x86: allow extend_brk users to reserve brk space

Impact: new interface; remove hard-coded limit

Add RESERVE_BRK(name, size) macro to reserve space in the brk
area.  This should be a conservative (ie, larger) estimate of
how much space might possibly be required from the brk area.
Any unused space will be freed, so there's no real downside
on making the reservation too large (within limits).

The name should be unique within a given file, and somewhat
descriptive.

The C definition of RESERVE_BRK() ends up being more complex than
one would expect to work around a cluster of gcc infelicities:

  The first attempt was to simply try putting __section(.brk_reservation)
  on a variable.  This doesn't work because it ends up making it a
  @progbits section, which gets actual space allocated in the vmlinux
  executable.

  The second attempt was to emit the space into a section using asm,
  but gcc doesn't allow arguments to be passed to file-level asm()
  statements, making it hard to pass in the size.

  The final attempt is to wrap the asm() in a function to allow
  it to have arguments, and put the function itself into the
  .discard section, which vmlinux*.lds drops entirely from the
  emitted vmlinux.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>

4 files changed, 10 insertions, 2 deletions

diff --git a/arch/x86/kernel/head_32.S b/arch/x86/kernel/head_32.S
index 3ce5456dfbe6..9e89f2a14b90 100644
--- a/arch/x86/kernel/head_32.S
+++ b/arch/x86/kernel/head_32.S
@@ -75,6 +75,8 @@ ALLOCATOR_SLOP = 4
 
 INIT_MAP_BEYOND_END = BOOTBITMAP_SIZE + (PAGE_TABLE_SIZE + ALLOCATOR_SLOP)*PAGE_SIZE_asm
 
+RESERVE_BRK(pagetables, PAGE_TABLE_SIZE * PAGE_SIZE)
+
 /*
  * 32-bit kernel entrypoint; only used by the boot CPU.  On entry,
  * %esi points to the real-mode code as a 32-bit pointer.
diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
index e894f36335f2..a0d26237d7cf 100644
--- a/arch/x86/kernel/setup.c
+++ b/arch/x86/kernel/setup.c
@@ -112,6 +112,8 @@
 #define ARCH_SETUP
 #endif
 
+RESERVE_BRK(dmi_alloc, 65536);
+
 unsigned int boot_cpu_id __read_mostly;
 
 static __initdata unsigned long _brk_start = (unsigned long)__brk_base;
diff --git a/arch/x86/kernel/vmlinux_32.lds.S b/arch/x86/kernel/vmlinux_32.lds.S
index 1063fbe93c73..a1f28b85fb34 100644
--- a/arch/x86/kernel/vmlinux_32.lds.S
+++ b/arch/x86/kernel/vmlinux_32.lds.S
@@ -192,7 +192,8 @@ SECTIONS
 
         . = ALIGN(PAGE_SIZE);
         __brk_base = . ;
-        . += 1024 * 1024 ;
+        . += 64 * 1024 ;        /* 64k slop space */
+        *(.brk_reservation)     /* areas brk users have reserved */
         __brk_limit = . ;
 
         _end = . ;
@@ -201,6 +202,7 @@ SECTIONS
   /* Sections to be discarded */
   /DISCARD/ : {
         *(.exitcall.exit)
+        *(.discard)
         }
 
   STABS_DEBUG
diff --git a/arch/x86/kernel/vmlinux_64.lds.S b/arch/x86/kernel/vmlinux_64.lds.S
index ff373423138c..7996687663a2 100644
--- a/arch/x86/kernel/vmlinux_64.lds.S
+++ b/arch/x86/kernel/vmlinux_64.lds.S
@@ -250,7 +250,8 @@ SECTIONS
 
         . = ALIGN(PAGE_SIZE);
         __brk_base = . ;
-        . += 1024 * 1024 ;
+        . += 64 * 1024;         /* 64k slop space */
+        *(.brk_reservation)     /* areas brk users have reserved */
         __brk_limit = . ;
   }
 
@@ -260,6 +261,7 @@ SECTIONS
   /DISCARD/ : {
         *(.exitcall.exit)
         *(.eh_frame)
+        *(.discard)
         }
 
   STABS_DEBUG