From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 5C6CF408032; Mon, 17 Aug 2026 11:37:47 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786966668; cv=none; b=pQaZa91btB7+IAr+zBIwC0ZUaHWfD3VWAy4WJjIEcPNb9WArVHCZg4Hq/Qj59gENja1yIpZqAawWXQ9YTBwKQgm00Fq4Sbzz23sNq91QhuvmTs4BOLM4863yYdD3ptFL6sqBiOKRHdYK8vXX8P3yqukpQ5OuGQ39Cg8hzA8gEAU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786966668; c=relaxed/simple; bh=Bxwm04HNz2+mNC/Hh75JBKdkoGAgln2WQQiW3h5oXTI=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=mXTWboxdOc66Kn8eNQ495DwmNTykCAH/8Gx6xNgIGA8P0yvbA+gCbUhdBbq3bNsFJ/M15qON3oOOCtKJB173A3+VV26HVzYW1kkoQqXwpqdyRqLwApGFflcZ3PX5DY0RAE81ufF0ouhFTApIhQQcBya+fQO7QhVuP1Fs0g01RWQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=A82khPjI; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="A82khPjI" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 9B7A11F00A3D; Mon, 17 Aug 2026 11:37:43 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1786966667; bh=dbSm7b5FCzZr7Y2/Maa9jFE0muBDPk7Odg1HUs0TTqk=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=A82khPjIfMjsPz4xnic/kCcq5dHmdnA/g7k14UxIjimMII9WqATs/aTgobYAVdhAi wSQVaalA4cS7o2CPnNB1U6oAl/QJScWOE3Mo4yDVUOfaw5beIswzgqw6KhRuWEENmq DsmiitAht7VY+wq5V5Jpq1PzZxigGiiwlZHnn48p+qx7f8xScbxUX7Yvb5vcDX0F14 4/zdpRSMUKPcx6tfqAQWVnZ3xTGzx2//UGgHR/PjTTvgIz3z88lcvPTXkox2o7FO5k SIR0Hg5C0GNG7VSXsUENI8dCb8KTH84UepL4edunSRMECxhYXRWOlLUYh1xqJ74+bl TK8DrfLy3nzgQ== Date: Mon, 17 Aug 2026 12:37:27 +0100 From: "Lorenzo Stoakes (ARM)" To: "Nico Pache (Red Hat)" Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, Andrew Morton , David Hildenbrand , 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 Subject: Re: [PATCH v4 4/7] mm/khugepaged: fix outdated comments Message-ID: References: <20260811-khugepaged_pte_refactor-v4-0-ddac39d61c4a@linux.dev> <20260811-khugepaged_pte_refactor-v4-4-ddac39d61c4a@linux.dev> Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260811-khugepaged_pte_refactor-v4-4-ddac39d61c4a@linux.dev> On Tue, Aug 11, 2026 at 06:48:36AM -0600, 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) Thanks, some nits below, but this is some good bit rot fixing here :) So: Reviewed-by: Lorenzo Stoakes (ARM) > --- > 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 Ah yeah I think there are still quite a few mmap sem's around and people often refer to it as that almost with nostalgia :) > * 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. NIT: 'but such case is' -> 'such cases are', 'we could always' -> 'we can always' and 'the risk seems low' -> 'so the risk seems to be 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 > -- Cheers, Lorenzo