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mm/khugepaged: generalize __collapse_huge_page_* for mTHP support

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generalize the order of the __collapse_huge_page_* and collapse_max_*
functions to support future mTHP collapse.

The current mechanism for determining collapse with the
khugepaged_max_ptes_none value is not designed with mTHP in mind.  This
raises a key design issue: if we support user defined max_pte_none values
(even those scaled by order), a collapse of a lower order can introduces
an feedback loop, or "creep", when max_ptes_none is set to a value greater
than HPAGE_PMD_NR / 2.  [1]

With this configuration, a successful collapse to order N will populate
enough pages to satisfy the collapse condition on order N+1 on the next
scan.  This leads to unnecessary work and memory churn.

To fix this issue introduce a helper function that will limit mTHP
collapse support to two max_ptes_none values, 0 and HPAGE_PMD_NR - 1. 
This effectively supports two modes: [2]

- max_ptes_none=0: never collapses if it encounters an empty PTE or a PTE
  that maps the shared zeropage. Consequently, no memory bloat.
- max_ptes_none=511 (on 4k pagesz): Always collapse to the highest
  available mTHP order.

This removes the possibility of "creep", and a warning will be emitted if
any non-supported max_ptes_none value is configured with mTHP enabled. 
Any intermediate value will default mTHP collapse to max_ptes_none=0.

mTHP collapse will not honor the khugepaged_max_ptes_shared or
khugepaged_max_ptes_swap parameters, and will fail if it encounters a
shared or swapped entry.

No functional changes in this patch; however it defines future behavior
for mTHP collapse.

Link: https://lore.kernel.org/20260605161422.213817-5-npache@redhat.com
Link: https://lore.kernel.org/all/e46ab3ab-a3d7-4fb7-9970-d0704bd5d05a@arm.com [1]
Link: https://lore.kernel.org/all/37375ace-5601-4d6c-9dac-d1c8268698e9@redhat.com [2]
Co-developed-by: Dev Jain <dev.jain@arm.com>
Signed-off-by: Dev Jain <dev.jain@arm.com>
Signed-off-by: Nico Pache <npache@redhat.com>
Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>
Acked-by: David Hildenbrand (arm) <david@kernel.org>
Reviewed-by: Lance Yang <lance.yang@linux.dev>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Bagas Sanjaya <bagasdotme@gmail.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Brendan Jackman <jackmanb@google.com>
Cc: Byungchul Park <byungchul@sk.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Gregory Price <gourry@gourry.net>
Cc: "Huang, Ying" <ying.huang@linux.alibaba.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Jann Horn <jannh@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Joshua Hahn <joshua.hahnjy@gmail.com>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: "Masami Hiramatsu (Google)" <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Nanyong Sun <sunnanyong@huawei.com>
Cc: Pedro Falcato <pfalcato@suse.de>
Cc: Peter Xu <peterx@redhat.com>
Cc: Rafael Aquini <raquini@redhat.com>
Cc: Rakie Kim <rakie.kim@sk.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Shivank Garg <shivankg@amd.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Takashi Iwai (SUSE) <tiwai@suse.de>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Usama Arif <usamaarif642@gmail.com>
Cc: Usama Arif <usama.arif@linux.dev>
Cc: Vishal Moola (Oracle) <vishal.moola@gmail.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Wei Yang <richard.weiyang@gmail.com>
Cc: Will Deacon <will@kernel.org>
Cc: Yang Shi <yang@os.amperecomputing.com>
Cc: Zach O'Keefe <zokeefe@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Nico Pache 2026-06-05 10:14:11 -06:00 committed by Andrew Morton
parent 45b310faee
commit 5a9c05d866
1 changed files with 93 additions and 33 deletions
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126
mm/khugepaged.c
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@ -353,30 +353,51 @@ static bool pte_none_or_zero(pte_t pte)
* the shared zeropage for the given collapse operation.
* @cc: The collapse control struct
* @vma: The vma to check for userfaultfd
* @order: The folio order being collapsed to
*
* Return: Maximum number of empty/shared zeropage PTEs for the collapse operation
*/
static unsigned int collapse_max_ptes_none(struct collapse_control *cc,
struct vm_area_struct *vma)
struct vm_area_struct *vma, unsigned int order)
{
const unsigned int max_ptes_none = khugepaged_max_ptes_none;
if (vma && userfaultfd_armed(vma))
return 0;
/* for MADV_COLLAPSE, allow any empty/shared zeropage PTEs */
if (!cc->is_khugepaged)
return HPAGE_PMD_NR;
/* For all other cases respect the user defined maximum */
return khugepaged_max_ptes_none;
/* for PMD collapse, respect the user defined maximum */
if (is_pmd_order(order))
return max_ptes_none;
/*
* for mTHP collapse with the sysctl value set to KHUGEPAGED_MAX_PTES_LIMIT,
* scale the maximum number of PTEs to the order of the collapse.
*/
if (max_ptes_none == KHUGEPAGED_MAX_PTES_LIMIT)
return (1 << order) - 1;
/*
* For mTHP collapse of values other than 0 or KHUGEPAGED_MAX_PTES_LIMIT,
* emit a warning and return 0.
*/
if (max_ptes_none)
pr_warn_once("mTHP collapse does not support max_ptes_none"
" values other than 0 or %u, defaulting to 0.\n",
KHUGEPAGED_MAX_PTES_LIMIT);
return 0;
}
/**
* collapse_max_ptes_shared - Calculate maximum allowed PTEs that map shared
* anonymous pages for the given collapse operation.
* @cc: The collapse control struct
* @order: The folio order being collapsed to
*
* Return: Maximum number of PTEs that map shared anonymous pages for the
* collapse operation
*/
static unsigned int collapse_max_ptes_shared(struct collapse_control *cc)
static unsigned int collapse_max_ptes_shared(struct collapse_control *cc,
unsigned int order)
{
/*
* For MADV_COLLAPSE, do not restrict the number of PTEs that map shared
@ -384,6 +405,13 @@ static unsigned int collapse_max_ptes_shared(struct collapse_control *cc)
*/
if (!cc->is_khugepaged)
return HPAGE_PMD_NR;
/*
* for mTHP collapse do not allow collapsing anonymous memory pages that
* are shared between processes.
*/
if (!is_pmd_order(order))
return 0;
/* for PMD collapse, respect the user defined maximum */
return khugepaged_max_ptes_shared;
}
@ -391,11 +419,13 @@ static unsigned int collapse_max_ptes_shared(struct collapse_control *cc)
* collapse_max_ptes_swap - Calculate the maximum allowed non-present PTEs or the
* maximum allowed non-present pagecache entries for the given collapse operation.
* @cc: The collapse control struct
* @order: The folio order being collapsed to
*
* Return: Maximum number of non-present PTEs or the maximum allowed non-present
* pagecache entries for the collapse operation.
*/
static unsigned int collapse_max_ptes_swap(struct collapse_control *cc)
static unsigned int collapse_max_ptes_swap(struct collapse_control *cc,
unsigned int order)
{
/*
* For MADV_COLLAPSE, do not restrict the number PTEs entries or
@ -403,6 +433,10 @@ static unsigned int collapse_max_ptes_swap(struct collapse_control *cc)
*/
if (!cc->is_khugepaged)
return HPAGE_PMD_NR;
/* for mTHP collapse do not allow any non-present PTEs or pagecache entries */
if (!is_pmd_order(order))
return 0;
/* for PMD collapse, respect the user defined maximum */
return khugepaged_max_ptes_swap;
}
@ -599,10 +633,11 @@ static void release_pte_pages(pte_t *pte, pte_t *_pte,
static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct *vma,
unsigned long start_addr, pte_t *pte, struct collapse_control *cc,
struct list_head *compound_pagelist)
unsigned int order, struct list_head *compound_pagelist)
{
const unsigned int max_ptes_none = collapse_max_ptes_none(cc, vma);
const unsigned int max_ptes_shared = collapse_max_ptes_shared(cc);
const unsigned int max_ptes_none = collapse_max_ptes_none(cc, vma, order);
const unsigned int max_ptes_shared = collapse_max_ptes_shared(cc, order);
const unsigned long nr_pages = 1UL << order;
struct page *page = NULL;
struct folio *folio = NULL;
unsigned long addr = start_addr;
@ -610,7 +645,7 @@ static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct *vma,
int none_or_zero = 0, shared = 0, referenced = 0;
enum scan_result result = SCAN_FAIL;
for (_pte = pte; _pte < pte + HPAGE_PMD_NR;
for (_pte = pte; _pte < pte + nr_pages;
_pte++, addr += PAGE_SIZE) {
pte_t pteval = ptep_get(_pte);
if (pte_none_or_zero(pteval)) {
@ -650,6 +685,12 @@ static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct *vma,
/* See collapse_scan_pmd(). */
if (folio_maybe_mapped_shared(folio)) {
/*
* TODO: Support shared pages without leading to further
* mTHP collapses. Currently bringing in new pages via
* shared may cause a future higher order collapse on a
* rescan of the same range.
*/
if (++shared > max_ptes_shared) {
result = SCAN_EXCEED_SHARED_PTE;
count_vm_event(THP_SCAN_EXCEED_SHARED_PTE);
@ -743,18 +784,18 @@ static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct *vma,
}
static void __collapse_huge_page_copy_succeeded(pte_t *pte,
struct vm_area_struct *vma,
unsigned long address,
spinlock_t *ptl,
struct list_head *compound_pagelist)
struct vm_area_struct *vma, unsigned long address,
spinlock_t *ptl, unsigned int order,
struct list_head *compound_pagelist)
{
unsigned long end = address + HPAGE_PMD_SIZE;
const unsigned long nr_pages = 1UL << order;
unsigned long end = address + (PAGE_SIZE * nr_pages);
struct folio *src, *tmp;
pte_t pteval;
pte_t *_pte;
unsigned int nr_ptes;
for (_pte = pte; _pte < pte + HPAGE_PMD_NR; _pte += nr_ptes,
for (_pte = pte; _pte < pte + nr_pages; _pte += nr_ptes,
address += nr_ptes * PAGE_SIZE) {
nr_ptes = 1;
pteval = ptep_get(_pte);
@ -807,11 +848,10 @@ static void __collapse_huge_page_copy_succeeded(pte_t *pte,
}
static void __collapse_huge_page_copy_failed(pte_t *pte,
pmd_t *pmd,
pmd_t orig_pmd,
struct vm_area_struct *vma,
struct list_head *compound_pagelist)
pmd_t *pmd, pmd_t orig_pmd, struct vm_area_struct *vma,
unsigned int order, struct list_head *compound_pagelist)
{
const unsigned long nr_pages = 1UL << order;
spinlock_t *pmd_ptl;
/*
@ -827,7 +867,7 @@ static void __collapse_huge_page_copy_failed(pte_t *pte,
* Release both raw and compound pages isolated
* in __collapse_huge_page_isolate.
*/
release_pte_pages(pte, pte + HPAGE_PMD_NR, compound_pagelist);
release_pte_pages(pte, pte + nr_pages, compound_pagelist);
}
/*
@ -847,16 +887,17 @@ static void __collapse_huge_page_copy_failed(pte_t *pte,
*/
static enum scan_result __collapse_huge_page_copy(pte_t *pte, struct folio *folio,
pmd_t *pmd, pmd_t orig_pmd, struct vm_area_struct *vma,
unsigned long address, spinlock_t *ptl,
unsigned long address, spinlock_t *ptl, unsigned int order,
struct list_head *compound_pagelist)
{
const unsigned long nr_pages = 1UL << order;
unsigned int i;
enum scan_result result = SCAN_SUCCEED;
/*
* Copying pages' contents is subject to memory poison at any iteration.
*/
for (i = 0; i < HPAGE_PMD_NR; i++) {
for (i = 0; i < nr_pages; i++) {
pte_t pteval = ptep_get(pte + i);
struct page *page = folio_page(folio, i);
unsigned long src_addr = address + i * PAGE_SIZE;
@ -875,10 +916,10 @@ static enum scan_result __collapse_huge_page_copy(pte_t *pte, struct folio *foli
if (likely(result == SCAN_SUCCEED))
__collapse_huge_page_copy_succeeded(pte, vma, address, ptl,
compound_pagelist);
order, compound_pagelist);
else
__collapse_huge_page_copy_failed(pte, pmd, orig_pmd, vma,
compound_pagelist);
order, compound_pagelist);
return result;
}
@ -1051,16 +1092,20 @@ static enum scan_result check_pmd_still_valid(struct mm_struct *mm,
* Bring missing pages in from swap, to complete THP collapse.
* Only done if khugepaged_scan_pmd believes it is worthwhile.
*
* For mTHP orders the function bails on the first swap entry, because
* faulting pages back in during collapse could re-populate PTEs that
* push a later scan over the threshold for a higher-order collapse.
*
* Called and returns without pte mapped or spinlocks held.
* Returns result: if not SCAN_SUCCEED, mmap_lock has been released.
*/
static enum scan_result __collapse_huge_page_swapin(struct mm_struct *mm,
struct vm_area_struct *vma, unsigned long start_addr, pmd_t *pmd,
int referenced)
struct vm_area_struct *vma, unsigned long start_addr,
pmd_t *pmd, int referenced, unsigned int order)
{
int swapped_in = 0;
vm_fault_t ret = 0;
unsigned long addr, end = start_addr + (HPAGE_PMD_NR * PAGE_SIZE);
unsigned long addr, end = start_addr + (PAGE_SIZE << order);
enum scan_result result;
pte_t *pte = NULL;
spinlock_t *ptl;
@ -1092,6 +1137,19 @@ static enum scan_result __collapse_huge_page_swapin(struct mm_struct *mm,
pte_present(vmf.orig_pte))
continue;
/*
* TODO: Support swapin without leading to further mTHP
* collapses. Currently bringing in new pages via swapin may
* cause a future higher order collapse on a rescan of the same
* range.
*/
if (!is_pmd_order(order)) {
pte_unmap(pte);
mmap_read_unlock(mm);
result = SCAN_EXCEED_SWAP_PTE;
goto out;
}
vmf.pte = pte;
vmf.ptl = ptl;
ret = do_swap_page(&vmf);
@ -1217,7 +1275,7 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a
* that case. Continuing to collapse causes inconsistency.
*/
result = __collapse_huge_page_swapin(mm, vma, address, pmd,
referenced);
referenced, HPAGE_PMD_ORDER);
if (result != SCAN_SUCCEED)
goto out_nolock;
}
@ -1265,6 +1323,7 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a
pte = pte_offset_map_lock(mm, &_pmd, address, &pte_ptl);
if (pte) {
result = __collapse_huge_page_isolate(vma, address, pte, cc,
HPAGE_PMD_ORDER,
&compound_pagelist);
spin_unlock(pte_ptl);
} else {
@ -1295,6 +1354,7 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a
result = __collapse_huge_page_copy(pte, folio, pmd, _pmd,
vma, address, pte_ptl,
HPAGE_PMD_ORDER,
&compound_pagelist);
pte_unmap(pte);
if (unlikely(result != SCAN_SUCCEED))
@ -1330,9 +1390,9 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
struct vm_area_struct *vma, unsigned long start_addr,
bool *lock_dropped, struct collapse_control *cc)
{
const unsigned int max_ptes_none = collapse_max_ptes_none(cc, vma);
const unsigned int max_ptes_shared = collapse_max_ptes_shared(cc);
const unsigned int max_ptes_swap = collapse_max_ptes_swap(cc);
const unsigned int max_ptes_none = collapse_max_ptes_none(cc, vma, HPAGE_PMD_ORDER);
const unsigned int max_ptes_shared = collapse_max_ptes_shared(cc, HPAGE_PMD_ORDER);
const unsigned int max_ptes_swap = collapse_max_ptes_swap(cc, HPAGE_PMD_ORDER);
pmd_t *pmd;
pte_t *pte, *_pte;
int none_or_zero = 0, shared = 0, referenced = 0;
@ -2386,8 +2446,8 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm,
unsigned long addr, struct file *file, pgoff_t start,
struct collapse_control *cc)
{
const unsigned int max_ptes_none = collapse_max_ptes_none(cc, NULL);
const unsigned int max_ptes_swap = collapse_max_ptes_swap(cc);
const unsigned int max_ptes_none = collapse_max_ptes_none(cc, NULL, HPAGE_PMD_ORDER);
const unsigned int max_ptes_swap = collapse_max_ptes_swap(cc, HPAGE_PMD_ORDER);
struct folio *folio = NULL;
struct address_space *mapping = file->f_mapping;
XA_STATE(xas, &mapping->i_pages, start);