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

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Message-ID: <28c47a9f-5ee4-425e-bd8d-bd251634534c@suse.cz>
Date: Fri, 9 Feb 2024 21:46:09 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Zi Yan <ziy@...dia.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 "Huang, Ying" <ying.huang@...el.com>, Ryan Roberts <ryan.roberts@....com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 "Matthew Wilcox (Oracle)" <willy@...radead.org>,
 David Hildenbrand <david@...hat.com>, "Yin, Fengwei"
 <fengwei.yin@...el.com>, Yu Zhao <yuzhao@...gle.com>,
 "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
 Johannes Weiner <hannes@...xchg.org>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>,
 Kemeng Shi <shikemeng@...weicloud.com>,
 Mel Gorman <mgorman@...hsingularity.net>, Rohan Puri
 <rohan.puri15@...il.com>, Mcgrof Chamberlain <mcgrof@...nel.org>,
 Adam Manzanares <a.manzanares@...sung.com>,
 "Vishal Moola (Oracle)" <vishal.moola@...il.com>
Subject: Re: [PATCH v3 2/3] mm/compaction: add support for >0 order folio
 memory compaction.

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