linux-kernel - Re: [PATCH 4/5] mm: add mm_get_static_huge_zero

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Message-ID: <a65ee315-23b7-4058-895a-69045829bd01@redhat.com>
Date: Mon, 16 Jun 2025 11:14:07 +0200
From: David Hildenbrand <david@...hat.com>
To: "Pankaj Raghav (Samsung)" <kernel@...kajraghav.com>,
 Dave Hansen <dave.hansen@...el.com>
Cc: Pankaj Raghav <p.raghav@...sung.com>,
 Suren Baghdasaryan <surenb@...gle.com>, Ryan Roberts <ryan.roberts@....com>,
 Mike Rapoport <rppt@...nel.org>, Michal Hocko <mhocko@...e.com>,
 Thomas Gleixner <tglx@...utronix.de>, Nico Pache <npache@...hat.com>,
 Dev Jain <dev.jain@....com>, Baolin Wang <baolin.wang@...ux.alibaba.com>,
 Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...hat.com>,
 "H . Peter Anvin" <hpa@...or.com>, Vlastimil Babka <vbabka@...e.cz>,
 Zi Yan <ziy@...dia.com>, Dave Hansen <dave.hansen@...ux.intel.com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 "Liam R . Howlett" <Liam.Howlett@...cle.com>, Jens Axboe <axboe@...nel.dk>,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org, willy@...radead.org,
 x86@...nel.org, linux-block@...r.kernel.org, linux-fsdevel@...r.kernel.org,
 "Darrick J . Wong" <djwong@...nel.org>, mcgrof@...nel.org,
 gost.dev@...sung.com, hch@....de
Subject: Re: [PATCH 4/5] mm: add mm_get_static_huge_zero_folio() routine

On 12.06.25 22:54, Pankaj Raghav (Samsung) wrote:
> On Thu, Jun 12, 2025 at 07:09:34AM -0700, Dave Hansen wrote:
>> On 6/12/25 03:50, Pankaj Raghav wrote:
>>> +/*
>>> + * mm_get_static_huge_zero_folio - Get a PMD sized zero folio
>>
>> Isn't that a rather inaccurate function name and comment?
> I agree. I also felt it was not a good name for the function.
> 
>>
>> The third line of the function literally returns a non-PMD-sized zero folio.
>>
>>> + * This function will return a PMD sized zero folio if CONFIG_STATIC_PMD_ZERO_PAGE
>>> + * is enabled. Otherwise, a ZERO_PAGE folio is returned.
>>> + *
>>> + * Deduce the size of the folio with folio_size instead of assuming the
>>> + * folio size.
>>> + */
>>> +static inline struct folio *mm_get_static_huge_zero_folio(void)
>>> +{
>>> +	if(IS_ENABLED(CONFIG_STATIC_PMD_ZERO_PAGE))
>>> +		return READ_ONCE(huge_zero_folio);
>>> +	return page_folio(ZERO_PAGE(0));
>>> +}
>>
>> This doesn't tell us very much about when I should use:
>>
>> 	mm_get_static_huge_zero_folio()
>> vs.
>> 	mm_get_huge_zero_folio(mm)
>> vs.
>> 	page_folio(ZERO_PAGE(0))
>>
>> What's with the "mm_" in the name? Usually "mm" means "mm_struct" not
>> Memory Management. It's really weird to prefix something that doesn't
>> take an "mm_struct" with "mm_"
> 
> Got it. Actually, I was not aware of this one.
> 
>>
>> Isn't the "get_" also a bad idea since mm_get_huge_zero_folio() does its
>> own refcounting but this interface does not?
>>
> 
> Agree.
> 
>> Shouldn't this be something more along the lines of:
>>
>> /*
>>   * pick_zero_folio() - Pick and return the largest available zero folio
>>   *
>>   * mm_get_huge_zero_folio() is preferred over this function. It is more
>>   * flexible and can provide a larger zero page under wider
>>   * circumstances.
>>   *
>>   * Only use this when there is no mm available.
>>   *
>>   * ... then other comments
>>   */
>> static inline struct folio *pick_zero_folio(void)
>> {
>> 	if (IS_ENABLED(CONFIG_STATIC_PMD_ZERO_PAGE))
>> 		return READ_ONCE(huge_zero_folio);
>> 	return page_folio(ZERO_PAGE(0));
>> }
>>
>> Or, maybe even name it _just_: zero_folio()
> 
> I think zero_folio() sounds like a good and straightforward name. In
> most cases it will return a ZERO_PAGE() folio. If
> CONFIG_STATIC_PMD_ZERO_PAGE is enabled, then we return a PMD page.

"zero_folio" would be confusing I'm afraid.

At least with current "is_zero_folio" etc.

"largest_zero_folio" or sth. like that might make it clearer that the 
size we are getting back might actually differ.

-- 
Cheers,

David / dhildenb