Re: [PATCH] mm/migrate_device: avoid overflowing migrate_vma collection arrays

[Zi Yan]

On Tue Jul 7, 2026 at 9:50 PM EDT, Zi Yan wrote:
> migrate_vma_collect_pmd() can drop pte lock to split a large folio and
> restart. But the code does not handle restart properly when pmd becomes
> huge or cleared. It can overflow migrate->dst and migrate->src arrays
> during the hole or skip collection. Fix it by:
> 1. avoiding migrate_vma_collect_huge_pmd() if some collection is done,
> 2. skipping the rest of the range if pmd no longer points to a pte page
>    table.
>
> Fixes: a30b48bf1b244 ("mm/migrate_device: implement THP migration of zone device pages")
> Reported-by: Sashiko <sashiko-bot@xxxxxxxxxx>
> Closes: https://sashiko.dev/#/patchset/20260706111958.3649651-1-wangkefeng.wang@xxxxxxxxxx
> Assisted-by: Claude:claude-opus-4-8
> Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
> ---
> The issue is spot by Sashiko during a patch[1] review as a pre-existing one.
> This patch has the minimal change. An alternative is to reset
> migrate->cpages and migrate->npages and restart the whole range from the
> beginning, but that also requires a restoration of no-longer-present PTEs.
>
> Link: https://lore.kernel.org/all/20260706111958.3649651-1-wangkefeng.wang@xxxxxxxxxx/ [1]
> ---
>  mm/migrate_device.c | 19 +++++++++++++++++--
>  1 file changed, 17 insertions(+), 2 deletions(-)
>

Sashiko has a new concern:

https://sashiko.dev/#/patchset/20260707-fix-array-overflow-in-migrate_vma_collect_pmd-v1-1-ce3ff4627653@xxxxxxxxxx

Q: migrate_vma_collect_pmd() drops the page table lock without flushing
the TLB leave stale entries active? When an order-0 page is collected and
its present PTE becomes a migration entry without flushing TLBs, later
CPUs with the stale TLB can still write to the page, potentially leading
to data corruption.

Answer: No, migrate_vma_collect_pmd() adds a reference to the collected
pages, so these pages are not going away. In addition, unmapped is
incremented after a page collection and warrants a TLB flush before the
code exits migrate_vma_collect_pmd():

1. pte_offset_map_lock() fails, this patch adds a flush.
2. split fails, a flush is already in the code.
3. the whole range is processed, a flush is at the end of the function.


-- 
Best Regards,
Yan, Zi