From: David Hildenbrand <david@redhat.com>
To: Miaohe Lin <linmiaohe@huawei.com>, akpm@linux-foundation.org
Cc: vbabka@suse.cz, iamjoonsoo.kim@lge.com, linux-mm@kvack.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm/page_isolation: don't putback unisolated page
Date: Mon, 6 Sep 2021 14:49:33 +0200 [thread overview]
Message-ID: <c60dc5e2-6f19-3be8-56be-555033cc9ca4@redhat.com> (raw)
In-Reply-To: <b0a2947b-360a-40c2-03e4-f0f67845f4c3@huawei.com>
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@huawei.com>
>>>> ---
>>>> 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@redhat.com>
>>>
>>
>> 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);
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.
--
Thanks,
David / dhildenb
next prev parent reply other threads:[~2021-09-06 12:49 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-09-04 9:18 [PATCH] mm/page_isolation: don't putback unisolated page Miaohe Lin
2021-09-06 9:40 ` David Hildenbrand
2021-09-06 11:32 ` Miaohe Lin
2021-09-06 11:50 ` David Hildenbrand
2021-09-06 12:01 ` David Hildenbrand
2021-09-06 12:08 ` Miaohe Lin
2021-09-06 12:02 ` David Hildenbrand
2021-09-06 12:11 ` David Hildenbrand
2021-09-06 12:45 ` Miaohe Lin
2021-09-06 12:49 ` David Hildenbrand [this message]
2021-09-07 1:46 ` Miaohe Lin
2021-09-07 9:52 ` David Hildenbrand
2021-09-07 8:08 ` Vlastimil Babka
2021-09-07 9:56 ` David Hildenbrand
2021-09-08 22:42 ` John Hubbard
2021-09-09 8:56 ` David Hildenbrand
2021-09-09 9:07 ` Vlastimil Babka
2021-09-09 9:37 ` David Hildenbrand
2021-09-09 16:50 ` John Hubbard
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