On 6/1/26 06:56, Dev Jain wrote:
>
>
> On 01/06/26 12:41 am, David Hildenbrand (Arm) wrote:
>> On 5/29/26 13:17, Dev Jain wrote:
>>> pagemap_pmd_range_thp() warns if a non-present PMD is not a migration
>>> entry. This became false once device-private entries at the PMD level were
>>> added.
>>>
>>> One can hit the warning by patching hmm-tests.c with the following:
>>>
>>> diff --git a/tools/testing/selftests/mm/hmm-tests.c
>>> b/tools/testing/selftests/mm/hmm-tests.c
>>> index e1c8a679a4cf3..7f0a3384f3c5f 100644
>>> --- a/tools/testing/selftests/mm/hmm-tests.c
>>> +++ b/tools/testing/selftests/mm/hmm-tests.c
>>> @@ -209,6 +209,37 @@ static int hmm_dmirror_cmd(int fd,
>>> return 0;
>>> }
>>>
>>> +static int hmm_read_self_pagemap(void *addr, unsigned long npages,
>>> + unsigned long page_size)
>>> +{
>>> + const size_t entry_size = sizeof(uint64_t);
>>> + const off_t offset = ((uintptr_t)addr / page_size) * entry_size;
>>> + uint64_t *entries;
>>> + ssize_t nread;
>>> + int fd;
>>> +
>>> + entries = malloc(npages * entry_size);
>>> + if (!entries)
>>> + return -ENOMEM;
>>> +
>>> + fd = open("/proc/self/pagemap", O_RDONLY);
>>> + if (fd < 0) {
>>> + free(entries);
>>> + return -errno;
>>> + }
>>> +
>>> + nread = pread(fd, entries, npages * entry_size, offset);
>>> + close(fd);
>>> + free(entries);
>>> +
>>> + if (nread < 0)
>>> + return -errno;
>>> + if ((size_t)nread != npages * entry_size)
>>> + return -EIO;
>>> +
>>> + return 0;
>>> +}
>>> +
>>> static void hmm_buffer_free(struct hmm_buffer *buffer)
>>> {
>>> if (buffer == NULL)
>>> @@ -2314,6 +2345,10 @@ TEST_F(hmm, migrate_anon_huge_fault)
>>> ASSERT_EQ(ret, 0);
>>> ASSERT_EQ(buffer->cpages, npages);
>>>
>>> + /* Exercise pagemap on a PMD device-private entry. */
>>> + ret = hmm_read_self_pagemap(buffer->ptr, npages, self->page_size);
>>> + ASSERT_EQ(ret, 0);
>>> +
>>> /* Check what the device read. */
>>> for (i = 0, ptr = buffer->mirror; i < size / sizeof(*ptr); ++i)
>>> ASSERT_EQ(ptr[i], i);
>>>
>>>
>>
>>
>>> Therefore, remove the stale migration-only assertion.
>>>
>>> Fixes: a30b48bf1b24 ("mm/migrate_device: implement THP migration of zone
>>> device pages")
>>> Cc: [email protected]
>>> Signed-off-by: Dev Jain <[email protected]>
>>> ---
>>> Applies on mm-unstable (404fb4f38e8f).
>>>
>>> fs/proc/task_mmu.c | 1 -
>>> 1 file changed, 1 deletion(-)
>>>
>>> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
>>> index 1e3a15bf46f4e..58938e62154d9 100644
>>> --- a/fs/proc/task_mmu.c
>>> +++ b/fs/proc/task_mmu.c
>>> @@ -2129,7 +2129,6 @@ static int pagemap_pmd_range_thp(pmd_t *pmdp,
>>> unsigned long addr,
>>> flags |= PM_SOFT_DIRTY;
>>> if (pmd_swp_uffd_wp(pmd))
>>> flags |= PM_UFFD_WP;
>>> - VM_WARN_ON_ONCE(!pmd_is_migration_entry(pmd));
>>> page = softleaf_to_page(entry);
>>> }
>>>
>>
>> The whole thp_migration_supported() guard is a bit shaky, right?
>
> I think if you remove this, then you will trigger a WARN_ON in
> softleaf_to_page(),
> for the case of !CONFIG_ARCH_ENABLE_THP_MIGRATION.
Right, what I noted below.
Right now it's all a bit hacked on top of initial migration entry support.
>>
>> I guess device-private entries currently imply thp_migration_supported(), but
>> that thp_migration_supported() check is really questionable and should likely
>> just go away (else if -> else).
>>
>> Staring at pte_to_pagemap_entry(), likely we'd also want
>>
>> if (softleaf_has_pfn(entry))
>> page = softleaf_to_page(entry);
>>
>> to prepare for PMD swap entries.
>
> Correct, and this is done in
> https://lore.kernel.org/all/[email protected]/
>
> I think in addition to that, Usama can also remove the
> thp_migration_supported() check.
Yes, that one should go away.
--
Cheers,
David
