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Message-ID: <91c443e1-3d7d-4b95-ab62-b970b047936f@lucifer.local>
Date: Tue, 14 Oct 2025 12:19:11 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Lance Yang <lance.yang@...ux.dev>
Cc: akpm@...ux-foundation.org, david@...hat.com, peterx@...hat.com,
ziy@...dia.com, baolin.wang@...ux.alibaba.com, baohua@...nel.org,
ryan.roberts@....com, dev.jain@....com, npache@...hat.com,
riel@...riel.com, Liam.Howlett@...cle.com, vbabka@...e.cz,
harry.yoo@...cle.com, jannh@...gle.com, matthew.brost@...el.com,
joshua.hahnjy@...il.com, rakie.kim@...com, byungchul@...com,
gourry@...rry.net, ying.huang@...ux.alibaba.com, apopple@...dia.com,
usamaarif642@...il.com, yuzhao@...gle.com,
linux-kernel@...r.kernel.org, linux-mm@...ck.org, ioworker0@...il.com,
stable@...r.kernel.org
Subject: Re: [PATCH v5 1/1] mm/rmap: fix soft-dirty and uffd-wp bit loss when
remapping zero-filled mTHP subpage to shared zeropage
Feels like the mTHP implementation is hitting up on the buffers of the THP code
being something of a mess... :)
On Tue, Sep 30, 2025 at 04:10:40PM +0800, Lance Yang wrote:
> From: Lance Yang <lance.yang@...ux.dev>
>
> When splitting an mTHP and replacing a zero-filled subpage with the shared
> zeropage, try_to_map_unused_to_zeropage() currently drops several important
> PTE bits.
>
> For userspace tools like CRIU, which rely on the soft-dirty mechanism for
It's slightly by-the-by, but CRIU in my view - as it relies on kernel
implementation details that can always change to operate - is not actually
something we have to strictly keep working.
HOWEVER, if we can reasonably do so without causing issues for us in the kernel
we ought to do so.
> incremental snapshots, losing the soft-dirty bit means modified pages are
> missed, leading to inconsistent memory state after restore.
>
> As pointed out by David, the more critical uffd-wp bit is also dropped.
> This breaks the userfaultfd write-protection mechanism, causing writes
> to be silently missed by monitoring applications, which can lead to data
> corruption.
Again, uffd-wp is a total mess. We shouldn't be in a position where its state
being correctly retained relies on everybody always getting the subtle,
uncommented, open-coded details right everywhere all the time.
But this is again a general comment... :)
>
> Preserve both the soft-dirty and uffd-wp bits from the old PTE when
> creating the new zeropage mapping to ensure they are correctly tracked.
>
> Cc: <stable@...r.kernel.org>
> Fixes: b1f202060afe ("mm: remap unused subpages to shared zeropage when splitting isolated thp")
> Suggested-by: David Hildenbrand <david@...hat.com>
> Suggested-by: Dev Jain <dev.jain@....com>
> Acked-by: David Hildenbrand <david@...hat.com>
> Reviewed-by: Dev Jain <dev.jain@....com>
> Signed-off-by: Lance Yang <lance.yang@...ux.dev>
Overall LGTM, so:
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> ---
> v4 -> v5:
> - Move ptep_get() call after the !pvmw.pte check, which handles PMD-mapped
> THP migration entries.
> - https://lore.kernel.org/linux-mm/20250930071053.36158-1-lance.yang@linux.dev/
>
> v3 -> v4:
> - Minor formatting tweak in try_to_map_unused_to_zeropage() function
> signature (per David and Dev)
> - Collect Reviewed-by from Dev - thanks!
> - https://lore.kernel.org/linux-mm/20250930060557.85133-1-lance.yang@linux.dev/
>
> v2 -> v3:
> - ptep_get() gets called only once per iteration (per Dev)
> - https://lore.kernel.org/linux-mm/20250930043351.34927-1-lance.yang@linux.dev/
>
> v1 -> v2:
> - Avoid calling ptep_get() multiple times (per Dev)
> - Double-check the uffd-wp bit (per David)
> - Collect Acked-by from David - thanks!
> - https://lore.kernel.org/linux-mm/20250928044855.76359-1-lance.yang@linux.dev/
>
> mm/migrate.c | 15 ++++++++++-----
> 1 file changed, 10 insertions(+), 5 deletions(-)
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index ce83c2c3c287..e3065c9edb55 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -296,8 +296,7 @@ bool isolate_folio_to_list(struct folio *folio, struct list_head *list)
> }
>
> static bool try_to_map_unused_to_zeropage(struct page_vma_mapped_walk *pvmw,
> - struct folio *folio,
> - unsigned long idx)
> + struct folio *folio, pte_t old_pte, unsigned long idx)
> {
> struct page *page = folio_page(folio, idx);
> pte_t newpte;
> @@ -306,7 +305,7 @@ static bool try_to_map_unused_to_zeropage(struct page_vma_mapped_walk *pvmw,
> return false;
> VM_BUG_ON_PAGE(!PageAnon(page), page);
> VM_BUG_ON_PAGE(!PageLocked(page), page);
> - VM_BUG_ON_PAGE(pte_present(ptep_get(pvmw->pte)), page);
> + VM_BUG_ON_PAGE(pte_present(old_pte), page);
Kinda ugly that we pass old_pte when it's avaiable via the shared state object,
but probably nothing to really concern ourselves about.
Guess you could argue it both ways :)
It'd be good to convert these VM_BUG_ON_*() to VM_WARN_ON_*() but I guess that's
somewhat out of the scope of the code here and would be inconsistent to change
it for just one condition.
>
> if (folio_test_mlocked(folio) || (pvmw->vma->vm_flags & VM_LOCKED) ||
> mm_forbids_zeropage(pvmw->vma->vm_mm))
> @@ -322,6 +321,12 @@ static bool try_to_map_unused_to_zeropage(struct page_vma_mapped_walk *pvmw,
>
> newpte = pte_mkspecial(pfn_pte(my_zero_pfn(pvmw->address),
> pvmw->vma->vm_page_prot));
> +
> + if (pte_swp_soft_dirty(old_pte))
> + newpte = pte_mksoft_dirty(newpte);
> + if (pte_swp_uffd_wp(old_pte))
> + newpte = pte_mkuffd_wp(newpte);
> +
> set_pte_at(pvmw->vma->vm_mm, pvmw->address, pvmw->pte, newpte);
>
> dec_mm_counter(pvmw->vma->vm_mm, mm_counter(folio));
> @@ -364,13 +369,13 @@ static bool remove_migration_pte(struct folio *folio,
> continue;
> }
> #endif
> + old_pte = ptep_get(pvmw.pte);
> if (rmap_walk_arg->map_unused_to_zeropage &&
> - try_to_map_unused_to_zeropage(&pvmw, folio, idx))
> + try_to_map_unused_to_zeropage(&pvmw, folio, old_pte, idx))
> continue;
>
> folio_get(folio);
> pte = mk_pte(new, READ_ONCE(vma->vm_page_prot));
> - old_pte = ptep_get(pvmw.pte);
>
> entry = pte_to_swp_entry(old_pte);
> if (!is_migration_entry_young(entry))
> --
> 2.49.0
>
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