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mm/khugepaged: fix outdated comments

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Fix comment in collapse_scan_pmd() that still described the old
folio_mapcount() > folio_ref_count() check and a "512" false-positive
scenario.  The code now uses folio_expected_ref_count() !=
folio_ref_count() which doesn't suffer from the same limitation.

Fix comment in collapse_huge_page() that referenced ptep_clear_flush, when
the code actually uses pmdp_collapse_flush.

Fix comment in __collapse_huge_page_swapin() that referenced the old
function name khugepaged_scan_pmd, now collapse_scan_pmd.

Also clean up some simple typos and stale terminology (mmap_sem ->
mmap_lock, PG_lock -> folio lock, page -> folio, grammar).

We also clarify a comment regarding where the max_ptes_none check is
deferred to in mthp_collapse() from the original collapse_scan_pmd check.

Update all comments that references a function to include parentheses.

[nico.pache@linux.dev: fix outdated comments]
  Link: https://lore.kernel.org/1c96e2f3-802f-472b-81e6-4af17a721a3c@linux.dev
Link: https://lore.kernel.org/20260811-khugepaged_pte_refactor-v4-4-ddac39d61c4a@linux.dev
Signed-off-by: Nico Pache (Red Hat) <nico.pache@linux.dev>
Acked-by: Usama Arif <usama.arif@linux.dev>
Assisted-by: Cursor(claude-sonnet-4):4.6
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Acked-by: Pedro Falcato <pfalcato@suse.de>
Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Reviewed-by: Lance Yang <lance.yang@linux.dev>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Nico Pache (Red Hat)
2026-08-11 06:48:36 -06:00
committed by Andrew Morton
parent 948ec48e56
commit cc044178ed
1 changed files with 21 additions and 23 deletions
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44
mm/khugepaged.c
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@@ -620,7 +620,7 @@ void __khugepaged_exit(struct mm_struct *mm)
/*
* This is required to serialize against
* collapse_test_exit() (which is guaranteed to run
* under mmap sem read mode). Stop here (after we return all
* under mmap_lock read mode). Stop here (after we return all
* pagetables will be destroyed) until khugepaged has finished
* working on the pagetables under the mmap_lock.
*/
@@ -782,8 +782,8 @@ static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct *vma,
/*
* We can do it before folio_isolate_lru because the
* folio can't be freed from under us. NOTE: PG_lock
* is needed to serialize against split_huge_page
* folio can't be freed from under us. NOTE: folio lock
* is needed to serialize against split_huge_page()
* when invoked from the VM.
*/
if (!folio_trylock(folio)) {
@@ -809,7 +809,7 @@ static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct *vma,
}
/*
* Isolate the page to avoid collapsing an hugepage
* Isolate the folio to avoid collapsing a hugepage
* currently in use by the VM.
*/
if (!folio_isolate_lru(folio)) {
@@ -921,7 +921,7 @@ static void __collapse_huge_page_copy_failed(pte_t *pte,
* Re-establish the PMD to point to the original page table
* entry. Restoring PMD needs to be done prior to releasing
* pages. Since pages are still isolated and locked here,
* acquiring anon_vma_lock_write is unnecessary.
* acquiring anon_vma_lock_write() is unnecessary.
*/
pmd_ptl = pmd_lock(vma->vm_mm, pmd);
pmd_populate(vma->vm_mm, pmd, pmd_pgtable(orig_pmd));
@@ -1095,9 +1095,9 @@ static enum scan_result hugepage_vma_revalidate(struct mm_struct *mm, unsigned l
return SCAN_VMA_CHECK;
/*
* Anon VMA expected, the address may be unmapped then
* remapped to file after khugepaged reaquired the mmap_lock.
* remapped to file after khugepaged reacquired the mmap_lock.
*
* thp_vma_allowable_orders may return true for qualified file
* thp_vma_allowable_orders() may return true for qualified file
* vmas.
*/
if (expect_anon && (!(*vmap)->anon_vma || !vma_is_anonymous(*vmap)))
@@ -1153,7 +1153,7 @@ 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.
* Only done if collapse_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
@@ -1221,7 +1221,7 @@ static enum scan_result __collapse_huge_page_swapin(struct mm_struct *mm,
pte = NULL;
/*
* do_swap_page returns VM_FAULT_RETRY with released mmap_lock.
* do_swap_page() returns VM_FAULT_RETRY with released mmap_lock.
* Note we treat VM_FAULT_RETRY as VM_FAULT_ERROR here because
* we do not retry here and swap entry will remain in pagetable
* resulting in later failure.
@@ -1285,7 +1285,7 @@ static enum scan_result alloc_charge_folio(struct folio **foliop, struct mm_stru
}
/*
* collapse_huge_page expects the mmap_lock to be unlocked before entering and
* collapse_huge_page() expects the mmap_lock to be unlocked before entering and
* will always return with the lock unlocked, to avoid holding the mmap_lock
* while allocating a THP, as that could trigger direct reclaim/compaction.
* Note that the VMA must be rechecked after grabbing the mmap_lock again.
@@ -1332,7 +1332,7 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long s
if (unmapped) {
/*
* __collapse_huge_page_swapin will return with mmap_lock
* __collapse_huge_page_swapin() will return with mmap_lock
* released when it fails. So we jump out_nolock directly in
* that case. Continuing to collapse causes inconsistency.
*/
@@ -1345,8 +1345,8 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long s
mmap_read_unlock(mm);
/*
* Prevent all access to pagetables with the exception of
* gup_fast later handled by the ptep_clear_flush and the VM
* handled by the anon_vma lock + PG_lock.
* gup_fast later handled by the pmdp_collapse_flush() and the VM
* handled by the anon_vma lock + folio lock.
*
* UFFDIO_MOVE is prevented to race as well thanks to the
* mmap_lock.
@@ -1403,9 +1403,9 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long s
spin_lock(pmd_ptl);
VM_WARN_ON_ONCE(!pmd_none(*pmd));
/*
* We can only use set_pmd_at when establishing
* We can only use set_pmd_at() when establishing
* hugepmds and never for establishing regular pmds that
* points to regular pagetables. Use pmd_populate for that
* points to regular pagetables. Use pmd_populate() for that
*/
pmd_populate(mm, pmd, pmd_pgtable(_pmd));
spin_unlock(pmd_ptl);
@@ -1637,7 +1637,8 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
/*
* If PMD is the only enabled order, enforce max_ptes_none, otherwise
* scan all pages to populate the bitmap for mTHP collapse.
* scan all pages to populate the bitmap for mTHP collapse. The bitmap
* is then checked again in mthp_collapse() for each attempted order.
*/
if (enabled_orders != BIT(HPAGE_PMD_ORDER))
max_ptes_none = KHUGEPAGED_MAX_PTES_LIMIT;
@@ -1758,12 +1759,9 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
/*
* Check if the page has any GUP (or other external) pins.
*
* Here the check may be racy:
* it may see folio_mapcount() > folio_ref_count().
* But such case is ephemeral we could always retry collapse
* later. However it may report false positive if the page
* has excessive GUP pins (i.e. 512). Anyway the same check
* will be done again later the risk seems low.
* Here the check is racy, but such cases are ephemeral and
* we can always retry collapse later. Anyway the same
* check will be done again later, so the risk seems to be low.
*/
if (folio_expected_ref_count(folio) != folio_ref_count(folio)) {
result = SCAN_PAGE_COUNT;
@@ -1784,7 +1782,7 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
out_unmap:
pte_unmap_unlock(pte, ptl);
if (result == SCAN_SUCCEED) {
/* collapse_huge_page expects the lock to be dropped before calling */
/* collapse_huge_page() expects the lock to be dropped before calling */
mmap_read_unlock(mm);
result = mthp_collapse(mm, start_addr, referenced,
unmapped, cc, enabled_orders);