From: Balbir Singh <balbirs@nvidia.com>
To: Wei Yang <richard.weiyang@gmail.com>
Cc: "David Hildenbrand (Arm)" <david@kernel.org>,
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 09:19:16 +1000 [thread overview]
Message-ID: <18ac53cf-72b3-4322-846e-7f6fd614172c@nvidia.com> (raw)
In-Reply-To: <20260512231442.53qwj37fbykp2qus@master>
On 5/13/26 09:14, Wei Yang wrote:
> On Wed, May 13, 2026 at 09:03:47AM +1000, Balbir Singh wrote:
>> 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?
>>
>
> Not get you clearly.
>
> You mean skip device-private pmd page in remove_migration_ptes()?
>
Yes, could you elaborate on why it's OK to skip device-private pmd
pages in remove_migration_ptes() and if so what is the impact?
>>>> 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 23:19 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
2026-05-12 23:14 ` Wei Yang
2026-05-12 23:19 ` Balbir Singh [this message]
2026-05-13 1:47 ` Balbir Singh
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