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DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1786452569; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=S0naQZB/DpgBMpRSB18moV5dYuwkkCVZRrrnKUkrNGc=; b=NUHw6mgBxJCSbQkBC2lPDn37fHG4Qr2OrJl1bVwuRY5/JrSTKjQJlrUb6FCXScmuLpLmOT 7GXOPIq5oRWik7b6gpAVWFavUX/TH6GFRb34FNHHjLK0hEbI1LQn7bZ7l2CJbzD+NS3PnR mFcwR2B02FA0LFe8uVmEjzcAhvZMj6U= From: "Nico Pache (Red Hat)" Date: Tue, 11 Aug 2026 06:48:36 -0600 Subject: [PATCH v4 4/7] mm/khugepaged: fix outdated comments MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Message-Id: <20260811-khugepaged_pte_refactor-v4-4-ddac39d61c4a@linux.dev> References: <20260811-khugepaged_pte_refactor-v4-0-ddac39d61c4a@linux.dev> In-Reply-To: <20260811-khugepaged_pte_refactor-v4-0-ddac39d61c4a@linux.dev> To: linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org Cc: Andrew Morton , David Hildenbrand , Lorenzo Stoakes , Zi Yan , Baolin Wang , "Liam R. Howlett" , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Jonathan Corbet , Shuah Khan , "Nico Pache (Red Hat)" X-Migadu-Flow: FLOW_OUT X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: E3E63140004 X-Stat-Signature: qptt15m4rnkc3n1x6m1tcdm9wbuy7myp X-Rspam-User: X-HE-Tag: 1786452570-910465 X-HE-Meta: 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 NHp2u3+D pkcSjUkaCvPBiG8cuSnd0tt3vP6IemQ7O6v22amkHGV6OJ2//kOPKbiS+CgAsv5C7PGSzzCny9UqogMlEetWvR/p7fTOhdhvf/Ntk7y1ULV+oPqV0hnmE6AkAJUXv1/tuXSH34WvG7swQQwLn4OdK1Tq8Yp4rW7AgFfWpcTxk40hs1ifihuofcxYiCiLRXgQCievf Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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. Acked-by: Usama Arif Assisted-by: Cursor(claude-sonnet-4):4.6 Acked-by: David Hildenbrand (Arm) Signed-off-by: Nico Pache (Red Hat) --- mm/khugepaged.c | 44 +++++++++++++++++++++----------------------- 1 file changed, 21 insertions(+), 23 deletions(-) diff --git a/mm/khugepaged.c b/mm/khugepaged.c index cae510aa2914..90d6e595d282 100644 --- a/mm/khugepaged.c +++ b/mm/khugepaged.c @@ -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. */ @@ -788,8 +788,8 @@ 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)) { @@ -815,7 +815,7 @@ 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)) { @@ -927,7 +927,7 @@ 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)); @@ -1101,9 +1101,9 @@ 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))) @@ -1159,7 +1159,7 @@ 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 @@ -1227,7 +1227,7 @@ 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. @@ -1291,7 +1291,7 @@ 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. @@ -1338,7 +1338,7 @@ 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. */ @@ -1351,8 +1351,8 @@ 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. @@ -1409,9 +1409,9 @@ 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); @@ -1643,7 +1643,8 @@ 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; @@ -1764,12 +1765,9 @@ 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 case is ephemeral and + * we could always retry collapse later. Anyway the same + * check will be done again later the risk seems low. */ if (folio_expected_ref_count(folio) != folio_ref_count(folio)) { result = SCAN_PAGE_COUNT; @@ -1790,7 +1788,7 @@ 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); -- 2.55.0