Messages in this thread | | | Date | Mon, 15 Sep 2008 09:17:23 +0100 | From | "Jan Beulich" <> | Subject | Re: [PATCH] x86-64: fix combining of regions in init_memory_mapping() |
| |
>>> "Yinghai Lu" <yhlu.kernel@gmail.com> 14.09.08 20:20 >>> >On Fri, Sep 12, 2008 at 7:43 AM, Jan Beulich <jbeulich@novell.com> wrote: >> When nr_range gets decremented, the same slot must be considered for >> coalescing with its new successor again. >> >> Signed-off-by: Jan Beulich <jbeulich@novell.com> >> >> --- >> arch/x86/mm/init_64.c | 2 +- >> 1 file changed, 1 insertion(+), 1 deletion(-) >> >> --- linux-2.6.27-rc6/arch/x86/mm/init_64.c 2008-08-29 10:53:00.000000000 +0200 >> +++ 2.6.27-rc6-x86_64-mr-coalesce/arch/x86/mm/init_64.c 2008-09-12 11:58:45.000000000 +0200 >> @@ -636,7 +636,7 @@ unsigned long __init_refok init_memory_m >> old_start = mr[i].start; >> memmove(&mr[i], &mr[i+1], >> (nr_range - 1 - i) * sizeof (struct map_range)); >> - mr[i].start = old_start; >> + mr[i--].start = old_start; >> nr_range--; >> } >> > >this patch seems not right. >Ingo, please don't apply it. > >original code: > /* try to merge same page size and continuous */ > for (i = 0; nr_range > 1 && i < nr_range - 1; i++) { > unsigned long old_start; > if (mr[i].end != mr[i+1].start || > mr[i].page_size_mask != mr[i+1].page_size_mask) > continue; > /* move it */ > old_start = mr[i].start; > memmove(&mr[i], &mr[i+1], > (nr_range - 1 - i) * sizeof (struct map_range)); > mr[i].start = old_start; > nr_range--; > } > >so it save old_start and first, and move entries forward (so old one >is overwriten), and put back old_start ...
Old and new code are not different in any way in this respect - both overwrite the old entry at index i with the entry at index i+1 and then set the start of the i-th entry back to what it was before the overwrite, effectively combining them. The patch just makes sure that the index isn't being updated at the same time as nr_range (because if you update both you effectively skip one).
The issue is apparently pretty benign to native code, but surfaces as a boot time crash in our forward ported Xen tree (where the page table setup overall works differently than in native). Since the underlying issue was present in native (and since I assume if there is an attempt to merge subsequent regions, then it should work right), I nevertheless submitted the patch for native inclusion.
Jan
| |