From: "David Hildenbrand (Arm)" <david@kernel.org>
To: Wei Yang <richard.weiyang@gmail.com>
Cc: Balbir Singh <balbirs@nvidia.com>,
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: Tue, 12 May 2026 20:55:47 +0200 [thread overview]
Message-ID: <113dddc5-27e3-4e9e-a90c-f076a4629f51@kernel.org> (raw)
In-Reply-To: <20260512143542.izpp3gu4iqxttw3f@master>
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);
>
> 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.
--
Cheers,
David
prev parent reply other threads:[~2026-05-12 18:55 UTC|newest]
Thread overview: 8+ 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) [this message]
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