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Message-ID: <a21a4dbe-ce2f-417b-8e21-3e846d7202d7@redhat.com>
Date: Thu, 18 Sep 2025 13:57:20 +0200
From: David Hildenbrand <david@...hat.com>
To: Usama Arif <usamaarif642@...il.com>,
Qun-wei Lin (林群崴) <Qun-wei.Lin@...iatek.com>,
"catalin.marinas@....com" <catalin.marinas@....com>,
"linux-mm@...ck.org" <linux-mm@...ck.org>,
"yuzhao@...gle.com" <yuzhao@...gle.com>,
"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
Cc: "corbet@....net" <corbet@....net>,
Andrew Yang (楊智強) <Andrew.Yang@...iatek.com>,
"npache@...hat.com" <npache@...hat.com>, "rppt@...nel.org"
<rppt@...nel.org>, "willy@...radead.org" <willy@...radead.org>,
"kernel-team@...a.com" <kernel-team@...a.com>,
"roman.gushchin@...ux.dev" <roman.gushchin@...ux.dev>,
"hannes@...xchg.org" <hannes@...xchg.org>,
"cerasuolodomenico@...il.com" <cerasuolodomenico@...il.com>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"ryncsn@...il.com" <ryncsn@...il.com>, "surenb@...gle.com"
<surenb@...gle.com>, "riel@...riel.com" <riel@...riel.com>,
"shakeel.butt@...ux.dev" <shakeel.butt@...ux.dev>,
Chinwen Chang (張錦文)
<chinwen.chang@...iatek.com>,
"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
Casper Li (李中榮) <casper.li@...iatek.com>,
"ryan.roberts@....com" <ryan.roberts@....com>,
"linux-mediatek@...ts.infradead.org" <linux-mediatek@...ts.infradead.org>,
"baohua@...nel.org" <baohua@...nel.org>,
"kaleshsingh@...gle.com" <kaleshsingh@...gle.com>,
"zhais@...gle.com" <zhais@...gle.com>,
"linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v5 2/6] mm: remap unused subpages to shared zeropage when
splitting isolated thp
On 18.09.25 13:42, Usama Arif wrote:
>
>
> On 18/09/2025 09:56, David Hildenbrand wrote:
>> On 18.09.25 10:53, Qun-wei Lin (林群崴) wrote:
>>> On Fri, 2024-08-30 at 11:03 +0100, Usama Arif wrote:
>>>> From: Yu Zhao <yuzhao@...gle.com>
>>>>
>>>> Here being unused means containing only zeros and inaccessible to
>>>> userspace. When splitting an isolated thp under reclaim or migration,
>>>> the unused subpages can be mapped to the shared zeropage, hence
>>>> saving
>>>> memory. This is particularly helpful when the internal
>>>> fragmentation of a thp is high, i.e. it has many untouched subpages.
>>>>
>>>> This is also a prerequisite for THP low utilization shrinker which
>>>> will
>>>> be introduced in later patches, where underutilized THPs are split,
>>>> and
>>>> the zero-filled pages are freed saving memory.
>>>>
>>>> Signed-off-by: Yu Zhao <yuzhao@...gle.com>
>>>> Tested-by: Shuang Zhai <zhais@...gle.com>
>>>> Signed-off-by: Usama Arif <usamaarif642@...il.com>
>>>> ---
>>>> include/linux/rmap.h | 7 ++++-
>>>> mm/huge_memory.c | 8 ++---
>>>> mm/migrate.c | 72 ++++++++++++++++++++++++++++++++++++++----
>>>> --
>>>> mm/migrate_device.c | 4 +--
>>>> 4 files changed, 75 insertions(+), 16 deletions(-)
>>>>
>>>> diff --git a/include/linux/rmap.h b/include/linux/rmap.h
>>>> index 91b5935e8485..d5e93e44322e 100644
>>>> --- a/include/linux/rmap.h
>>>> +++ b/include/linux/rmap.h
>>>> @@ -745,7 +745,12 @@ int folio_mkclean(struct folio *);
>>>> int pfn_mkclean_range(unsigned long pfn, unsigned long nr_pages,
>>>> pgoff_t pgoff,
>>>> struct vm_area_struct *vma);
>>>> -void remove_migration_ptes(struct folio *src, struct folio *dst,
>>>> bool locked);
>>>> +enum rmp_flags {
>>>> + RMP_LOCKED = 1 << 0,
>>>> + RMP_USE_SHARED_ZEROPAGE = 1 << 1,
>>>> +};
>>>> +
>>>> +void remove_migration_ptes(struct folio *src, struct folio *dst, int
>>>> flags);
>>>> /*
>>>> * rmap_walk_control: To control rmap traversing for specific needs
>>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>>> index 0c48806ccb9a..af60684e7c70 100644
>>>> --- a/mm/huge_memory.c
>>>> +++ b/mm/huge_memory.c
>>>> @@ -3020,7 +3020,7 @@ bool unmap_huge_pmd_locked(struct
>>>> vm_area_struct *vma, unsigned long addr,
>>>> return false;
>>>> }
>>>> -static void remap_page(struct folio *folio, unsigned long nr)
>>>> +static void remap_page(struct folio *folio, unsigned long nr, int
>>>> flags)
>>>> {
>>>> int i = 0;
>>>> @@ -3028,7 +3028,7 @@ static void remap_page(struct folio *folio,
>>>> unsigned long nr)
>>>> if (!folio_test_anon(folio))
>>>> return;
>>>> for (;;) {
>>>> - remove_migration_ptes(folio, folio, true);
>>>> + remove_migration_ptes(folio, folio, RMP_LOCKED |
>>>> flags);
>>>> i += folio_nr_pages(folio);
>>>> if (i >= nr)
>>>> break;
>>>> @@ -3240,7 +3240,7 @@ static void __split_huge_page(struct page
>>>> *page, struct list_head *list,
>>>> if (nr_dropped)
>>>> shmem_uncharge(folio->mapping->host, nr_dropped);
>>>> - remap_page(folio, nr);
>>>> + remap_page(folio, nr, PageAnon(head) ?
>>>> RMP_USE_SHARED_ZEROPAGE : 0);
>>>> /*
>>>> * set page to its compound_head when split to non order-0
>>>> pages, so
>>>> @@ -3542,7 +3542,7 @@ int split_huge_page_to_list_to_order(struct
>>>> page *page, struct list_head *list,
>>>> if (mapping)
>>>> xas_unlock(&xas);
>>>> local_irq_enable();
>>>> - remap_page(folio, folio_nr_pages(folio));
>>>> + remap_page(folio, folio_nr_pages(folio), 0);
>>>> ret = -EAGAIN;
>>>> }
>>>> diff --git a/mm/migrate.c b/mm/migrate.c
>>>> index 6f9c62c746be..d039863e014b 100644
>>>> --- a/mm/migrate.c
>>>> +++ b/mm/migrate.c
>>>> @@ -204,13 +204,57 @@ bool isolate_folio_to_list(struct folio *folio,
>>>> struct list_head *list)
>>>> return true;
>>>> }
>>>> +static bool try_to_map_unused_to_zeropage(struct
>>>> page_vma_mapped_walk *pvmw,
>>>> + struct folio *folio,
>>>> + unsigned long idx)
>>>> +{
>>>> + struct page *page = folio_page(folio, idx);
>>>> + bool contains_data;
>>>> + pte_t newpte;
>>>> + void *addr;
>>>> +
>>>> + VM_BUG_ON_PAGE(PageCompound(page), page);
>>>> + VM_BUG_ON_PAGE(!PageAnon(page), page);
>>>> + VM_BUG_ON_PAGE(!PageLocked(page), page);
>>>> + VM_BUG_ON_PAGE(pte_present(*pvmw->pte), page);
>>>> +
>>>> + if (folio_test_mlocked(folio) || (pvmw->vma->vm_flags &
>>>> VM_LOCKED) ||
>>>> + mm_forbids_zeropage(pvmw->vma->vm_mm))
>>>> + return false;
>>>> +
>>>> + /*
>>>> + * The pmd entry mapping the old thp was flushed and the pte
>>>> mapping
>>>> + * this subpage has been non present. If the subpage is only
>>>> zero-filled
>>>> + * then map it to the shared zeropage.
>>>> + */
>>>> + addr = kmap_local_page(page);
>>>> + contains_data = memchr_inv(addr, 0, PAGE_SIZE);
>>>> + kunmap_local(addr);
>>>> +
>>>> + if (contains_data)
>>>> + return false;
>>>> +
>>>> + newpte = pte_mkspecial(pfn_pte(my_zero_pfn(pvmw->address),
>>>> + pvmw->vma->vm_page_prot));
>>>> + set_pte_at(pvmw->vma->vm_mm, pvmw->address, pvmw->pte,
>>>> newpte);
>>>> +
>>>> + dec_mm_counter(pvmw->vma->vm_mm, mm_counter(folio));
>>>> + return true;
>>>> +}
>>>> +
>>>> +struct rmap_walk_arg {
>>>> + struct folio *folio;
>>>> + bool map_unused_to_zeropage;
>>>> +};
>>>> +
>>>> /*
>>>> * Restore a potential migration pte to a working pte entry
>>>> */
>>>> static bool remove_migration_pte(struct folio *folio,
>>>> - struct vm_area_struct *vma, unsigned long addr, void
>>>> *old)
>>>> + struct vm_area_struct *vma, unsigned long addr, void
>>>> *arg)
>>>> {
>>>> - DEFINE_FOLIO_VMA_WALK(pvmw, old, vma, addr, PVMW_SYNC |
>>>> PVMW_MIGRATION);
>>>> + struct rmap_walk_arg *rmap_walk_arg = arg;
>>>> + DEFINE_FOLIO_VMA_WALK(pvmw, rmap_walk_arg->folio, vma, addr,
>>>> PVMW_SYNC | PVMW_MIGRATION);
>>>> while (page_vma_mapped_walk(&pvmw)) {
>>>> rmap_t rmap_flags = RMAP_NONE;
>>>> @@ -234,6 +278,9 @@ static bool remove_migration_pte(struct folio
>>>> *folio,
>>>> continue;
>>>> }
>>>> #endif
>>>> + if (rmap_walk_arg->map_unused_to_zeropage &&
>>>> + try_to_map_unused_to_zeropage(&pvmw, folio,
>>>> idx))
>>>> + continue;
>>>> folio_get(folio);
>>>> pte = mk_pte(new, READ_ONCE(vma->vm_page_prot));
>>>> @@ -312,14 +359,21 @@ static bool remove_migration_pte(struct folio
>>>> *folio,
>>>> * Get rid of all migration entries and replace them by
>>>> * references to the indicated page.
>>>> */
>>>> -void remove_migration_ptes(struct folio *src, struct folio *dst,
>>>> bool locked)
>>>> +void remove_migration_ptes(struct folio *src, struct folio *dst, int
>>>> flags)
>>>> {
>>>> + struct rmap_walk_arg rmap_walk_arg = {
>>>> + .folio = src,
>>>> + .map_unused_to_zeropage = flags &
>>>> RMP_USE_SHARED_ZEROPAGE,
>>>> + };
>>>> +
>>>> struct rmap_walk_control rwc = {
>>>> .rmap_one = remove_migration_pte,
>>>> - .arg = src,
>>>> + .arg = &rmap_walk_arg,
>>>> };
>>>> - if (locked)
>>>> + VM_BUG_ON_FOLIO((flags & RMP_USE_SHARED_ZEROPAGE) && (src !=
>>>> dst), src);
>>>> +
>>>> + if (flags & RMP_LOCKED)
>>>> rmap_walk_locked(dst, &rwc);
>>>> else
>>>> rmap_walk(dst, &rwc);
>>>> @@ -934,7 +988,7 @@ static int writeout(struct address_space
>>>> *mapping, struct folio *folio)
>>>> * At this point we know that the migration attempt cannot
>>>> * be successful.
>>>> */
>>>> - remove_migration_ptes(folio, folio, false);
>>>> + remove_migration_ptes(folio, folio, 0);
>>>> rc = mapping->a_ops->writepage(&folio->page, &wbc);
>>>> @@ -1098,7 +1152,7 @@ static void migrate_folio_undo_src(struct folio
>>>> *src,
>>>> struct list_head *ret)
>>>> {
>>>> if (page_was_mapped)
>>>> - remove_migration_ptes(src, src, false);
>>>> + remove_migration_ptes(src, src, 0);
>>>> /* Drop an anon_vma reference if we took one */
>>>> if (anon_vma)
>>>> put_anon_vma(anon_vma);
>>>> @@ -1336,7 +1390,7 @@ static int migrate_folio_move(free_folio_t
>>>> put_new_folio, unsigned long private,
>>>> lru_add_drain();
>>>> if (old_page_state & PAGE_WAS_MAPPED)
>>>> - remove_migration_ptes(src, dst, false);
>>>> + remove_migration_ptes(src, dst, 0);
>>>> out_unlock_both:
>>>> folio_unlock(dst);
>>>> @@ -1474,7 +1528,7 @@ static int unmap_and_move_huge_page(new_folio_t
>>>> get_new_folio,
>>>> if (page_was_mapped)
>>>> remove_migration_ptes(src,
>>>> - rc == MIGRATEPAGE_SUCCESS ? dst : src,
>>>> false);
>>>> + rc == MIGRATEPAGE_SUCCESS ? dst : src, 0);
>>>> unlock_put_anon:
>>>> folio_unlock(dst);
>>>> diff --git a/mm/migrate_device.c b/mm/migrate_device.c
>>>> index 8d687de88a03..9cf26592ac93 100644
>>>> --- a/mm/migrate_device.c
>>>> +++ b/mm/migrate_device.c
>>>> @@ -424,7 +424,7 @@ static unsigned long
>>>> migrate_device_unmap(unsigned long *src_pfns,
>>>> continue;
>>>> folio = page_folio(page);
>>>> - remove_migration_ptes(folio, folio, false);
>>>> + remove_migration_ptes(folio, folio, 0);
>>>> src_pfns[i] = 0;
>>>> folio_unlock(folio);
>>>> @@ -840,7 +840,7 @@ void migrate_device_finalize(unsigned long
>>>> *src_pfns,
>>>> dst = src;
>>>> }
>>>> - remove_migration_ptes(src, dst, false);
>>>> + remove_migration_ptes(src, dst, 0);
>>>> folio_unlock(src);
>>>> if (folio_is_zone_device(src))
>>>
>>> Hi,
>>>
>>> This patch has been in the mainline for some time, but we recently
>>> discovered an issue when both mTHP and MTE (Memory Tagging Extension)
>>> are enabled.
>>>
>>> It seems that remapping to the same zeropage might causes MTE tag
>>> mismatches, since MTE tags are associated with physical addresses.
>>
>> Does this only trigger when the VMA has mte enabled? Maybe we'll have to bail out if we detect that mte is enabled.
>>
>
> I believe MTE is all or nothing? i.e. all the memory is tagged when enabled, but will let the arm folks confirm.
>
I think for a process it's only active when PROT_MTE was called.
In khugepaged, I recall that we use
copy_mc_user_highpage()->copy_user_highpage() that translates on arm64
to its custom copy_highpage() where tags are copied as well.
So that's why khugepaged works.
KSM? Hmmm. Not sure how that is fenced off. We'd likely need some hidden
page_mte_tagged() check somewhere.
--
Cheers
David / dhildenb
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