From: Balbir Singh <balbirs@nvidia.com>
To: "David Hildenbrand (Arm)" <david@kernel.org>,
Wei Yang <richard.weiyang@gmail.com>
Cc: akpm@linux-foundation.org, ljs@kernel.org, riel@surriel.com,
liam@infradead.org, vbabka@kernel.org, harry@kernel.org,
jannh@google.com, sj@kernel.org, ziy@nvidia.com,
linux-mm@kvack.org, Lorenzo Stoakes <lorenzo.stoakes@oracle.com>,
stable@vger.kernel.org
Subject: Re: [PATCH] mm/page_vma_mapped: revalidate and do proper check before return device-private pmd
Date: Wed, 13 May 2026 09:03:47 +1000 [thread overview]
Message-ID: <9a56d762-ebe5-429e-9fc8-a9c9e5d0d434@nvidia.com> (raw)
In-Reply-To: <113dddc5-27e3-4e9e-a90c-f076a4629f51@kernel.org>
On 5/13/26 04:55, David Hildenbrand (Arm) wrote:
> On 5/12/26 16:35, Wei Yang wrote:
>> On Tue, May 12, 2026 at 02:43:54PM +0200, David Hildenbrand (Arm) wrote:
>>> On 5/9/26 00:48, Balbir Singh wrote:
>>>>
>>>> Could you elaborate a more on the improper situation?
>>>>
>>>>
>>>> Do we need to check softleaf_is_device_private() twice, can't we hold the pmd
>>>> lock and check once?
>>>
>>> I think what we try to do here is, is to only grab the lock if we verified that there is something of interest in there.
>>>
>>> I wonder if we should rewrite that whole thing to just do a pmd_same() check after grabbing the lock.
>>>
>>> Something a lot cleaner like:
>>>
>>> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
>>> index a4d52fdb3056..de6a255cc847 100644
>>> --- a/mm/page_vma_mapped.c
>>> +++ b/mm/page_vma_mapped.c
>>> @@ -242,40 +242,28 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
>>> */
>>> 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;
>>> - }
>>> - if (likely(pmd_trans_huge(pmde))) {
>>> - if (pvmw->flags & PVMW_MIGRATION)
>>> - return not_found(pvmw);
>>> - if (!check_pmd(pmd_pfn(pmde), pvmw))
>>> - return not_found(pvmw);
>>> - return true;
>>> - }
>>> - /* 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);
>>> -
>>> - if (softleaf_is_device_private(entry)) {
>>> - pvmw->ptl = pmd_lock(mm, pvmw->pmd);
>>> - return true;
>>> - }
>>> + if (pmd_present(pmde)) {
>>> + if (!pmd_leaf(pmde))
>>> + goto pte_table;
>>> + if (pvmw->flags & PVMW_MIGRATION)
>>> + return not_found(pvmw);
>>> + if (!check_pmd(pmd_pfn(pmde), pvmw))
>>> + return not_found(pvmw);
>>> + } else if (pmd_is_migration_entry(pmde)) {
>>> + softleaf_t entry = softleaf_from_pmd(pmde);
>>> +
>>> + if (!(pvmw->flags & PVMW_MIGRATION))
>>> + return not_found(pvmw);
>>> + if (!check_pmd(softleaf_to_pfn(entry), pvmw))
>>> + return not_found(pvmw);
>>> + } else if (pmd_is_device_private_entry(pmde)) {
>>> + softleaf_t entry = softleaf_from_pmd(pmde);
>>>
>>> + if (pvmw->flags & PVMW_MIGRATION)
>>> + return not_found(pvmw);
>>> + if (!check_pmd(softleaf_to_pfn(entry), pvmw))
>>> + return not_found(pvmw);
>>> + } else {
>>> if ((pvmw->flags & PVMW_SYNC) &&
>>> thp_vma_suitable_order(vma, pvmw->address,
>>> PMD_ORDER) &&
>>> @@ -285,6 +273,15 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
>>> step_forward(pvmw, PMD_SIZE);
>>> continue;
>>> }
>>> +
>>> + /* Double-check under PTL that the PMD didn't change. */
>>> + pvmw->ptl = pmd_lock(mm, pvmw->pmd);
>>> + if (pmd_same(pmde, pmdp_get(pvmw->pmd)))
>>> + return true;
>>> + spin_unlock(pvmw->ptl);
>>> + pvmw->ptl = NULL;
>>> + goto restart;
>>> +pte_table:
>>> if (!map_pte(pvmw, &pmde, &ptl)) {
>>> if (!pvmw->pte)
>>>
>>>
>>>
>>>
>>> There is likely room to clean this up / compress it further.
>>
>> I tried to compress above logic like this, hope it could look cleaner.
>>
>> if (pmd_trans_huge(pmde) || pmd_is_valid_softleaf(pmde)) {
>> unsigned long pfn;
>> bool is_migration = pmd_is_migration_entry(pmde);
>> bool for_migration = !!(pvmw->flags & PVMW_MIGRATION);
>>
>> if (is_migration != for_migration)
>> return not_found(pvmw);
>>
I got some time to look at PVMW_MIGRATION, remove_migration_ptes
is invoked for device private pages, would we want them to skip
device private pmd pages?
>> if (pmd_trans_huge(pmde))
>> pfn = pmd_pfn(pmde);
>> else
>> pfn = softleaf_to_pfn(softleaf_from_pmd(pmde));
>>
>> if (!check_pmd(pfn, pvmw))
>> return not_found(pvmw);
>> } else if (!pmd_present(pmde)) {
>
> It's more compact, but not necessarily cleaner. In particular, I detest
> pmd_trans_huge(), we should phase it out.
>
> if (pmd_present(pmde) && !pmd_leaf(pmde)) {
> goto pte_table;
> } else if (pmd_present(pmde) || pmd_is_valid_softleaf(pmde))
>
> ...
>
> Might work as well. But once we add support for other softleaf types, we'll have
> to touch it again. So I'd rather just list what we actually expect.
>
Balbir
next prev parent reply other threads:[~2026-05-12 23:04 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-05-08 1:37 [PATCH] mm/page_vma_mapped: revalidate and do proper check before return device-private pmd Wei Yang
2026-05-08 21:51 ` Andrew Morton
2026-05-10 1:22 ` Wei Yang
2026-05-08 22:48 ` Balbir Singh
2026-05-10 1:20 ` Wei Yang
2026-05-12 12:43 ` David Hildenbrand (Arm)
2026-05-12 14:35 ` Wei Yang
2026-05-12 18:55 ` David Hildenbrand (Arm)
2026-05-12 23:03 ` Balbir Singh [this message]
2026-05-12 23:14 ` Wei Yang
2026-05-12 23:19 ` Balbir Singh
2026-05-13 1:47 ` Balbir Singh
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