap 0.9.1 mmap regression in linux-user
From: Vince Weaver
Date: Thu, 17 Jan 2008 13:35:07 -0500 EST

The 0.9.1 release segfaults on certain benchmarks under linux-user
emulation (x86 on x86).

Specifically "facerec" from the SPEC2000 benchmarks, and over half of the
SPEC2006 benchmarks.

I tracked this down to a problem in the mmap() code in linux-user.

I've attached a patch that reverts behavior back to that from 0.9.0, which
makes the benchmarks all run for me.

The problem is that after a large number of mmap() calls, the
mmap_find_vma() fuction sometimes completely destroys the memory map
table.  I can produce debug info on this if needed.  What the patch does
is make the code ignore the results of mmap_find_vma().

I'm not familiar enough with the qemu mmap() code to fix this properly...



--- ./qemu-0.9.1-stock/linux-user/mmap.c        2008-01-06 14:38:42.000000000 
+++ ./qemu-0.9.1/linux-user/mmap.c      2008-01-17 13:27:11.000000000 -0500
@@ -250,8 +250,8 @@
         /* Note: we prefer to control the mapping address. It is
            especially important if qemu_host_page_size >
            qemu_real_host_page_size */
-        p = mmap(g2h(mmap_start),
-                 host_len, prot, flags | MAP_FIXED, fd, host_offset);
+        p = mmap(real_start ? g2h(real_start) : NULL,
+                 host_len, prot, flags, fd, host_offset);
         if (p == MAP_FAILED)
             return -1;
         /* update start so that it points to the file position at 'offset' */

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