linux-kernel - Re: [PATCH] mm/hugetlb: Convert &folio->page to folio

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Message-ID: <aAk2dCTru-C595oY@casper.infradead.org>
Date: Wed, 23 Apr 2025 19:50:28 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Fan Ni <nifan.cxl@...il.com>
Cc: muchun.song@...ux.dev, mcgrof@...nel.org, a.manzanares@...sung.com,
	dave@...olabs.net, akpm@...ux-foundation.org, david@...hat.com,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/hugetlb: Convert &folio->page to folio_page(folio, 0)

On Fri, Apr 18, 2025 at 03:42:45PM -0700, Fan Ni wrote:
> > >  	result = install_pmd
> > > -			? set_huge_pmd(vma, haddr, pmd, &folio->page)
> > > +			? set_huge_pmd(vma, haddr, pmd, folio_page(folio, 0))
> > >  			: SCAN_SUCCEED;
> > 
> > I feel that set_huge_pmd() should take a folio.
> There is a patch on the mailing list for it,
> https://lore.kernel.org/linux-mm/20240817095122.2460977-5-wangkefeng.wang@huawei.com/
> 
> If the above patch is needed, do_set_pmd() should be converted to use folio.

Maybe?  I think we'll eventually want to support folios larger than PMD
size.  So I don't want to pass in a folio here unless we can calculate the
precise PMD-size chunk we want to map from this folio given the
information available in vmf.  I know that today the implementation
does this:

        if (folio_order(folio) != HPAGE_PMD_ORDER)
                return ret;
        page = &folio->page;

but eventually we should do better than that.  And I don't want to
lose the information about which page in the folio we really want
to map.  So have a think about what the right interface should be
(passing in a page?  folio + offset-in-number-of-pages?  Something
involving the PFN?)

The obvious hurdle is folio_add_file_rmap_pmd() which today takes both a
folio and a page.