From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 D7ECD3E6DF0 for ; Mon, 16 Mar 2026 18:54:05 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773687246; cv=none; b=gNdSl5vbPtmimP+m0Q/6YZDXvNqYzdGoAzsXSWrmU2AiPvOnA6geEl09dz8d/fUvO63RJfUzzPGKyALf5fuhY6Eoqtv+GBf00TSJlIlyd62anheA17WndsJd8wMyao7HM5gQvg4nZheuQ2IiwHte76C109XWMgo3NtxVKB0Tkh8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773687246; c=relaxed/simple; bh=7+1pt7j7D2qnVA9uCZuqifM+Dqo+V4s9K4ZaH0VJI+k=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=FNHr79YuVxqAI1Cbc9EfOakS07de3N6dKcXQGxFxg3AXaVFD8BQmFeVZHgeaFQytfMNWDbYLZ1ZDX6aIL1sEZtE8vw1Pzoylje66y2GtAnonzIuQ0oE3kwVUB7KkNFMV3TfstVQQT0rwNy1opqLOYNvxSM83v6m4HJflByNa+iQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Q3xuz+1w; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="Q3xuz+1w" Received: by smtp.kernel.org (Postfix) with ESMTPSA id B3420C19421; Mon, 16 Mar 2026 18:54:04 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1773687245; bh=7+1pt7j7D2qnVA9uCZuqifM+Dqo+V4s9K4ZaH0VJI+k=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=Q3xuz+1wz2sLcyvTchb22arPvEzlHz06YTP2geWVo1K7sUmkzHj6gG8mmQQxr4NMZ mR/fu0j0FFXr7PmfcyIMT9xHwPx/flspE/xXySgKXV85LGS8opm0IdtKBBoq4E3ToM JCgvbuJBvB5UpJMlbY+PA/6cmenGAvZivzcyOOLKyAV+Vs3PLyoEqUDpbsii2AhwSQ lrJ5JmlwV8+Xa0RKB1rlV5RjiD36Nug9aGDCuLZ7UplFEEjuBwogZEVbvYcXWhNgnk 7e2N2IS/KbiuC+NNDCwM9fTQSx4MDZZZ0I9ROZR7k/0RraLRILImserDtkPGGKBUfQ IDa63AhiAnjZw== Date: Mon, 16 Mar 2026 18:54:03 +0000 From: "Lorenzo Stoakes (Oracle)" To: Nico Pache Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, aarcange@redhat.com, akpm@linux-foundation.org, anshuman.khandual@arm.com, apopple@nvidia.com, baohua@kernel.org, baolin.wang@linux.alibaba.com, byungchul@sk.com, catalin.marinas@arm.com, cl@gentwo.org, corbet@lwn.net, dave.hansen@linux.intel.com, david@kernel.org, dev.jain@arm.com, gourry@gourry.net, hannes@cmpxchg.org, hughd@google.com, jackmanb@google.com, jack@suse.cz, jannh@google.com, jglisse@google.com, joshua.hahnjy@gmail.com, kas@kernel.org, lance.yang@linux.dev, Liam.Howlett@oracle.com, lorenzo.stoakes@oracle.com, mathieu.desnoyers@efficios.com, matthew.brost@intel.com, mhiramat@kernel.org, mhocko@suse.com, peterx@redhat.com, pfalcato@suse.de, rakie.kim@sk.com, raquini@redhat.com, rdunlap@infradead.org, richard.weiyang@gmail.com, rientjes@google.com, rostedt@goodmis.org, rppt@kernel.org, ryan.roberts@arm.com, shivankg@amd.com, sunnanyong@huawei.com, surenb@google.com, thomas.hellstrom@linux.intel.com, tiwai@suse.de, usamaarif642@gmail.com, vbabka@suse.cz, vishal.moola@gmail.com, wangkefeng.wang@huawei.com, will@kernel.org, willy@infradead.org, yang@os.amperecomputing.com, ying.huang@linux.alibaba.com, ziy@nvidia.com, zokeefe@google.com Subject: Re: [PATCH mm-unstable v3 5/5] mm/khugepaged: unify khugepaged and madv_collapse with collapse_single_pmd() Message-ID: References: <20260311211315.450947-1-npache@redhat.com> <20260311211315.450947-6-npache@redhat.com> Precedence: bulk X-Mailing-List: linux-kernel@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: <20260311211315.450947-6-npache@redhat.com> On Wed, Mar 11, 2026 at 03:13:15PM -0600, Nico Pache wrote: > The khugepaged daemon and madvise_collapse have two different > implementations that do almost the same thing. Create collapse_single_pmd > to increase code reuse and create an entry point to these two users. Ah this is nice :) Thanks! > > Refactor madvise_collapse and collapse_scan_mm_slot to use the new > collapse_single_pmd function. This introduces a minor behavioral change > that is most likely an undiscovered bug. The current implementation of > khugepaged tests collapse_test_exit_or_disable before calling > collapse_pte_mapped_thp, but we weren't doing it in the madvise_collapse > case. By unifying these two callers madvise_collapse now also performs > this check. We also modify the return value to be SCAN_ANY_PROCESS which > properly indicates that this process is no longer valid to operate on. > > By moving the madvise_collapse writeback-retry logic into the helper > function we can also avoid having to revalidate the VMA. > > We also guard the khugepaged_pages_collapsed variable to ensure its only > incremented for khugepaged. > > Signed-off-by: Nico Pache The logic all seems correct to me, just a bunch of nits below really. This is a really nice refactoring! :) With them addressed: Reviewed-by: Lorenzo Stoakes (Oracle) Cheers, Lorenzo > --- > mm/khugepaged.c | 120 +++++++++++++++++++++++++----------------------- > 1 file changed, 63 insertions(+), 57 deletions(-) > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > index 33ae56e313ed..733c4a42c2ce 100644 > --- a/mm/khugepaged.c > +++ b/mm/khugepaged.c > @@ -2409,6 +2409,65 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm, > return result; > } > > +/* > + * Try to collapse a single PMD starting at a PMD aligned addr, and return > + * the results. > + */ > +static enum scan_result collapse_single_pmd(unsigned long addr, > + struct vm_area_struct *vma, bool *mmap_locked, mmap_locked seems mildly pointless here, and it's a semi-code smell to pass 'is locked' flags I think. You never read this, but the parameter implies somebody might pass in mmaplocked == false, but you know it's always true here. Anyway I think it makes more sense to pass in lock_dropped and get rid of mmap_locked in madvise_collapse() and just pass in lock_dropped directly (setting it false if anon). Also obviously update collapse_scan_mm_slot() to use lock_dropped instead just inverted. That's clearer I think since it makes it a verb rather than a noun and the function is dictating whether or not the lock is dropped, it also implies the lock is held on entry. > + struct collapse_control *cc) > +{ > + struct mm_struct *mm = vma->vm_mm; > + bool triggered_wb = false; > + enum scan_result result; > + struct file *file; > + pgoff_t pgoff; > + Maybe move the mmap_assert_locked() from madvise_collapse() to here? Then we assert it in both cases. > + if (vma_is_anonymous(vma)) { > + result = collapse_scan_pmd(mm, vma, addr, mmap_locked, cc); > + goto end; > + } > + > + file = get_file(vma->vm_file); > + pgoff = linear_page_index(vma, addr); > + > + mmap_read_unlock(mm); > + *mmap_locked = false; > +retry: > + result = collapse_scan_file(mm, addr, file, pgoff, cc); > + > + /* > + * For MADV_COLLAPSE, when encountering dirty pages, try to writeback, > + * then retry the collapse one time. > + */ > + if (!cc->is_khugepaged && result == SCAN_PAGE_DIRTY_OR_WRITEBACK && > + !triggered_wb && mapping_can_writeback(file->f_mapping)) { > + const loff_t lstart = (loff_t)pgoff << PAGE_SHIFT; > + const loff_t lend = lstart + HPAGE_PMD_SIZE - 1; > + > + filemap_write_and_wait_range(file->f_mapping, lstart, lend); > + triggered_wb = true; > + goto retry; Thinking through this logic I do agree that we don't need to revalidate here, which should be quite a nice win, I just don't know why we previously assumed we'd have to... or maybe it was just because it became too spaghetti to goto around it somehow?? > + } > + fput(file); > + > + if (result == SCAN_PTE_MAPPED_HUGEPAGE) { > + mmap_read_lock(mm); > + if (collapse_test_exit_or_disable(mm)) > + result = SCAN_ANY_PROCESS; > + else > + result = try_collapse_pte_mapped_thp(mm, addr, > + !cc->is_khugepaged); > + if (result == SCAN_PMD_MAPPED) > + result = SCAN_SUCCEED; > + mmap_read_unlock(mm); > + } > +end: > + if (cc->is_khugepaged && result == SCAN_SUCCEED) > + ++khugepaged_pages_collapsed; > + return result; > +} > + > static void collapse_scan_mm_slot(unsigned int progress_max, > enum scan_result *result, struct collapse_control *cc) > __releases(&khugepaged_mm_lock) > @@ -2479,34 +2538,9 @@ static void collapse_scan_mm_slot(unsigned int progress_max, > VM_BUG_ON(khugepaged_scan.address < hstart || > khugepaged_scan.address + HPAGE_PMD_SIZE > > hend); Nice-to-have, but could we convert these VM_BUG_ON()'s to VM_WARN_ON_ONCE()'s while we're passing? > - if (!vma_is_anonymous(vma)) { > - struct file *file = get_file(vma->vm_file); > - pgoff_t pgoff = linear_page_index(vma, > - khugepaged_scan.address); > - > - mmap_read_unlock(mm); > - mmap_locked = false; > - *result = collapse_scan_file(mm, > - khugepaged_scan.address, file, pgoff, cc); > - fput(file); > - if (*result == SCAN_PTE_MAPPED_HUGEPAGE) { > - mmap_read_lock(mm); > - if (collapse_test_exit_or_disable(mm)) > - goto breakouterloop; > - *result = try_collapse_pte_mapped_thp(mm, > - khugepaged_scan.address, false); > - if (*result == SCAN_PMD_MAPPED) > - *result = SCAN_SUCCEED; > - mmap_read_unlock(mm); > - } > - } else { > - *result = collapse_scan_pmd(mm, vma, > - khugepaged_scan.address, &mmap_locked, cc); > - } > - > - if (*result == SCAN_SUCCEED) > - ++khugepaged_pages_collapsed; > > + *result = collapse_single_pmd(khugepaged_scan.address, > + vma, &mmap_locked, cc); > /* move to next address */ > khugepaged_scan.address += HPAGE_PMD_SIZE; > if (!mmap_locked) > @@ -2806,9 +2840,7 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start, > > for (addr = hstart; addr < hend; addr += HPAGE_PMD_SIZE) { > enum scan_result result = SCAN_FAIL; > - bool triggered_wb = false; > > -retry: > if (!mmap_locked) { > cond_resched(); > mmap_read_lock(mm); > @@ -2823,46 +2855,20 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start, > hend = min(hend, vma->vm_end & HPAGE_PMD_MASK); > } > mmap_assert_locked(mm); > - if (!vma_is_anonymous(vma)) { > - struct file *file = get_file(vma->vm_file); > - pgoff_t pgoff = linear_page_index(vma, addr); > > - mmap_read_unlock(mm); > - mmap_locked = false; > - *lock_dropped = true; > - result = collapse_scan_file(mm, addr, file, pgoff, cc); > - > - if (result == SCAN_PAGE_DIRTY_OR_WRITEBACK && !triggered_wb && > - mapping_can_writeback(file->f_mapping)) { > - loff_t lstart = (loff_t)pgoff << PAGE_SHIFT; > - loff_t lend = lstart + HPAGE_PMD_SIZE - 1; > + result = collapse_single_pmd(addr, vma, &mmap_locked, cc); > > - filemap_write_and_wait_range(file->f_mapping, lstart, lend); > - triggered_wb = true; > - fput(file); > - goto retry; > - } > - fput(file); > - } else { > - result = collapse_scan_pmd(mm, vma, addr, &mmap_locked, cc); > - } > if (!mmap_locked) > *lock_dropped = true; > > -handle_result: > switch (result) { > case SCAN_SUCCEED: > case SCAN_PMD_MAPPED: > ++thps; > break; > - case SCAN_PTE_MAPPED_HUGEPAGE: > - BUG_ON(mmap_locked); > - mmap_read_lock(mm); > - result = try_collapse_pte_mapped_thp(mm, addr, true); > - mmap_read_unlock(mm); > - goto handle_result; > /* Whitelisted set of results where continuing OK */ > case SCAN_NO_PTE_TABLE: > + case SCAN_PTE_MAPPED_HUGEPAGE: > case SCAN_PTE_NON_PRESENT: > case SCAN_PTE_UFFD_WP: > case SCAN_LACK_REFERENCED_PAGE: > -- > 2.53.0 >