linux-kernel - Re: [PATCH] mm/huge_memory: softleaf device private fixes in remove_migration

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Message-ID: <c5ebefad-2b1c-45c1-9a82-17a024d52d1c@lucifer.local>
Date: Wed, 12 Nov 2025 13:43:12 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Balbir Singh <balbirs@...dia.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        akpm@...ux-foundation.org, David Hildenbrand <david@...nel.org>,
        Zi Yan <ziy@...dia.com>, Baolin Wang <baolin.wang@...ux.alibaba.com>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Nico Pache <npache@...hat.com>, Ryan Roberts <ryan.roberts@....com>,
        Dev Jain <dev.jain@....com>, Barry Song <baohua@...nel.org>,
        Lance Yang <lance.yang@...ux.dev>
Subject: Re: [PATCH] mm/huge_memory: softleaf device private fixes in
 remove_migration_pmd()

On Wed, Nov 12, 2025 at 03:46:34PM +1100, Balbir Singh wrote:
> commit a6ca2ba46390 ("mm: replace pmd_to_swp_entry() with softleaf_from_pmd()")
> does not work with device private THP entries. softleaf_is_migration_young()
> asserts that the entry be a migration entry, but in the current code, the
> entry might already be replaced by a device private entry by the time the
> check is made. The issue exists with commit
> 7385dbdbf841 ("mm/rmap: extend rmap and migration support device-private entries")

OK this is _hugely_ confusing.

Is the bug in my patch or in yours?

Why are you replying to your own series with this patch?

You shouldn't reference non-upstream commit messages in general.

If the bug is in 7385dbdbf841, fix it in your series, then perhaps send a
suggested fix-patch to the appropriate patch in my series to make life easier
for Andrew.

As mine I think in this case was purely a mechanical replacement of function
calls I'm guessing it's a bug in yours? So I think this is probably the best
way.

>
> Fix this by processing the migration entries prior to conversion to
> device private if the folio is device private.
>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: David Hildenbrand <david@...nel.org>
> Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> Cc: Zi Yan <ziy@...dia.com>
> Cc: Baolin Wang <baolin.wang@...ux.alibaba.com>
> Cc: "Liam R. Howlett" <Liam.Howlett@...cle.com>
> Cc: Nico Pache <npache@...hat.com>
> Cc: Ryan Roberts <ryan.roberts@....com>
> Cc: Dev Jain <dev.jain@....com>
> Cc: Barry Song <baohua@...nel.org>
> Cc: Lance Yang <lance.yang@...ux.dev>
>
> Signed-off-by: Balbir Singh <balbirs@...dia.com>
> ---
>  mm/huge_memory.c | 25 ++++++++++++++++++-------
>  1 file changed, 18 insertions(+), 7 deletions(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 942bd8410c54..82b019205216 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -4939,6 +4939,7 @@ void remove_migration_pmd(struct page_vma_mapped_walk *pvmw, struct page *new)
>  	unsigned long haddr = address & HPAGE_PMD_MASK;
>  	pmd_t pmde;
>  	softleaf_t entry;
> +	bool old = false, dirty = false, migration_read_entry = false;
>
>  	if (!(pvmw->pmd && !pvmw->pte))
>  		return;
> @@ -4947,6 +4948,19 @@ void remove_migration_pmd(struct page_vma_mapped_walk *pvmw, struct page *new)
>  	folio_get(folio);
>  	pmde = folio_mk_pmd(folio, READ_ONCE(vma->vm_page_prot));
>
> +	if (!softleaf_is_migration_young(entry))
> +		old = true;
> +
> +	/* NOTE: this may contain setting soft-dirty on some archs */

'This may contain setting soft-dirty' is confusing. 'This may set soft-dirty on some arches' perhaps?

> +	if (folio_test_dirty(folio) && softleaf_is_migration_dirty(entry))
> +		dirty = true;
> +
> +	if (softleaf_is_migration_write(entry))
> +		pmde = pmd_mkwrite(pmde, vma);
> +
> +	if (!softleaf_is_migration_read(entry))
> +		migration_read_entry = true;
> +
>  	if (folio_is_device_private(folio)) {
>  		if (pmd_write(pmde))
>  			entry = make_writable_device_private_entry(
> @@ -4959,20 +4973,17 @@ void remove_migration_pmd(struct page_vma_mapped_walk *pvmw, struct page *new)
>
>  	if (pmd_swp_soft_dirty(*pvmw->pmd))
>  		pmde = pmd_mksoft_dirty(pmde);
> -	if (softleaf_is_migration_write(entry))
> -		pmde = pmd_mkwrite(pmde, vma);
> +	if (old)
> +		pmde = pmd_mkold(pmde);
>  	if (pmd_swp_uffd_wp(*pvmw->pmd))
>  		pmde = pmd_mkuffd_wp(pmde);
> -	if (!softleaf_is_migration_young(entry))
> -		pmde = pmd_mkold(pmde);
> -	/* NOTE: this may contain setting soft-dirty on some archs */
> -	if (folio_test_dirty(folio) && softleaf_is_migration_dirty(entry))
> +	if (dirty)
>  		pmde = pmd_mkdirty(pmde);
>
>  	if (folio_test_anon(folio)) {
>  		rmap_t rmap_flags = RMAP_NONE;
>
> -		if (!softleaf_is_migration_read(entry))
> +		if (migration_read_entry)
>  			rmap_flags |= RMAP_EXCLUSIVE;
>
>  		folio_add_anon_rmap_pmd(folio, new, vma, haddr, rmap_flags);
> --
> 2.51.1
>

Thanks, Lorenzo