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Date: Wed, 30 Mar 2022 18:03:36 -0400
From: Zi Yan <ziy@...dia.com>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
Steven Rostedt <rostedt@...dmis.org>,
LKML <linux-kernel@...r.kernel.org>,
Mel Gorman <mgorman@...hsingularity.net>,
David Hildenbrand <david@...hat.com>,
Mike Rapoport <rppt@...ux.ibm.com>,
Oscar Salvador <osalvador@...e.de>,
Andrew Morton <akpm@...ux-foundation.org>,
Linux-MM <linux-mm@...ck.org>
Subject: Re: [BUG] Crash on x86_32 for: mm: page_alloc: avoid merging non-fallbackable pageblocks with others
On 30 Mar 2022, at 17:43, Vlastimil Babka wrote:
> On 3/30/22 22:05, Linus Torvalds wrote:
>> On Wed, Mar 30, 2022 at 12:42 PM Steven Rostedt <rostedt@...dmis.org> wrote:
>>>
>>> I started testing new patches and it crashed when doing the x86-32 test on
>>> boot up.
>>>
>>> Initializing HighMem for node 0 (000375fe:0021ee00)
>>> BUG: kernel NULL pointer dereference, address: 00000878
>>> #PF: supervisor read access in kernel mode
>>> #PF: error_code(0x0000) - not-present page
>>> *pdpt = 0000000000000000 *pde = f0000000f000eef3
>>> Oops: 0000 [#1] PREEMPT SMP PTI
>>> CPU: 0 PID: 0 Comm: swapper Not tainted 5.17.0-test+ #469
>>> Hardware name: MSI MS-7823/CSM-H87M-G43 (MS-7823), BIOS V1.6 02/22/2014
>>> EIP: get_pfnblock_flags_mask+0x2c/0x36
>>> Code: 6d ea ff 55 89 e5 56 89 ce 53 8b 18 89 d8 c1 eb 1e e8 f7 fb ff ff 69 db c0 02 00 00 89 c1 89 c2 c1 ea 05 8b 83 7c d7 79 c1 5b <8b> 04 90 d3 e8 21 f0 5e 5d c3 55 89 e5 57 56 89 d6 53 89 c3 64 a1
>>
>> The whole function is in that Code: thing, and it decodes to:
>>
>> 0: 55 push %ebp
>> 1: 89 e5 mov %esp,%ebp
>> 3: 56 push %esi
>> 4: 89 ce mov %ecx,%esi
>> 6: 53 push %ebx
>> 7: 8b 18 mov (%eax),%ebx
>> 9: 89 d8 mov %ebx,%eax
>> b: c1 eb 1e shr $0x1e,%ebx
>> e: e8 f7 fb ff ff call 0xfffffc0a
>> 13: 69 db c0 02 00 00 imul $0x2c0,%ebx,%ebx
>> 19: 89 c1 mov %eax,%ecx
>> 1b: 89 c2 mov %eax,%edx
>> 1d: c1 ea 05 shr $0x5,%edx
>> 20: 8b 83 7c d7 79 c1 mov -0x3e862884(%ebx),%eax
>> 26: 5b pop %ebx
>> 27:* 8b 04 90 mov (%eax,%edx,4),%eax <-- trapping instruction
>> 2a: d3 e8 shr %cl,%eax
>> 2c: 21 f0 and %esi,%eax
>> 2e: 5e pop %esi
>> 2f: 5d pop %ebp
>> 30: c3 ret
>>
>> with '%eax' being NULL, and %edx being 0x21e.
>>
>> (The call seems to be to 'pfn_to_bitidx().isra.0' if my compiler does
>> similar code generation, so it's out-of-lined part of pfn_to_bitidx()
>> despite being marked inline)
>>
>> So that oops is that
>>
>> word = bitmap[word_bitidx];
>>
>> line, with 'bitmap' being NULL (and %edx contains 'word_bitidx').
>>
>> Looking around, your 'config-bad' doesn't even have
>> CONFIG_MEMORY_ISOLATION enabled, and so I suspect the culprit is this
>> part of the change:
>>
>> - if (unlikely(has_isolate_pageblock(zone))) {
>>
>> which used to always be false for that config, and now the code is
>> suddenly enabled.
>
> If CONFIG_MEMORY_ISOLATION was enabled then the zone layout would be the
> same, so I think it's not simply that. I think it's the timing -
> has_isolate_pageblock(zone) would only be possible to become true later
> in runtime when some isolation is ongoing, but here it seems we are
> still in the early boot. Probably at a boundary of highmem with another
> zone that doesn't have the pageblock bitmap yet initialized? While later
> it would have, and all would be fine.
It seems so based on the boot log from Steven.
>
> As Zi Yan said, the usual merging code will, through page_is_buddy()
> find safely enough the buddy is not applicable, so I agree with his
> patch direction. Seems this also shows the code tried to become too
> smart and for the next merge window we should try just move the
> migratetype checks into the main while loop (under something like "if
> (order >= max_order)") and simplify the function a lot, hopefully with
> negligible perf impact.
Something like this?
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 6c6af8658775..568ecaf5700d 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1053,7 +1053,6 @@ static inline void __free_one_page(struct page *page,
int migratetype, fpi_t fpi_flags)
{
struct capture_control *capc = task_capc(zone);
- unsigned int max_order = pageblock_order;
unsigned long buddy_pfn;
unsigned long combined_pfn;
struct page *buddy;
@@ -1069,8 +1068,7 @@ static inline void __free_one_page(struct page *page,
VM_BUG_ON_PAGE(pfn & ((1 << order) - 1), page);
VM_BUG_ON_PAGE(bad_range(zone, page), page);
-continue_merging:
- while (order < max_order) {
+ while (order < MAX_ORDER - 1) {
if (compaction_capture(capc, page, order, migratetype)) {
__mod_zone_freepage_state(zone, -(1 << order),
migratetype);
@@ -1081,6 +1079,27 @@ static inline void __free_one_page(struct page *page,
if (!page_is_buddy(page, buddy, order))
goto done_merging;
+
+ if (order >= pageblock_order) {
+ /* If we are here, it means order is >= pageblock_order.
+ * We want to prevent merge between freepages on pageblock
+ * without fallbacks and normal pageblock. Without this,
+ * pageblock isolation could cause incorrect freepage or CMA
+ * accounting or HIGHATOMIC accounting.
+ *
+ * We don't want to hit this code for the more frequent
+ * low-order merging.
+ */
+ int buddy_mt;
+
+ buddy_mt = get_pageblock_migratetype(buddy);
+
+ if (migratetype != buddy_mt
+ && (!migratetype_is_mergeable(migratetype) ||
+ !migratetype_is_mergeable(buddy_mt)))
+ goto done_merging;
+ }
+
/*
* Our buddy is free or it is CONFIG_DEBUG_PAGEALLOC guard page,
* merge with it and move up one order.
@@ -1094,32 +1113,6 @@ static inline void __free_one_page(struct page *page,
pfn = combined_pfn;
order++;
}
- if (order < MAX_ORDER - 1) {
- /* If we are here, it means order is >= pageblock_order.
- * We want to prevent merge between freepages on pageblock
- * without fallbacks and normal pageblock. Without this,
- * pageblock isolation could cause incorrect freepage or CMA
- * accounting or HIGHATOMIC accounting.
- *
- * We don't want to hit this code for the more frequent
- * low-order merging.
- */
- int buddy_mt;
-
- buddy_pfn = __find_buddy_pfn(pfn, order);
- buddy = page + (buddy_pfn - pfn);
-
- if (!page_is_buddy(page, buddy, order))
- goto done_merging;
- buddy_mt = get_pageblock_migratetype(buddy);
-
- if (migratetype != buddy_mt
- && (!migratetype_is_mergeable(migratetype) ||
- !migratetype_is_mergeable(buddy_mt)))
- goto done_merging;
- max_order = order + 1;
- goto continue_merging;
- }
done_merging:
set_buddy_order(page, order);
--
Best Regards,
Yan, Zi
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