linux-kernel - Re: [PATCH v2 01/14] mm: Convert pXd_devmap checks to vma_is

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Message-ID: <bf855ce0-d0ba-4bd6-bfc1-8be2fdbdfe70@redhat.com>
Date: Tue, 17 Jun 2025 11:19:34 +0200
From: David Hildenbrand <david@...hat.com>
To: Alistair Popple <apopple@...dia.com>, akpm@...ux-foundation.org
Cc: linux-mm@...ck.org, 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,
 m.szyprowski@...sung.com, Jason Gunthorpe <jgg@...dia.com>
Subject: Re: [PATCH v2 01/14] mm: Convert pXd_devmap checks to vma_is_dax

On 16.06.25 13:58, 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>
> Reviewed-by: Jason Gunthorpe <jgg@...dia.com>
> Reviewed-by: Dan Williams <dan.j.williams@...el.com>
> 
> ---
> 
> Changes from v1:
> 
>   - Remove vma_is_dax() check from mm/userfaultfd.c as
>     validate_move_areas() will already skip DAX VMA's on account of them
>     not being anonymous.

This should be documented in the patch description above.

> ---
>   fs/userfaultfd.c | 2 +-
>   mm/hmm.c         | 2 +-
>   mm/userfaultfd.c | 6 ------
>   3 files changed, 2 insertions(+), 8 deletions(-)
> 
> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index ef054b3..a886750 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;

VMA checks should be done before doing any page table walk.

>   
>   	if (pmd_trans_huge(_pmd)) {
> diff --git a/mm/hmm.c b/mm/hmm.c
> index feac861..5311753 100644
> --- a/mm/hmm.c
> +++ b/mm/hmm.c
> @@ -441,7 +441,7 @@ 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;

Dito.

> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> index 58b3ad6..8395db2 100644
> --- a/mm/userfaultfd.c
> +++ b/mm/userfaultfd.c
> @@ -1818,12 +1818,6 @@ 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)) {
> -				spin_unlock(ptl);
> -				err = -ENOENT;
> -				break;
> -			}
> -
>   			/* Check if we can move the pmd without splitting it. */
>   			if (move_splits_huge_pmd(dst_addr, src_addr, src_start + len) ||
>   			    !pmd_none(dst_pmdval)) {


-- 
Cheers,

David / dhildenb