Re: [PATCH v8 05/10] mm: thp: enable thp migration in generic path

[Zi Yan]

On 13 Jul 2017, at 5:30, Naoya Horiguchi wrote:

> On Tue, Jul 11, 2017 at 10:00:30AM -0400, Zi Yan wrote:
>> On 11 Jul 2017, at 2:47, Naoya Horiguchi wrote:
>>
>>> On Sat, Jul 01, 2017 at 09:40:03AM -0400, Zi Yan wrote:
>>>> From: Zi Yan <zi.yan@xxxxxxxxxxxxxx>
>>>>
>>>> This patch adds thp migration's core code, including conversions
>>>> between a PMD entry and a swap entry, setting PMD migration entry,
>>>> removing PMD migration entry, and waiting on PMD migration entries.
>>>>
>>>> This patch makes it possible to support thp migration.
>>>> If you fail to allocate a destination page as a thp, you just split
>>>> the source thp as we do now, and then enter the normal page migration.
>>>> If you succeed to allocate destination thp, you enter thp migration.
>>>> Subsequent patches actually enable thp migration for each caller of
>>>> page migration by allowing its get_new_page() callback to
>>>> allocate thps.
>>>>
>>>> ChangeLog v1 -> v2:
>>>> - support pte-mapped thp, doubly-mapped thp
>>>>
>>>> Signed-off-by: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
>>>>
>>>> ChangeLog v2 -> v3:
>>>> - use page_vma_mapped_walk()
>>>> - use pmdp_huge_clear_flush() instead of pmdp_huge_get_and_clear() in
>>>>   set_pmd_migration_entry()
>>>>
>>>> ChangeLog v3 -> v4:
>>>> - factor out the code of removing pte pgtable page in zap_huge_pmd()
>>>>
>>>> ChangeLog v4 -> v5:
>>>> - remove unnecessary PTE-mapped THP code in remove_migration_pmd()
>>>>   and set_pmd_migration_entry()
>>>> - restructure the code in zap_huge_pmd() to avoid factoring out
>>>>   the pte pgtable page code
>>>> - in zap_huge_pmd(), check that PMD swap entries are migration entries
>>>> - change author information
>>>>
>>>> ChangeLog v5 -> v7
>>>> - use macro to disable the code when thp migration is not enabled
>>>>
>>>> ChangeLog v7 -> v8
>>>> - use IS_ENABLED instead of macro to make code look clean in
>>>>   zap_huge_pmd() and page_vma_mapped_walk()
>>>> - remove BUILD_BUG() in pmd_to_swp_entry() and swp_entry_to_pmd() to
>>>>   avoid compilation error
>>>> - rename variable 'migration' to 'flush_needed' and invert the logic in
>>>>   zap_huge_pmd() to make code more descriptive
>>>> - use pmdp_invalidate() in set_pmd_migration_entry() to avoid race
>>>>   with MADV_DONTNEED
>>>> - remove unnecessary tlb flush in remove_migration_pmd()
>>>> - add the missing migration flag check in page_vma_mapped_walk()
>>>>
>>>> Signed-off-by: Zi Yan <zi.yan@xxxxxxxxxxxxxx>
>>>> Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
>>>> ---
>>>>  arch/x86/include/asm/pgtable_64.h |  2 +
>>>>  include/linux/swapops.h           | 67 ++++++++++++++++++++++++++++++-
>>>>  mm/huge_memory.c                  | 84 ++++++++++++++++++++++++++++++++++++---
>>>>  mm/migrate.c                      | 32 ++++++++++++++-
>>>>  mm/page_vma_mapped.c              | 18 +++++++--
>>>>  mm/pgtable-generic.c              |  3 +-
>>>>  mm/rmap.c                         | 13 ++++++
>>>>  7 files changed, 207 insertions(+), 12 deletions(-)
>>>>
>>> ...
>>>
>>>> diff --git a/mm/rmap.c b/mm/rmap.c
>>>> index 91948fbbb0bb..b28f633cd569 100644
>>>> --- a/mm/rmap.c
>>>> +++ b/mm/rmap.c
>>>> @@ -1302,6 +1302,7 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
>>>>  	bool ret = true;
>>>>  	enum ttu_flags flags = (enum ttu_flags)arg;
>>>>
>>>> +
>>>>  	/* munlock has nothing to gain from examining un-locked vmas */
>>>>  	if ((flags & TTU_MUNLOCK) && !(vma->vm_flags & VM_LOCKED))
>>>>  		return true;
>>>> @@ -1312,6 +1313,18 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
>>>>  	}
>>>>
>>>>  	while (page_vma_mapped_walk(&pvmw)) {
>>>> +#ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION
>>>> +		/* PMD-mapped THP migration entry */
>>>> +		if (flags & TTU_MIGRATION) {
>>>
>>> My testing based on mmotm-2017-07-06-16-18 showed that migrating shmem thp
>>> caused kernel crash. I don't think this is critical because that case is
>>> just not-prepared yet. So in order to avoid the crash, please add
>>> PageAnon(page) check here. This makes shmem thp migration just fail.
>>>
>>> +			if (!PageAnon(page))
>>> +				continue;
>>>
>>
>> Thanks for your testing. I will add this check in my next version.
>
> Sorry, the code I'm suggesting above doesn't work because it makes normal
> pagecache migration fail.  This check should come after making sure that
> pvmw.pte is NULL.

Right. I think the two ifs are confusing. Replacing the chunk with:

if (!pvmw.pte && (flags & TTU_MIGRATION)) {
    VM_BUG_ON_PAGE(PageHuge(page) || !PageTransCompound(page),
            page);

    if (!PageAnon(page))
        continue;

    set_pmd_migration_entry(&pvmw, page);
    continue;
}

would be better.

BTW, is your page migration test suite available online? If so, I could use
it to test my code.

Thanks.



â
Best Regards,
Yan Zi

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