linux-kernel - Re: [PATCH v1 01/10] mm: Expose clear_huge

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Message-ID: <2ff8ccf6-bf36-48b2-7dc2-e6c0d962f8b7@arm.com>
Date:   Tue, 27 Jun 2023 08:21:22 +0100
From:   Ryan Roberts <ryan.roberts@....com>
To:     Yu Zhao <yuzhao@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Yin Fengwei <fengwei.yin@...el.com>,
        David Hildenbrand <david@...hat.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Sven Schnelle <svens@...ux.ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-alpha@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-ia64@...r.kernel.org,
        linux-m68k@...ts.linux-m68k.org, linux-s390@...r.kernel.org
Subject: Re: [PATCH v1 01/10] mm: Expose clear_huge_page() unconditionally

On 27/06/2023 02:55, Yu Zhao wrote:
> On Mon, Jun 26, 2023 at 11:14 AM Ryan Roberts <ryan.roberts@....com> wrote:
>>
>> In preparation for extending vma_alloc_zeroed_movable_folio() to
>> allocate a arbitrary order folio, expose clear_huge_page()
>> unconditionally, so that it can be used to zero the allocated folio in
>> the generic implementation of vma_alloc_zeroed_movable_folio().
>>
>> Signed-off-by: Ryan Roberts <ryan.roberts@....com>
>> ---
>>  include/linux/mm.h | 3 ++-
>>  mm/memory.c        | 2 +-
>>  2 files changed, 3 insertions(+), 2 deletions(-)
>>
>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>> index 7f1741bd870a..7e3bf45e6491 100644
>> --- a/include/linux/mm.h
>> +++ b/include/linux/mm.h
>> @@ -3684,10 +3684,11 @@ enum mf_action_page_type {
>>   */
>>  extern const struct attribute_group memory_failure_attr_group;
>>
>> -#if defined(CONFIG_TRANSPARENT_HUGEPAGE) || defined(CONFIG_HUGETLBFS)
>>  extern void clear_huge_page(struct page *page,
>>                             unsigned long addr_hint,
>>                             unsigned int pages_per_huge_page);
>> +
>> +#if defined(CONFIG_TRANSPARENT_HUGEPAGE) || defined(CONFIG_HUGETLBFS)
> 
> We might not want to depend on THP eventually. Right now, we still
> have to, unless splitting is optional, which seems to contradict
> 06/10. (deferred_split_folio()  is a nop without THP.)

Yes, I agree - for large anon folios to work, we depend on THP. But I don't
think that helps us here.

In the next patch, I give vma_alloc_zeroed_movable_folio() an extra `order`
parameter. So the generic/default version of the function now needs a way to
clear a compound page.

I guess I could do something like:

 static inline
 struct folio *vma_alloc_zeroed_movable_folio(struct vm_area_struct *vma,
				   unsigned long vaddr, gfp_t gfp, int order)
 {
 	struct folio *folio;

	folio = vma_alloc_folio(GFP_HIGHUSER_MOVABLE | gfp,
					order, vma, vaddr, false);
 	if (folio) {
#ifdef CONFIG_LARGE_FOLIO
		clear_huge_page(&folio->page, vaddr, 1U << order);
#else
		BUG_ON(order != 0);
		clear_user_highpage(&folio->page, vaddr);
#endif
	}

 	return folio;
 }

But that's pretty messy and there's no reason why other users might come along
that pass order != 0 and will be surprised by the BUG_ON.