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Message-Id: <D2WJD87NON2T.ZE0SCIMOBRIH@nvidia.com>
Date: Mon, 22 Jul 2024 21:43:43 -0400
From: "Zi Yan" <ziy@...dia.com>
To: "Huang, Ying" <ying.huang@...el.com>
Cc: "David Hildenbrand" <david@...hat.com>, <linux-mm@...ck.org>, "Andrew
Morton" <akpm@...ux-foundation.org>, "Baolin Wang"
<baolin.wang@...ux.alibaba.com>, <linux-kernel@...r.kernel.org>, "Mel
Gorman" <mgorman@...hsingularity.net>
Subject: Re: [RFC PATCH 3/3] mm/migrate: move common code to
numa_migrate_check (was numa_migrate_prep)
On Mon Jul 22, 2024 at 9:16 PM EDT, Huang, Ying wrote:
> Zi Yan <ziy@...dia.com> writes:
>
> > On 22 Jul 2024, at 10:01, Zi Yan wrote:
> >
> >> On 21 Jul 2024, at 21:47, Huang, Ying wrote:
> >>
> >>> Zi Yan <ziy@...dia.com> writes:
> >>>
> >>>> On 18 Jul 2024, at 4:36, Huang, Ying wrote:
> >>>>
> >>>>> Zi Yan <zi.yan@...t.com> writes:
> >>>>>
> >>>>>> From: Zi Yan <ziy@...dia.com>
> >>>>>>
> >>>>>> do_numa_page() and do_huge_pmd_numa_page() share a lot of common code. To
> >>>>>> reduce redundancy, move common code to numa_migrate_prep() and rename
> >>>>>> the function to numa_migrate_check() to reflect its functionality.
> >>>>>>
> >>>>>> There is some code difference between do_numa_page() and
> >>>>>> do_huge_pmd_numa_page() before the code move:
> >>>>>>
> >>>>>> 1. do_huge_pmd_numa_page() did not check shared folios to set TNF_SHARED.
> >>>>>> 2. do_huge_pmd_numa_page() did not check and skip zone device folios.
> >>>>>>
> >>>>>> Signed-off-by: Zi Yan <ziy@...dia.com>
> >>>>>> ---
> >>>>>> mm/huge_memory.c | 28 ++++++-----------
> >>>>>> mm/internal.h | 5 +--
> >>>>>> mm/memory.c | 81 +++++++++++++++++++++++-------------------------
> >>>>>> 3 files changed, 52 insertions(+), 62 deletions(-)
> >>>>>>
> >>>>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> >>>>>> index 8c11d6da4b36..66d67d13e0dc 100644
> >>>>>> --- a/mm/huge_memory.c
> >>>>>> +++ b/mm/huge_memory.c
> >>>>>> @@ -1670,10 +1670,10 @@ vm_fault_t do_huge_pmd_numa_page(struct vm_fault *vmf)
> >>>>>> pmd_t pmd;
> >>>>>> struct folio *folio;
> >>>>>> unsigned long haddr = vmf->address & HPAGE_PMD_MASK;
> >>>>>> - int nid = NUMA_NO_NODE;
> >>>>>> - int target_nid, last_cpupid = (-1 & LAST_CPUPID_MASK);
> >>>>>> + int target_nid = NUMA_NO_NODE;
> >>>>>> + int last_cpupid = (-1 & LAST_CPUPID_MASK);
> >>>>>> bool writable = false;
> >>>>>> - int flags = 0;
> >>>>>> + int flags = 0, nr_pages;
> >>>>>>
> >>>>>> vmf->ptl = pmd_lock(vma->vm_mm, vmf->pmd);
> >>>>>> if (unlikely(!pmd_same(oldpmd, *vmf->pmd))) {
> >>>>>> @@ -1693,21 +1693,13 @@ vm_fault_t do_huge_pmd_numa_page(struct vm_fault *vmf)
> >>>>>> writable = true;
> >>>>>>
> >>>>>> folio = vm_normal_folio_pmd(vma, haddr, pmd);
> >>>>>> - if (!folio)
> >>>>>> + if (!folio || folio_is_zone_device(folio))
> >>>>>
> >>>>> This change appears unrelated. Can we put it in a separate patch?
> >>>>>
> >>>>> IIUC, this isn't necessary even in do_numa_page()? Because in
> >>>>> change_pte_range(), folio_is_zone_device() has been checked already.
> >>>>> But It doesn't hurt too.
> >>>>>
> >>>>>> goto out_map;
> >>>>>>
> >>>>>> - /* See similar comment in do_numa_page for explanation */
> >>>>>> - if (!writable)
> >>>>>> - flags |= TNF_NO_GROUP;
> >>>>>> + nr_pages = folio_nr_pages(folio);
> >>>>>>
> >>>>>> - nid = folio_nid(folio);
> >>>>>> - /*
> >>>>>> - * For memory tiering mode, cpupid of slow memory page is used
> >>>>>> - * to record page access time. So use default value.
> >>>>>> - */
> >>>>>> - if (folio_has_cpupid(folio))
> >>>>>> - last_cpupid = folio_last_cpupid(folio);
> >>>>>> - target_nid = numa_migrate_prep(folio, vmf, haddr, nid, &flags);
> >>>>>> + target_nid = numa_migrate_check(folio, vmf, haddr, writable,
> >>>>>> + &flags, &last_cpupid);
> >>>>>> if (target_nid == NUMA_NO_NODE)
> >>>>>> goto out_map;
> >>>>>> if (migrate_misplaced_folio_prepare(folio, vma, target_nid)) {
> >>>>>> @@ -1720,8 +1712,8 @@ vm_fault_t do_huge_pmd_numa_page(struct vm_fault *vmf)
> >>>>>>
> >>>>>> if (!migrate_misplaced_folio(folio, vma, target_nid)) {
> >>>>>> flags |= TNF_MIGRATED;
> >>>>>> - nid = target_nid;
> >>>>>> } else {
> >>>>>> + target_nid = NUMA_NO_NODE;
> >>>>>> flags |= TNF_MIGRATE_FAIL;
> >>>>>> vmf->ptl = pmd_lock(vma->vm_mm, vmf->pmd);
> >>>>>> if (unlikely(!pmd_same(oldpmd, *vmf->pmd))) {
> >>>>>> @@ -1732,8 +1724,8 @@ vm_fault_t do_huge_pmd_numa_page(struct vm_fault *vmf)
> >>>>>> }
> >>>>>>
> >>>>>> out:
> >>>>>> - if (nid != NUMA_NO_NODE)
> >>>>>> - task_numa_fault(last_cpupid, nid, HPAGE_PMD_NR, flags);
> >>>>>> + if (target_nid != NUMA_NO_NODE)
> >>>>>> + task_numa_fault(last_cpupid, target_nid, nr_pages, flags);
> >>>>>
> >>>>> This appears a behavior change. IIUC, there are 2 possible issues.
> >>>>>
> >>>>> 1) if migrate_misplaced_folio() fails, folio_nid() should be used as
> >>>>> nid. "target_nid" as variable name here is confusing, because
> >>>>> folio_nid() is needed in fact.
> >>>>>
> >>>>> 2) if !pmd_same(), task_numa_fault() should be skipped. The original
> >>>>> code is buggy.
> >>>>>
> >>>>> Similar issues for do_numa_page().
> >>>>>
> >>>>> If my understanding were correct, we should implement a separate patch
> >>>>> to fix 2) above. And that may need to be backported.
> >>>>
> >>>> Hmm, the original code seems OK after I checked the implementation.
> >>>> There are two possible !pte_same()/!pmd_same() locations:
> >>>> 1) at the beginning of do_numa_page() and do_huge_pmd_numa_page() and the faulted
> >>>> PTE/PMD changed before the folio can be checked, task_numa_fault() should not be
> >>>> called.
> >>>
> >>> Yes.
> >>>
> >>>> 2) when migrate_misplaced_folio() failed and the PTE/PMD changed, but the folio
> >>>> has been determined and checked. task_numa_fault() should be called even if
> >>>> !pte_same()/!pmd_same(),
> >>>
> >>> IIUC, if !pte_same()/!pmd_same(), the fault has been processed on
> >>> another CPU. For example, do_numa_page()/do_huge_pmd_numa_page() has
> >>> been called on another CPU and task_numa_fault() has been called for the
> >>> PTE/PMD already.
> >>
> >> Hmm, this behavior at least dates back to 2015 at
> >> commit 074c238177a7 ("mm: numa: slow PTE scan rate if migration failures occur”).
> >> So cc Mel.
> >>
> >> The code is https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/mm/memory.c?id=074c238177a75f5e79af3b2cb6a84e54823ef950#n3102. I have not checked older
> >> commits.
> >>
> >> I wonder how far we should trace back.
> >
> > OK, I find the commit where task_numa_fault policy settled:
> > 8191acbd30c7 ("mm: numa: Sanitize task_numa_fault() callsites”).
> >
> > It says:
> > “So modify all three sites to always account; we did after all receive
> > the fault; and always account to where the page is after migration,
> > regardless of success.“, where the three call sites were:
> > do_huge_pmd_numa_page(), do_numa_page(), and do_pmd_numa_page().
> >
> > The current code still follows what the commit log does.
>
> Per my understanding, the issue is introduced in commit b99a342d4f11
> ("NUMA balancing: reduce TLB flush via delaying mapping on hint page
> fault"). Before that, the PTE is restored before migration, so
> task_numa_fault() should be called for migration failure too. After
> that, the PTE is restored after migration failure, if the PTE has been
> changed by someone else, someone else should have called
> task_numa_fault() if necessary, we shouldn't call it again.
You are right. Will fix the issue. Thank you for the explanation.
--
Best Regards,
Yan, Zi
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