Date:   Fri, 14 Jul 2017 00:06:42 +0000
From:   Naoya Horiguchi <n-horiguchi@...jp.nec.com>
To:     Zi Yan <zi.yan@...rutgers.edu>
CC:     "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "minchan@...nel.org" <minchan@...nel.org>,
        "vbabka@...e.cz" <vbabka@...e.cz>,
        "mgorman@...hsingularity.net" <mgorman@...hsingularity.net>,
        "mhocko@...nel.org" <mhocko@...nel.org>,
        "khandual@...ux.vnet.ibm.com" <khandual@...ux.vnet.ibm.com>,
        "dnellans@...dia.com" <dnellans@...dia.com>,
        "dave.hansen@...el.com" <dave.hansen@...el.com>
Subject: Re: [PATCH v8 05/10] mm: thp: enable thp migration in generic path

On Thu, Jul 13, 2017 at 07:28:24AM -0400, Zi Yan wrote:
> 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@...rutgers.edu>
> >>>>
> >>>> 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@...jp.nec.com>
> >>>>
> >>>> 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@...rutgers.edu>
> >>>> Cc: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> >>>> ---
> >>>>  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.

Yes, it looks good.

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

Please refer to https://github.com/Naoya-Horiguchi/mm_regression.

Thanks,
Naoya Horiguchi

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