From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-188.mta0.migadu.com (out-188.mta0.migadu.com [91.218.175.188]) (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 5A9662264AD for ; Fri, 19 Sep 2025 02:42:00 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.188 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1758249722; cv=none; b=hXKXcT8Fi3a95f4jnIPMPnDgp8qlieArGxERmX0xzBSjiQHES91mnS3x9fNtnRBnP0g+JkY5+GVwLb5mURbgpXLbrdrO7UDAgWLtOPp+gtVT33ndzyNKPIRkIKbUJGdAT/Jft49RFNHr/i/NB8FNvUAMcA8asB1HD0YkgMdxhDw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1758249722; c=relaxed/simple; bh=J5hxMnYbCOSaY19mDdzY05EZhi/RbVb37H/vBA2VIF8=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=JiGfaOsz6dpjz/ZlbCx3K7QGOl83b6qjTGiqv9/Xg4cnGztoUYYpAMhzecws5MsGaKnzw7sTU18XhWJnW2orpqRDuAnMTDhDdNF+tE2cuQirkd0QkQZYq8YAdGA4odEe0tynQSSrY/LQHpmL4CjPwfl/OPcLC7uJnpvcZccC7UM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=MKGEJivH; arc=none smtp.client-ip=91.218.175.188 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="MKGEJivH" Message-ID: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1758249718; 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=LOUYmorl2vXwXIh7NZzsymKhAVYTrNuSDHQ/iAUV2PM=; b=MKGEJivHc42R/NfCjKmZBd9XndL1e1w2iM+yb8YZwTHrId6TWcSYGjiKSX7sRiZAHwNOb9 qieUWzLl8HTtNL7lz++Hv9jmKKrj+ksJOqEEptglJbcoV1CkSnmO/1ZBNiyXMakjXcG+gC BEE1Xne7+KnPXT/NAt8Zeu8zjX08zjA= Date: Fri, 19 Sep 2025 10:41:47 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH mm-new v2 2/2] mm/khugepaged: abort collapse scan on guard PTEs Content-Language: en-US To: David Hildenbrand Cc: ziy@nvidia.com, baolin.wang@linux.alibaba.com, Liam.Howlett@oracle.com, npache@redhat.com, ryan.roberts@arm.com, dev.jain@arm.com, baohua@kernel.org, ioworker0@gmail.com, kirill@shutemov.name, hughd@google.com, mpenttil@redhat.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, akpm@linux-foundation.org, lorenzo.stoakes@oracle.com References: <20250918050431.36855-1-lance.yang@linux.dev> <20250918050431.36855-3-lance.yang@linux.dev> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Lance Yang In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT On 2025/9/19 02:47, David Hildenbrand wrote: > On 18.09.25 07:04, Lance Yang wrote: >> From: Lance Yang >> >> Guard PTE markers are installed via MADV_GUARD_INSTALL to create >> lightweight guard regions. >> >> Currently, any collapse path (khugepaged or MADV_COLLAPSE) will fail when >> encountering such a range. >> >> MADV_COLLAPSE fails deep inside the collapse logic when trying to swap-in >> the special marker in __collapse_huge_page_swapin(). >> >> hpage_collapse_scan_pmd() >>   `- collapse_huge_page() >>       `- __collapse_huge_page_swapin() -> fails! >> >> khugepaged's behavior is slightly different due to its max_ptes_swap >> limit >> (default 64). It won't fail as deep, but it will still needlessly scan up >> to 64 swap entries before bailing out. >> >> IMHO, we can and should detect this much earlier. >> >> This patch adds a check directly inside the PTE scan loop. If a guard >> marker is found, the scan is aborted immediately with >> SCAN_PTE_NON_PRESENT, >> avoiding wasted work. >> >> Suggested-by: Lorenzo Stoakes >> Signed-off-by: Lance Yang >> --- >>   mm/khugepaged.c | 10 ++++++++++ >>   1 file changed, 10 insertions(+) >> >> diff --git a/mm/khugepaged.c b/mm/khugepaged.c >> index 9ed1af2b5c38..70ebfc7c1f3e 100644 >> --- a/mm/khugepaged.c >> +++ b/mm/khugepaged.c >> @@ -1306,6 +1306,16 @@ static int hpage_collapse_scan_pmd(struct >> mm_struct *mm, >>                       result = SCAN_PTE_UFFD_WP; >>                       goto out_unmap; >>                   } >> +                /* >> +                 * Guard PTE markers are installed by >> +                 * MADV_GUARD_INSTALL. Any collapse path must >> +                 * not touch them, so abort the scan immediately >> +                 * if one is found. >> +                 */ >> +                if (is_guard_pte_marker(pteval)) { >> +                    result = SCAN_PTE_NON_PRESENT; >> +                    goto out_unmap; >> +                } > > Thinking about it, this is interesting. > > Essentially we track any non-swap swap entries towards > khugepaged_max_ptes_swap, which is rather weird. > > I think we might also run into migration entries here and hwpoison entries? > > So what about just generalizing this: > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > index af5f5c80fe4ed..28f1f4bf0e0a8 100644 > --- a/mm/khugepaged.c > +++ b/mm/khugepaged.c > @@ -1293,7 +1293,24 @@ static int hpage_collapse_scan_pmd(struct > mm_struct *mm, >         for (_address = address, _pte = pte; _pte < pte + HPAGE_PMD_NR; >              _pte++, _address += PAGE_SIZE) { >                 pte_t pteval = ptep_get(_pte); > -               if (is_swap_pte(pteval)) { > + > +               if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) { > +                       ++none_or_zero; > +                       if (!userfaultfd_armed(vma) && > +                           (!cc->is_khugepaged || > +                            none_or_zero <= khugepaged_max_ptes_none)) { > +                               continue; > +                       } else { > +                               result = SCAN_EXCEED_NONE_PTE; > +                               count_vm_event(THP_SCAN_EXCEED_NONE_PTE); > +                               goto out_unmap; > +                       } > +               } else if (!pte_present(pteval)) { > +                       if (non_swap_entry(pte_to_swp_entry(pteval))) { > +                               result = SCAN_PTE_NON_PRESENT; > +                               goto out_unmap; > +                       } > + >                         ++unmapped; >                         if (!cc->is_khugepaged || >                             unmapped <= khugepaged_max_ptes_swap) { > @@ -1313,18 +1330,7 @@ static int hpage_collapse_scan_pmd(struct > mm_struct *mm, >                                 goto out_unmap; >                         } >                 } > -               if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) { > -                       ++none_or_zero; > -                       if (!userfaultfd_armed(vma) && > -                           (!cc->is_khugepaged || > -                            none_or_zero <= khugepaged_max_ptes_none)) { > -                               continue; > -                       } else { > -                               result = SCAN_EXCEED_NONE_PTE; > -                               count_vm_event(THP_SCAN_EXCEED_NONE_PTE); > -                               goto out_unmap; > -                       } > -               } > + >                 if (pte_uffd_wp(pteval)) { >                         /* >                          * Don't collapse the page if any of the small > > > With that, the function flow looks more similar to > __collapse_huge_page_isolate(), > except that we handle swap entries in there now. Ah, indeed. I like this crazy idea ;p > > > And with that in place, couldn't we factor out a huge chunk of both > scanning > functions into some helper (passing whether swap entries are allowed or > not?). Yes. Factoring out the common scanning logic into a new helper is a good suggestion. It would clean things up ;) > > Yes, I know, refactoring khugepaged, crazy idea. I'll look into that. But let's do this separately :) Cheers, Lance