linux-kernel - Re: [PATCH v2] kpageflags: respect folio head-page flag placement

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Date:   Tue, 7 Nov 2023 09:03:53 +0800
From:   Kefeng Wang <wangkefeng.wang@...wei.com>
To:     Gregory Price <gregory.price@...verge.com>,
        Matthew Wilcox <willy@...radead.org>
CC:     Gregory Price <gourry.memverge@...il.com>, <linux-mm@...ck.org>,
        <akpm@...ux-foundation.org>, <david@...hat.com>, <vbabka@...e.cz>,
        <naoya.horiguchi@...ux.dev>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] kpageflags: respect folio head-page flag placement



On 2023/10/31 12:34, Gregory Price wrote:
> On Tue, Oct 31, 2023 at 09:13:44AM +0000, Matthew Wilcox wrote:
>> On Mon, Oct 30, 2023 at 07:41:23PM -0400, Gregory Price wrote:
>>> On Mon, Oct 30, 2023 at 11:22:18PM +0000, Matthew Wilcox wrote:
>>>> On Mon, Oct 30, 2023 at 02:00:05PM -0400, Gregory Price wrote:
>>>>> kpageflags reads page-flags directly from the page, even when the
>>>>> respective flag is only updated on the headpage of a folio.
>>>>>
>>>>> Update bitchecks to use PAGEFLAG() interfaces to check folio for the
>>>>> referenced, dirty, lru, active, and unevictable bits.
>>>>
>>>> But uptodate, writeback and reclaim (amongst others) are also defined
>>>> only on the head page.
>>>>
>>>
>>> Ah yes i was only looking at the things defined w/ PAGEFLAG defines in
>>> page-flags.h. I'll give it full once over can collect them all, my bad.
>>>
>>> (also i forgot to update my commit message)
>>>
>>> Quick question here since i have your attention: any recommendation on
>>> what to do for ONLY_HEAD flags?  If the provided page is not the head,
>>> should the flag report 0... or whatever the head says?
>>
>> Thinking about it some more, really almost all flags are per-folio, not
>> per-page.  The only exceptions are HWPoison and AnonExclusive.  So
>> probably the right way to do this is to make k = folio->flags, and
>> then just change a few places rather than changing all the places that
>> test 'k'.
> 
> Funny enough that's what i originally did but was confident it was
> correct so walked it back.  I'll take another crack at it.

Hi Gregory, any update?
I changed stable_page_flags[1] when try to remove page idle wrapper,
Matthew pointed it will conflict with this, I could redo my patch
based on your new version:)

[1]https://lore.kernel.org/linux-mm/20231103072906.2000381-5-wangkefeng.wang@huawei.com/

Thanks.
> 
> ~Gregory
>