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Howlett" , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Jonathan Corbet , Shuah Khan References: <20260811-khugepaged_pte_refactor-v4-0-ddac39d61c4a@linux.dev> <20260811-khugepaged_pte_refactor-v4-4-ddac39d61c4a@linux.dev> From: "Nico Pache (Red Hat)" Content-Language: en-US, en-ZM In-Reply-To: <20260811-khugepaged_pte_refactor-v4-4-ddac39d61c4a@linux.dev> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: 47B6640003 X-Stat-Signature: szeazrt5d9nprt1b6zrcrd6hhhzc15nk X-Rspam-User: X-HE-Tag: 1787589013-360292 X-HE-Meta: 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 OaHcES9e cEgkOk1mzv+4UYt0+ApoGnMUH3i1n4L7BKAPgT+mdVhcEB7kbHwnPYTUtx2ebqiGEELTexpPnrprwOwmz8APv+j2takzdc9Scpng7Iztv5yNwX7X2qt0zZ+B62t+baK11QzQ1dEvCwvuJFchBfo77y2DwbIuMtj5v5CLM54QWVceV7j9EPz5CZDNqMOsMasdWbttr6LWppbZ14pzIbY7lRZ3K2B7Ie25ru7up+ed2ce4CIp3YAy00BxJvBAnhYyhtjJzaWjsc6ina9DZ1UdIBY14jOt9oxM2GsNqzH/Ggy2fDqFvlh2kZ5HSshLeYcTcRj/ifRnmPhdP4zQM= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 8/11/26 6:48 AM, Nico Pache (Red Hat) wrote: > 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) > --- Hi Andrew, Can you please append the following fixup! Thank you :) commit a5b3dde4f1b1657731dc7a1908ef60f6bd6283db Author: Nico Pache (Red Hat) Date: Fri Aug 21 04:27:28 2026 -0600 fixup! mm/khugepaged: fix outdated comments Polish the refreshed GUP-pin comment wording per review. Signed-off-by: Nico Pache (Red Hat) diff --git a/mm/khugepaged.c b/mm/khugepaged.c index ad7629a0216a..30f17c7494fa 100644 --- a/mm/khugepaged.c +++ b/mm/khugepaged.c @@ -1759,9 +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 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. + * 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; > 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); >