On 2021/9/6 20:49, David Hildenbrand wrote:
> On 06.09.21 14:45, Miaohe Lin wrote:
>> On 2021/9/6 20:11, David Hildenbrand wrote:
>>> On 06.09.21 14:02, David Hildenbrand wrote:
>>>> On 04.09.21 11:18, Miaohe Lin wrote:
>>>>> If __isolate_free_page() failed, due to zone watermark check, the page is
>>>>> still on the free list. But this page will be put back to free list again
>>>>> via __putback_isolated_page() now. This may trigger page->flags checks in
>>>>> __free_one_page() if PageReported is set. Or we will corrupt the free list
>>>>> because list_add() will be called for pages already on another list.
>>>>>
>>>>> Fixes: 3c605096d315 ("mm/page_alloc: restrict max order of merging on isolated pageblock")
>>>>> Signed-off-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>
>>>>> ---
>>>>> mm/page_isolation.c | 6 ++----
>>>>> 1 file changed, 2 insertions(+), 4 deletions(-)
>>>>>
>>>>> diff --git a/mm/page_isolation.c b/mm/page_isolation.c
>>>>> index 9bb562d5d194..7d70d772525c 100644
>>>>> --- a/mm/page_isolation.c
>>>>> +++ b/mm/page_isolation.c
>>>>> @@ -93,10 +93,8 @@ static void unset_migratetype_isolate(struct page *page, unsigned migratetype)
>>>>> buddy_pfn = __find_buddy_pfn(pfn, order);
>>>>> buddy = page + (buddy_pfn - pfn);
>>>>> - if (!is_migrate_isolate_page(buddy)) {
>>>>> - __isolate_free_page(page, order);
>>>>> - isolated_page = true;
>>>>> - }
>>>>> + if (!is_migrate_isolate_page(buddy))
>>>>> + isolated_page = !!__isolate_free_page(page, order);
>>>>> }
>>>>> }
>>>>>
>>>>
>>>> Thanks!
>>>>
>>>> Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>
>>>>
>>>
>>> To make the confusion perfect (sorry) :D I tripple-checked:
>>>
>>> In unset_migratetype_isolate() we check that is_migrate_isolate_page(page) holds, otherwise we return.
>>>
>>> We call __isolate_free_page() only for such pages.
>>>
>>> __isolate_free_page() won't perform watermark checks on is_migrate_isolate().
>>>
>>> Consequently, __isolate_free_page() should never fail when called from unset_migratetype_isolate()
>>>
>>> If that's correct then we could instead maybe add a VM_BUG_ON() and a comment why this can't fail.
>>>
>>>
>>> Makes sense or am I missing something?
>>
>> I think you're right. __isolate_free_page() should never fail when called from unset_migratetype_isolate()
>> as explained by you. But it might be too fragile to reply on the failure conditions of __isolate_free_page().
>> If that changes, VM_BUG_ON() here might trigger unexpectedly. Or am I just over-worried as failure conditions
>> of __isolate_free_page() can hardly change?
>
> Maybe
>
> isolated_page = !!__isolate_free_page(page, order);
> /*
> * Isolating a free page in an isolated pageblock is expected to always
> * work as watermarks don't apply here.
> */
> VM_BUG_ON(isolated_page);
Should this be VM_BUG_ON(!isolated_page) ?
>
>
> VM_BUG_ON() allows us to detect any issues when testing. Combined with the comment it tells everybody messing with __isolate_free_page() what we expect in this function.
>
> In production system, we would handle it gracefully.
>
Sounds reasonable. Will do it in v2. Many thanks for your suggestion and effort!
