Re: [PATCH v3 2/3] mm/compaction: add support for >0 order folio memory compaction.

[Vlastimil Babka <vbabka@suse.cz>]

On 2/9/24 20:40, Zi Yan wrote:
> On 9 Feb 2024, at 14:36, Zi Yan wrote:
> 
>> On 9 Feb 2024, at 11:37, Vlastimil Babka wrote:
>>
>>> On 2/2/24 17:15, Zi Yan wrote:
>>>
>>> ...
>>>
>>>>  /*
>>>> @@ -1835,9 +1857,17 @@ static struct folio *compaction_alloc(struct folio *src, unsigned long data)
>>>>  static void compaction_free(struct folio *dst, unsigned long data)
>>>>  {
>>>>  	struct compact_control *cc = (struct compact_control *)data;
>>>> +	int order = folio_order(dst);
>>>> +	struct page *page = &dst->page;
>>>> +
>>>> +	folio_set_count(dst, 0);
>>>
>>> We can't change refcount to 0 like this, after it was already set to 1 and
>>> somebody else might have done get_page_unless_zero(). You need to either
>>> put_page_testzero() and if it's false, consider the page lost, or leave it
>>> refcounted and adjust the code to handle both refcounted and non-refcounted
>>> pages on the lists (the first option is simpler and shouldn't be too bad).
>> Got it. Will fix it with the first option. Thanks.
> 
> Do you think we should have a WARN or WARN_ONCE if we lose a page here?

No, no WARN, it all happens legitimately. It's only our compaction losing
the page - whoever would do the get_page_unless_zero() to inspect that page
would then have to put_page() which will free it back to page allocator.

> --
> Best Regards,
> Yan, Zi