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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date: Fri, 09 Feb 2024 14:40:53 -0500
From: Zi Yan <ziy@...dia.com>
To: Vlastimil Babka <vbabka@...e.cz>
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 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:
>>> From: Zi Yan <ziy@...dia.com>
>>>
>>> Before last commit, memory compaction only migrates order-0 folios and
>>> skips >0 order folios. Last commit splits all >0 order folios during
>>> compaction. This commit migrates >0 order folios during compaction by
>>> keeping isolated free pages at their original size without splitting them
>>> into order-0 pages and using them directly during migration process.
>>>
>>> What is different from the prior implementation:
>>> 1. All isolated free pages are kept in a NR_PAGE_ORDERS array of page
>>>    lists, where each page list stores free pages in the same order.
>>> 2. All free pages are not post_alloc_hook() processed nor buddy pages,
>>>    although their orders are stored in first page's private like buddy
>>>    pages.
>>> 3. During migration, in new page allocation time (i.e., in
>>>    compaction_alloc()), free pages are then processed by post_alloc_hook().
>>>    When migration fails and a new page is returned (i.e., in
>>>    compaction_free()), free pages are restored by reversing the
>>>    post_alloc_hook() operations using newly added
>>>    free_pages_prepare_fpi_none().
>>>
>>> Step 3 is done for a latter optimization that splitting and/or merging free
>>> pages during compaction becomes easier.
>>>
>>> Note: without splitting free pages, compaction can end prematurely due to
>>> migration will return -ENOMEM even if there is free pages. This happens
>>> when no order-0 free page exist and compaction_alloc() return NULL.
>>>
>>> Signed-off-by: Zi Yan <ziy@...dia.com>
>>
>> ...
>>
>>>  /*
>>> @@ -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?

--
Best Regards,
Yan, Zi

Download attachment "signature.asc" of type "application/pgp-signature" (855 bytes)