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

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Date:   Wed, 8 Nov 2023 10:01:19 +0800
From:   Kefeng Wang <wangkefeng.wang@...wei.com>
To:     Gregory Price <gregory.price@...verge.com>
CC:     Matthew Wilcox <willy@...radead.org>,
        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 2023/11/7 23:34, Gregory Price wrote:
> On Tue, Nov 07, 2023 at 09:03:53AM +0800, Kefeng Wang wrote:
>>
>>
>> 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.
> 
> looks like we were noticing the same thing.  I haven't done any further
> work, got caught up in another project.

Yes, I see your "Node Weights and Weighted Interleave", this is an 
interesting topic, we need some easy and efficient way to use tiered
memory.

> 
> Matthew last pointed out:
> 
> "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'."
> 
> https://lore.kernel.org/linux-mm/ZUDFSEvpxxoGWmdG@casper.infradead.org/
> 
> I took a quick look, and the only thing I'm not confident about is that
> some flags are stored in the head page, and some are stored on the
> second page.
> 
> /* Which page is the flag stored in */
> #define FOLIO_PF_ANY            0
> #define FOLIO_PF_HEAD           0
> #define FOLIO_PF_ONLY_HEAD      0
> #define FOLIO_PF_NO_TAIL        0
> #define FOLIO_PF_NO_COMPOUND    0
> #define FOLIO_PF_SECOND         1
> 
> There's only a handful, so yeah the best way is probably to go ahead and
> swap k = page->flags for k = *folio_flags(folio, 0) and then handle the
> couple of outliars.

If you don't mind, I maybe try to convert it in my changes.

Thanks.
> 
> ~Gregory