linux-kernel - Re: [PATCH v3 14/25] huge_memory: Allow mappings of PUD sized pages

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Message-ID: <fc83a855-bb3f-4374-8896-579420732b25@redhat.com>
Date: Sat, 14 Dec 2024 16:32:00 +0100
From: David Hildenbrand <david@...hat.com>
To: Alistair Popple <apopple@...dia.com>, dan.j.williams@...el.com,
 linux-mm@...ck.org
Cc: lina@...hilina.net, zhang.lyra@...il.com, gerald.schaefer@...ux.ibm.com,
 vishal.l.verma@...el.com, dave.jiang@...el.com, logang@...tatee.com,
 bhelgaas@...gle.com, jack@...e.cz, jgg@...pe.ca, catalin.marinas@....com,
 will@...nel.org, mpe@...erman.id.au, npiggin@...il.com,
 dave.hansen@...ux.intel.com, ira.weiny@...el.com, willy@...radead.org,
 djwong@...nel.org, tytso@....edu, linmiaohe@...wei.com, peterx@...hat.com,
 linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org, linuxppc-dev@...ts.ozlabs.org,
 nvdimm@...ts.linux.dev, linux-cxl@...r.kernel.org,
 linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
 linux-xfs@...r.kernel.org, jhubbard@...dia.com, hch@....de,
 david@...morbit.com
Subject: Re: [PATCH v3 14/25] huge_memory: Allow mappings of PUD sized pages

On 22.11.24 02:40, Alistair Popple wrote:
> Currently DAX folio/page reference counts are managed differently to
> normal pages. To allow these to be managed the same as normal pages
> introduce vmf_insert_folio_pud. This will map the entire PUD-sized folio
> and take references as it would for a normally mapped page.
> 
> This is distinct from the current mechanism, vmf_insert_pfn_pud, which
> simply inserts a special devmap PUD entry into the page table without
> holding a reference to the page for the mapping.
> 
> Signed-off-by: Alistair Popple <apopple@...dia.com>
> ---

Hi,

The patch subject of this (and especially the next patch) is misleading. 
Likely you meant to have it as:

"mm/huge_memory: add vmf_insert_folio_pud() for mapping PUD sized pages"

>   	for (i = 0; i < nr_pages; i++) {
> @@ -1523,6 +1531,26 @@ void folio_add_file_rmap_pmd(struct folio *folio, struct page *page,
>   #endif
>   }
>   
> +/**
> + * folio_add_file_rmap_pud - add a PUD mapping to a page range of a folio
> + * @folio:	The folio to add the mapping to
> + * @page:	The first page to add
> + * @vma:	The vm area in which the mapping is added
> + *
> + * The page range of the folio is defined by [page, page + HPAGE_PUD_NR)
> + *
> + * The caller needs to hold the page table lock.
> + */
> +void folio_add_file_rmap_pud(struct folio *folio, struct page *page,
> +		struct vm_area_struct *vma)
> +{
> +#ifdef CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD
> +	__folio_add_file_rmap(folio, page, HPAGE_PUD_NR, vma, RMAP_LEVEL_PUD);
> +#else
> +	WARN_ON_ONCE(true);
> +#endif
> +}
> +
>   static __always_inline void __folio_remove_rmap(struct folio *folio,
>   		struct page *page, int nr_pages, struct vm_area_struct *vma,
>   		enum rmap_level level)
> @@ -1552,6 +1580,7 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
>   		partially_mapped = nr && atomic_read(mapped);
>   		break;
>   	case RMAP_LEVEL_PMD:
> +	case RMAP_LEVEL_PUD:
>   		atomic_dec(&folio->_large_mapcount);
>   		last = atomic_add_negative(-1, &folio->_entire_mapcount);
>   		if (last) {

If you simply reuse that code (here and on the adding path), you will 
end up effectively setting nr_pmdmapped to a very large value and 
passing that into __folio_mod_stat().

There, we will adjust NR_SHMEM_PMDMAPPED/NR_FILE_PMDMAPPED, which is 
wrong (it's PUD mapped ;) ).

It's probably best to split out the rmap changes from the other things 
in this patch.


-- 
Cheers,

David / dhildenb