linux-kernel - Re: [PATCH 2/2] mm/hugetlb: refactor subpage recording

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Date:   Tue, 26 Jan 2021 21:50:39 -0500
From:   Zi Yan <ziy@...dia.com>
To:     Matthew Wilcox <willy@...radead.org>
CC:     Jason Gunthorpe <jgg@...pe.ca>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Joao Martins <joao.m.martins@...cle.com>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        John Hubbard <jhubbard@...dia.com>
Subject: Re: [PATCH 2/2] mm/hugetlb: refactor subpage recording

On 26 Jan 2021, at 21:24, Matthew Wilcox wrote:

> On Tue, Jan 26, 2021 at 08:07:30PM -0400, Jason Gunthorpe wrote:
>> I'm looking at Matt's folio patches and see:
>>
>> +static inline struct folio *next_folio(struct folio *folio)
>> +{
>> +       return folio + folio_nr_pages(folio);
>> +}
>
> This is a replacement for places that would do 'page++'.  eg it's
> used by the bio iterator where we already checked that the phys addr
> and the struct page are contiguous.
>
>> And checking page_trans_huge_mapcount():
>>
>> 	for (i = 0; i < thp_nr_pages(page); i++) {
>> 		mapcount = atomic_read(&page[i]._mapcount) + 1;
>
> I think we are guaranteed this for transparent huge pages.  At least
> for now.  Zi Yan may have some thoughts for his work on 1GB transhuge
> pages ...

It should work for 1GB THP too. My implementation allocates 1GB pages
from cma_alloc(), which calls alloc_contig_range(). At least for now
subpages from a 1GB THP are physically contiguous.

It will be a concern if we use other ways (like migrating in-use pages)
of forming 1GB THPs. Thanks for pointing this out.

>
>> And we have the same logic in hmm_vma_walk_pud():
>>
>> 	if (pud_huge(pud) && pud_devmap(pud)) {
>> 		pfn = pud_pfn(pud) + ((addr & ~PUD_MASK) >> PAGE_SHIFT);
>> 		for (i = 0; i < npages; ++i, ++pfn)
>> 			hmm_pfns[i] = pfn | cpu_flags;
>>
>> So, if page[n] does not access the tail pages of a compound we have
>> many more people who are surprised by this than just GUP.
>>
>> Where are these special rules for hugetlb compound tails documented?
>> Why does it need to be like this?
>>
>> Isn't it saner to forbid a compound and its tails from being
>> non-linear in the page array? That limits when compounds can be
>> created, but seems more likely to happen than a full mm audit to find
>> all the places that assume linearity.
>>
>> Jason


—
Best Regards,
Yan Zi

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