From: Lance Yang <lance.yang@linux.dev>
To: richard.weiyang@gmail.com, david@kernel.org, balbirs@nvidia.com
Cc: akpm@linux-foundation.org, ljs@kernel.org, riel@surriel.com,
liam@infradead.org, vbabka@kernel.org, harry@kernel.org,
jannh@google.com, ziy@nvidia.com, sj@kernel.org,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
stable@vger.kernel.org, Lance Yang <lance.yang@linux.dev>
Subject: Re: [Patch mm-hotfixes v4] mm/page_vma_mapped: fix device-private PMD handling
Date: Thu, 25 Jun 2026 19:42:35 +0800 [thread overview]
Message-ID: <20260625114235.40611-1-lance.yang@linux.dev> (raw)
In-Reply-To: <20260624085756.6598-1-lance.yang@linux.dev>
On Wed, Jun 24, 2026 at 04:57:56PM +0800, Lance Yang wrote:
>
[...]
>>
>>Fixes: 65edfda6f3f2 ("mm/rmap: extend rmap and migration support device-private entries")
>>Cc: <stable@vger.kernel.org>
>>Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
>>Suggested-by: David Hildenbrand <david@kernel.org>
>
>Shouldn't we add
>
>Suggested-by: Lorenzo Stoakes <ljs@kernel.org>
>
>as well?
No need to resend. I think Andrew can add this when applying :)
>v4 mostly follows Lorenzo's comments, code bits included. Feels only fair.
>
>>Cc: David Hildenbrand <david@kernel.org>
>>Cc: Balbir Singh <balbirs@nvidia.com>
>>Cc: SeongJae Park <sj@kernel.org>
>>Cc: Zi Yan <ziy@nvidia.com>
>>Cc: Lorenzo Stoakes <ljs@kernel.org>
>>Cc: Lance Yang <lance.yang@linux.dev>
>>
>>---
>>v4:
>> * refine subject and commit log based on Lorenzo's suggestion
>> * put pmd device-private entry handling in its own if branch,
>> suggested by Lorenzo
>>
>>v3:
>> * remove cleanup part, only fix the issue for device-private entry
>> * refine user effect description based on Lorenzo's suggestion
>>
>>v2: https://lore.kernel.org/all/20260616063436.20455-1-richard.weiyang@gmail.com/T/#u
>> * specify the possible error case of current code and user visible effect
>> * besides fix, cleanup the pmd entry handling based on David's suggestion
>>
>>v1: https://lore.kernel.org/linux-mm/20260508013728.21285-1-richard.weiyang@gmail.com/
>>---
>> mm/page_vma_mapped.c | 20 +++++++++++++++-----
>> 1 file changed, 15 insertions(+), 5 deletions(-)
>>
>>diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
>>index 2ccbabfb2cc1..17dff8aab9f9 100644
>>--- a/mm/page_vma_mapped.c
>>+++ b/mm/page_vma_mapped.c
>>@@ -269,14 +269,24 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
>
Never mind my race comment below. Obviously missed folio lock there. My
bad. Don't have a caller like that. Nothing else jumped out, so:
Reviewed-by: Lance Yang <lance.yang@linux.dev>
Cheers, Lance
>
>Hmm ... looks like there may still be a race here ...
>
>Current code picks the branch from the lockless PMD value:
>
> pmde = pmdp_get_lockless(pvmw->pmd);
>
> if (pmd_trans_huge(pmde) || pmd_is_migration_entry(pmde)) {
> pvmw->ptl = pmd_lock(mm, pvmw->pmd);
> pmde = *pvmw->pmd;
> if (!pmd_present(pmde)) {
> softleaf_t entry;
>
> if (!thp_migration_supported() ||
> !(pvmw->flags & PVMW_MIGRATION))
> return not_found(pvmw);
> entry = softleaf_from_pmd(pmde);
>
> if (!softleaf_is_migration(entry) ||
> !check_pmd(softleaf_to_pfn(entry), pvmw))
> return not_found(pvmw);
> return true;
> }
> }
>
>But after taking PTL, the PMD may already be a different non-present PMD
>type:
>
>CPU0: pmde = pmdp_get_lockless(); // sees PMD migration entry
>
>CPU1: remove_migration_ptes(src, dst /* device-private */)
> ... via rmap_walk(dst) ...
> page_vma_mapped_walk(&pvmw /* src, PVMW_MIGRATION */)
> returns with PTL held for the PMD migration entry
> remove_migration_pmd(new = dst page)
> installs a device-private PMD
> next page_vma_mapped_walk()
> drops PTL via not_found()
>
>CPU0: takes PTL
> pmde = *pvmw->pmd; // now device-private PMD
>
>So when PVMW_MIGRATION is not set, current code can return not_found()
>before we even decode the locked PMD as a device-private entry.
>
>Commit 65edfda6f3f2 ("mm/rmap: extend rmap and migration support
>device-private entries") made the
>
>device-private PMD <-> PMD migration
>
>transition possible.
>
>set_pmd_migration_entry() can replace a device-private PMD with a PMD
>migration entry, and remove_migration_pmd() can restore a PMD migration
>entry back to a device-private PMD when the new folio is device-private.
>
>Maybe decode the locked softleaf entry first, before the migration-only
>checks? Something like this on top:
>
>---8<---
>diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
>index 17dff8aab9f9..97babd408dba 100644
>--- a/mm/page_vma_mapped.c
>+++ b/mm/page_vma_mapped.c
>@@ -249,10 +249,18 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
> if (!pmd_present(pmde)) {
> softleaf_t entry;
>
>+ entry = softleaf_from_pmd(pmde);
>+ if (softleaf_is_device_private(entry)) {
>+ if (pvmw->flags & PVMW_MIGRATION)
>+ return not_found(pvmw);
>+ if (!check_pmd(softleaf_to_pfn(entry), pvmw))
>+ return not_found(pvmw);
>+ return true;
>+ }
>+
> if (!thp_migration_supported() ||
> !(pvmw->flags & PVMW_MIGRATION))
> return not_found(pvmw);
>- entry = softleaf_from_pmd(pmde);
>
> if (!softleaf_is_migration(entry) ||
> !check_pmd(softleaf_to_pfn(entry), pvmw))
>@@ -266,7 +274,10 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
> return not_found(pvmw);
> return true;
> }
>- /* THP pmd was split under us: handle on pte level */
>+ /*
>+ * THP pmd was split under us, or device-private PMD
>+ * changed under us: handle on pte level.
>+ */
> spin_unlock(pvmw->ptl);
> pvmw->ptl = NULL;
> } else if (pmd_is_device_private_entry(pmde)) {
>--
>
>Anyway, that stuff is getting kinda messy now. Feels like it really needs
>a cleanup on top before it bites us again :)
>
>Cheers, Lance
>
>> /* THP pmd was split under us: handle on pte level */
>> spin_unlock(pvmw->ptl);
>> pvmw->ptl = NULL;
>>- } else if (!pmd_present(pmde)) {
>>- const softleaf_t entry = softleaf_from_pmd(pmde);
>>+ } else if (pmd_is_device_private_entry(pmde)) {
>>+ softleaf_t entry;
>>+
>>+ pvmw->ptl = pmd_lock(mm, pvmw->pmd);
>>+ pmde = *pvmw->pmd;
>>+ entry = softleaf_from_pmd(pmde);
>>
>>- if (softleaf_is_device_private(entry)) {
>>- pvmw->ptl = pmd_lock(mm, pvmw->pmd);
>>+ if (likely(softleaf_is_device_private(entry))) {
>>+ if (pvmw->flags & PVMW_MIGRATION)
>>+ return not_found(pvmw);
>>+ if (!check_pmd(softleaf_to_pfn(entry), pvmw))
>>+ return not_found(pvmw);
>> return true;
>> }
>>-
>>+ /* device-private pmd was split under us: handle on pte level */
>>+ spin_unlock(pvmw->ptl);
>>+ pvmw->ptl = NULL;
>>+ } else if (!pmd_present(pmde)) {
>> if ((pvmw->flags & PVMW_SYNC) &&
>> thp_vma_suitable_order(vma, pvmw->address,
>> PMD_ORDER) &&
>>--
>>2.34.1
>>
>>
>
next prev parent reply other threads:[~2026-06-25 11:42 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-06-24 6:53 [Patch mm-hotfixes v4] mm/page_vma_mapped: fix device-private PMD handling Wei Yang
2026-06-24 8:57 ` Lance Yang
2026-06-25 9:57 ` Wei Yang
2026-06-25 10:37 ` David Hildenbrand (Arm)
2026-06-25 11:25 ` Lance Yang
2026-06-25 11:42 ` Lance Yang [this message]
2026-06-25 13:12 ` Lorenzo Stoakes
2026-06-25 11:12 ` Balbir Singh
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