Re: [PATCH] x86/mm/init: respect memblock reserved regions whendestroying mappings
From: Yinghai Lu
Date: Sun Feb 06 2011 - 02:03:01 EST
On Fri, Feb 4, 2011 at 5:18 PM, Jeremy Fitzhardinge <jeremy@xxxxxxxx> wrote:
> On 02/04/2011 03:35 AM, Stefano Stabellini wrote:
>> On Thu, 3 Feb 2011, H. Peter Anvin wrote:
>>> On 02/03/2011 03:25 AM, Stefano Stabellini wrote:
>>>>> How on Earth would you end up with a reserved region *inside the BRK*?
>>>> I think in practice you cannot, but you can have reserved regions at
>>>> _end, that is the main problem I am trying to solve.
>>>> If we have a reserved region at _end and _end is not PMD aligned, then
>>>> we have a problem.
>>>>
>>>> I thought that checking for reserved regions before destroying the
>>>> mapping would be a decent solution (because it wouldn't affect the
>>>> normal case); so I ended up checking between _brk_end and _end too.
>>>>
>>>> Other alternative solutions I thought about but that I discarded because
>>>> they also affect the normal case are:
>>>>
>>>> - never destroy mappings that could go over _end;
>>>> - always PMD align _end.
>>>>
>>>> If none of the above are acceptable, I welcome other suggestions :-)
>>>>
>>> Sounds like the code does the right thing, but the description needs to
>>> be improved.
>>>
>> I tried to improve both the commit message and the comments within the
>> code, this is the result:
>>
>>
>> commit d0136be7b48953f27202dbde285a7379d06cfe98
>> Author: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
>> Date: Tue Jan 25 12:05:11 2011 +0000
>>
>> x86/mm/init: respect memblock reserved regions when destroying mappings
>>
>> In init_memory_mapping we destroy the mappings between _brk_end and
>> _end, but if _end is not PMD aligned we also destroy mappings for
>> potentially reserved regions between _end and the following PMD.
>>
>> In order to avoid this problem, before clearing any PMDs we check if the
>> corresponding memory area has been reserved and we only destroy the
>> mapping if hasn't.
>>
>> We found this issue because under Xen we have a valid mapping at _end,
>> and if _end is not PMD aligned the current code destroys the initial
>> part of it.
>
> This description makes more sense, even if the code does exactly the
> same thing.
>
>>
>> In practice this fix does not have any impact on native.
>>
>> Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
>>
>> diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c
>> index 947f42a..65c34f4 100644
>> --- a/arch/x86/mm/init.c
>> +++ b/arch/x86/mm/init.c
>> @@ -283,6 +283,8 @@ unsigned long __init_refok init_memory_mapping(unsigned long start,
>> if (!after_bootmem && !start) {
>> pud_t *pud;
>> pmd_t *pmd;
>> + unsigned long addr;
>> + u64 size, memblock_addr;
>>
>> mmu_cr4_features = read_cr4();
>>
>> @@ -291,11 +293,22 @@ unsigned long __init_refok init_memory_mapping(unsigned long start,
>> * located on different 2M pages. cleanup_highmap(), however,
>> * can only consider _end when it runs, so destroy any
>> * mappings beyond _brk_end here.
>> + *
>> + * If _end is not PMD aligned, we also destroy the mapping of
>> + * the memory area between _end the next PMD, so before clearing
>> + * the PMD we make sure that the corresponding memory region has
>> + * not been reserved.
>> */
>> pud = pud_offset(pgd_offset_k(_brk_end), _brk_end);
>> pmd = pmd_offset(pud, _brk_end - 1);
>> - while (++pmd <= pmd_offset(pud, (unsigned long)_end - 1))
>> - pmd_clear(pmd);
>> + addr = (_brk_end + PMD_SIZE - 1) & PMD_MASK;
>
> I guess its OK if this is >_end, because the pmd offset will be greater
> than _end's. But is there an edge condition if the pmd_offset goes off
> the end of the pud, and pud page itself happens to be at the end of the
> address space and it wraps?
>
>> + while (++pmd <= pmd_offset(pud, (unsigned long)_end - 1)) {
> Could _end be in a different pud from _brk_end? Could this walk off the
> pud page?
>
> Or is it moot, and there's a guarantee that the whole space is mapped
> out of the same pud page? I guess the original code has the same
> concern, so this patch leaves the status quo unchanged.
>
> J
>
>
>> + memblock_addr = memblock_x86_find_in_range_size(__pa(addr),
>> + &size, PMD_SIZE);
>> + if (memblock_addr == (u64) __pa(addr) && size >= PMD_SIZE)
>> + pmd_clear(pmd);
>> + addr += PMD_SIZE;
>> + }
>> }
>> #endif
>> __flush_tlb_all();
why not just move calling cleanup_highmap down?
something like attached patch.
---
arch/x86/include/asm/pgtable_64.h | 2 +-
arch/x86/kernel/head64.c | 3 ---
arch/x86/kernel/setup.c | 6 ++++++
arch/x86/mm/init.c | 19 -------------------
arch/x86/mm/init_64.c | 5 +++--
5 files changed, 10 insertions(+), 25 deletions(-)
Index: linux-2.6/arch/x86/include/asm/pgtable_64.h
===================================================================
--- linux-2.6.orig/arch/x86/include/asm/pgtable_64.h
+++ linux-2.6/arch/x86/include/asm/pgtable_64.h
@@ -165,7 +165,7 @@ static inline int pgd_large(pgd_t pgd) {
#define __swp_entry_to_pte(x) ((pte_t) { .pte = (x).val })
extern int kern_addr_valid(unsigned long addr);
-extern void cleanup_highmap(void);
+extern void cleanup_highmap(unsigned long end);
#define HAVE_ARCH_UNMAPPED_AREA
#define HAVE_ARCH_UNMAPPED_AREA_TOPDOWN
Index: linux-2.6/arch/x86/kernel/head64.c
===================================================================
--- linux-2.6.orig/arch/x86/kernel/head64.c
+++ linux-2.6/arch/x86/kernel/head64.c
@@ -77,9 +77,6 @@ void __init x86_64_start_kernel(char * r
/* Make NULL pointers segfault */
zap_identity_mappings();
- /* Cleanup the over mapped high alias */
- cleanup_highmap();
-
max_pfn_mapped = KERNEL_IMAGE_SIZE >> PAGE_SHIFT;
for (i = 0; i < NUM_EXCEPTION_VECTORS; i++) {
Index: linux-2.6/arch/x86/kernel/setup.c
===================================================================
--- linux-2.6.orig/arch/x86/kernel/setup.c
+++ linux-2.6/arch/x86/kernel/setup.c
@@ -297,6 +297,9 @@ static void __init init_gbpages(void)
static inline void init_gbpages(void)
{
}
+static void __init cleanup_highmap(unsigned long end)
+{
+}
#endif
static void __init reserve_brk(void)
@@ -922,6 +925,9 @@ void __init setup_arch(char **cmdline_p)
*/
reserve_brk();
+ /* Cleanup the over mapped high alias after _brk_end*/
+ cleanup_highmap(_brk_end);
+
memblock.current_limit = get_max_mapped();
memblock_x86_fill();
Index: linux-2.6/arch/x86/mm/init.c
===================================================================
--- linux-2.6.orig/arch/x86/mm/init.c
+++ linux-2.6/arch/x86/mm/init.c
@@ -279,25 +279,6 @@ unsigned long __init_refok init_memory_m
load_cr3(swapper_pg_dir);
#endif
-#ifdef CONFIG_X86_64
- if (!after_bootmem && !start) {
- pud_t *pud;
- pmd_t *pmd;
-
- mmu_cr4_features = read_cr4();
-
- /*
- * _brk_end cannot change anymore, but it and _end may be
- * located on different 2M pages. cleanup_highmap(), however,
- * can only consider _end when it runs, so destroy any
- * mappings beyond _brk_end here.
- */
- pud = pud_offset(pgd_offset_k(_brk_end), _brk_end);
- pmd = pmd_offset(pud, _brk_end - 1);
- while (++pmd <= pmd_offset(pud, (unsigned long)_end - 1))
- pmd_clear(pmd);
- }
-#endif
__flush_tlb_all();
if (!after_bootmem && e820_table_end > e820_table_start)
Index: linux-2.6/arch/x86/mm/init_64.c
===================================================================
--- linux-2.6.orig/arch/x86/mm/init_64.c
+++ linux-2.6/arch/x86/mm/init_64.c
@@ -297,13 +297,14 @@ void __init init_extra_mapping_uc(unsign
* rounded up to the 2MB boundary. This catches the invalid pmds as
* well, as they are located before _text:
*/
-void __init cleanup_highmap(void)
+void __init cleanup_highmap(unsigned long end)
{
unsigned long vaddr = __START_KERNEL_map;
- unsigned long end = roundup((unsigned long)_end, PMD_SIZE) - 1;
pmd_t *pmd = level2_kernel_pgt;
pmd_t *last_pmd = pmd + PTRS_PER_PMD;
+ end = roundup(end, PMD_SIZE) - 1;
+
for (; pmd < last_pmd; pmd++, vaddr += PMD_SIZE) {
if (pmd_none(*pmd))
continue;