linux-kernel - Re: [PATCH 02/12] mm: Convert pXd_devmap checks to vma_is

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Message-ID: <371b8fdd-129d-4fe3-bbc7-f0a1bc433b30@redhat.com>
Date: Fri, 30 May 2025 11:37:21 +0200
From: David Hildenbrand <david@...hat.com>
To: Alistair Popple <apopple@...dia.com>, linux-mm@...ck.org
Cc: gerald.schaefer@...ux.ibm.com, dan.j.williams@...el.com, jgg@...pe.ca,
 willy@...radead.org, linux-kernel@...r.kernel.org, nvdimm@...ts.linux.dev,
 linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
 linux-xfs@...r.kernel.org, jhubbard@...dia.com, hch@....de,
 zhang.lyra@...il.com, debug@...osinc.com, bjorn@...nel.org,
 balbirs@...dia.com, lorenzo.stoakes@...cle.com,
 linux-arm-kernel@...ts.infradead.org, loongarch@...ts.linux.dev,
 linuxppc-dev@...ts.ozlabs.org, linux-riscv@...ts.infradead.org,
 linux-cxl@...r.kernel.org, dri-devel@...ts.freedesktop.org, John@...ves.net
Subject: Re: [PATCH 02/12] mm: Convert pXd_devmap checks to vma_is_dax

On 29.05.25 08:32, Alistair Popple wrote:
> Currently dax is the only user of pmd and pud mapped ZONE_DEVICE
> pages. Therefore page walkers that want to exclude DAX pages can check
> pmd_devmap or pud_devmap. However soon dax will no longer set PFN_DEV,
> meaning dax pages are mapped as normal pages.
> 
> Ensure page walkers that currently use pXd_devmap to skip DAX pages
> continue to do so by adding explicit checks of the VMA instead.
> 
> Signed-off-by: Alistair Popple <apopple@...dia.com>
> ---
>   fs/userfaultfd.c | 2 +-
>   mm/hmm.c         | 2 +-
>   mm/userfaultfd.c | 2 +-
>   3 files changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index 22f4bf9..de671d3 100644
> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -304,7 +304,7 @@ static inline bool userfaultfd_must_wait(struct userfaultfd_ctx *ctx,
>   		goto out;
>   
>   	ret = false;
> -	if (!pmd_present(_pmd) || pmd_devmap(_pmd))
> +	if (!pmd_present(_pmd) || vma_is_dax(vmf->vma))
>   		goto out;
>   
>   	if (pmd_trans_huge(_pmd)) {
> diff --git a/mm/hmm.c b/mm/hmm.c
> index 082f7b7..db12c0a 100644
> --- a/mm/hmm.c
> +++ b/mm/hmm.c
> @@ -429,7 +429,7 @@ static int hmm_vma_walk_pud(pud_t *pudp, unsigned long start, unsigned long end,
>   		return hmm_vma_walk_hole(start, end, -1, walk);
>   	}
>   
> -	if (pud_leaf(pud) && pud_devmap(pud)) {
> +	if (pud_leaf(pud) && vma_is_dax(walk->vma)) {
>   		unsigned long i, npages, pfn;
>   		unsigned int required_fault;
>   		unsigned long *hmm_pfns;
> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> index e0db855..133f750 100644
> --- a/mm/userfaultfd.c
> +++ b/mm/userfaultfd.c
> @@ -1791,7 +1791,7 @@ ssize_t move_pages(struct userfaultfd_ctx *ctx, unsigned long dst_start,
>   
>   		ptl = pmd_trans_huge_lock(src_pmd, src_vma);
>   		if (ptl) {
> -			if (pmd_devmap(*src_pmd)) {
> +			if (vma_is_dax(src_vma)) {
>   				spin_unlock(ptl);
>   				err = -ENOENT;
>   				break;

I assume we could also just refuse dax folios, right?

If we decide to check VMAs, we should probably check earlier.

But I wonder, what about anonymous non-dax pages in COW mappings? Is it 
possible? Not supported?

If supported, checking the actual folio would be the right thing to do.

-- 
Cheers,

David / dhildenb