linux-kernel - Re: [PATCHv3 09/15] mm/hugetlb: Refactor code around vmemmap

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Message-ID: <aW5NDEuP_pRLsq9d@thinkstation>
Date: Mon, 19 Jan 2026 15:26:37 +0000
From: Kiryl Shutsemau <kas@...nel.org>
To: Muchun Song <muchun.song@...ux.dev>
Cc: Oscar Salvador <osalvador@...e.de>, Mike Rapoport <rppt@...nel.org>, 
	Vlastimil Babka <vbabka@...e.cz>, Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, 
	Zi Yan <ziy@...dia.com>, Baoquan He <bhe@...hat.com>, Michal Hocko <mhocko@...e.com>, 
	Johannes Weiner <hannes@...xchg.org>, Jonathan Corbet <corbet@....net>, kernel-team@...a.com, 
	linux-mm@...ck.org, linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org, 
	Andrew Morton <akpm@...ux-foundation.org>, David Hildenbrand <david@...nel.org>, 
	Matthew Wilcox <willy@...radead.org>, Usama Arif <usamaarif642@...il.com>, 
	Frank van der Linden <fvdl@...gle.com>
Subject: Re: [PATCHv3 09/15] mm/hugetlb: Refactor code around vmemmap_walk

On Mon, Jan 19, 2026 at 06:04:32PM +0800, Muchun Song wrote:
> > @@ -415,29 +377,27 @@ static int alloc_vmemmap_page_list(unsigned long start, unsigned long end,
> >    *		to remap.
> >    * @end:	end address of the vmemmap virtual address range that we want to
> >    *		remap.
> > - * @reuse:	reuse address.
> >    * @flags:	modifications to vmemmap_remap_walk flags
> >    *
> >    * Return: %0 on success, negative error code otherwise.
> >    */
> >   static int vmemmap_remap_alloc(unsigned long start, unsigned long end,
> > -			       unsigned long reuse, unsigned long flags)
> > +			       unsigned long flags)
> >   {
> >   	LIST_HEAD(vmemmap_pages);
> >   	struct vmemmap_remap_walk walk = {
> >   		.remap_pte	= vmemmap_restore_pte,
> > -		.reuse_addr	= reuse,
> > +		.vmemmap_start	= start,
> >   		.vmemmap_pages	= &vmemmap_pages,
> >   		.flags		= flags,
> >   	};
> > -	/* See the comment in the vmemmap_remap_free(). */
> > -	BUG_ON(start - reuse != PAGE_SIZE);
> > +	start += HUGETLB_VMEMMAP_RESERVE_SIZE;
> 
> From the @start comment above this function, we can see that the @start
> address is the starting address for the remap operation we want to perform.
> However, the modification here does not conform to the semantics here.
> Therefore, I suggest letting the caller __hugetlb_vmemmap_restore_folio()
> pass the correct parameters itself.
> 
> The reason we previously used the reuse address here is that the
> initialization of ->reuse_page was inside vmemmap_pte_entry. Now,
> ->reuse_page has been removed in your patch.

Fair enough.

> > @@ -466,18 +425,16 @@ static int __hugetlb_vmemmap_restore_folio(const struct hstate *h,
> >   	if (flags & VMEMMAP_SYNCHRONIZE_RCU)
> >   		synchronize_rcu();
> > +	vmemmap_start	= (unsigned long)folio;
> 
> (unsigned long)&folio->page？The folio is the metadata for contiguous pages,
> while the page is our optimization target. Although their values are the
> same now, their semantics are different.

Okay.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov