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Fri, 10 Oct 2025 06:29:50 -0700 (PDT) Date: Fri, 10 Oct 2025 13:29:50 +0000 From: Wei Yang To: Lance Yang Cc: akpm@linux-foundation.org, david@redhat.com, lorenzo.stoakes@oracle.com, 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, richard.weiyang@gmail.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH mm-new v3 3/3] mm/khugepaged: merge PTE scanning logic into a new helper Message-ID: <20251010132950.yx6bmvub3a46gcl3@master> Reply-To: Wei Yang References: <20251008043748.45554-1-lance.yang@linux.dev> <20251008043748.45554-4-lance.yang@linux.dev> 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: <20251008043748.45554-4-lance.yang@linux.dev> User-Agent: NeoMutt/20170113 (1.7.2) On Wed, Oct 08, 2025 at 12:37:48PM +0800, Lance Yang wrote: >From: Lance Yang > >As David suggested, the PTE scanning logic in hpage_collapse_scan_pmd() >and __collapse_huge_page_isolate() was almost duplicated. > >This patch cleans things up by moving all the common PTE checking logic >into a new shared helper, thp_collapse_check_pte(). While at it, we use >vm_normal_folio() instead of vm_normal_page(). > >Suggested-by: David Hildenbrand >Suggested-by: Dev Jain >Signed-off-by: Lance Yang >--- > mm/khugepaged.c | 243 ++++++++++++++++++++++++++---------------------- > 1 file changed, 130 insertions(+), 113 deletions(-) > >diff --git a/mm/khugepaged.c b/mm/khugepaged.c >index b5c0295c3414..7116caae1fa4 100644 >--- a/mm/khugepaged.c >+++ b/mm/khugepaged.c >@@ -61,6 +61,12 @@ enum scan_result { > SCAN_PAGE_FILLED, > }; > >+enum pte_check_result { >+ PTE_CHECK_SUCCEED, >+ PTE_CHECK_CONTINUE, >+ PTE_CHECK_FAIL, >+}; >+ > #define CREATE_TRACE_POINTS > #include > >@@ -533,62 +539,139 @@ static void release_pte_pages(pte_t *pte, pte_t *_pte, > } > } > >+/* >+ * thp_collapse_check_pte - Check if a PTE is suitable for THP collapse >+ * @pte: The PTE to check >+ * @vma: The VMA the PTE belongs to >+ * @addr: The virtual address corresponding to this PTE >+ * @foliop: On success, used to return a pointer to the folio >+ * Must be non-NULL >+ * @none_or_zero: Counter for none/zero PTEs. Must be non-NULL >+ * @unmapped: Counter for swap PTEs. Can be NULL if not scanning swaps >+ * @shared: Counter for shared pages. Must be non-NULL >+ * @scan_result: Used to return the failure reason (SCAN_*) on a >+ * PTE_CHECK_FAIL return. Must be non-NULL >+ * @cc: Collapse control settings >+ * >+ * Returns: >+ * PTE_CHECK_SUCCEED - PTE is suitable, proceed with further checks >+ * PTE_CHECK_CONTINUE - Skip this PTE and continue scanning >+ * PTE_CHECK_FAIL - Abort collapse scan >+ */ >+static inline int thp_collapse_check_pte(pte_t pte, struct vm_area_struct *vma, >+ unsigned long addr, struct folio **foliop, int *none_or_zero, >+ int *unmapped, int *shared, int *scan_result, >+ struct collapse_control *cc) >+{ >+ struct folio *folio = NULL; >+ >+ if (pte_none(pte) || is_zero_pfn(pte_pfn(pte))) { >+ (*none_or_zero)++; >+ if (!userfaultfd_armed(vma) && >+ (!cc->is_khugepaged || >+ *none_or_zero <= khugepaged_max_ptes_none)) { >+ return PTE_CHECK_CONTINUE; >+ } else { >+ *scan_result = SCAN_EXCEED_NONE_PTE; >+ count_vm_event(THP_SCAN_EXCEED_NONE_PTE); >+ return PTE_CHECK_FAIL; >+ } >+ } else if (!pte_present(pte)) { >+ if (!unmapped) { >+ *scan_result = SCAN_PTE_NON_PRESENT; >+ return PTE_CHECK_FAIL; >+ } >+ >+ if (non_swap_entry(pte_to_swp_entry(pte))) { >+ *scan_result = SCAN_PTE_NON_PRESENT; >+ return PTE_CHECK_FAIL; >+ } >+ >+ (*unmapped)++; >+ if (!cc->is_khugepaged || >+ *unmapped <= khugepaged_max_ptes_swap) { >+ /* >+ * Always be strict with uffd-wp enabled swap >+ * entries. Please see comment below for >+ * pte_uffd_wp(). >+ */ >+ if (pte_swp_uffd_wp(pte)) { >+ *scan_result = SCAN_PTE_UFFD_WP; >+ return PTE_CHECK_FAIL; >+ } >+ return PTE_CHECK_CONTINUE; >+ } else { >+ *scan_result = SCAN_EXCEED_SWAP_PTE; >+ count_vm_event(THP_SCAN_EXCEED_SWAP_PTE); >+ return PTE_CHECK_FAIL; >+ } >+ } else if (pte_uffd_wp(pte)) { >+ /* >+ * Don't collapse the page if any of the small PTEs are >+ * armed with uffd write protection. Here we can also mark >+ * the new huge pmd as write protected if any of the small >+ * ones is marked but that could bring unknown userfault >+ * messages that falls outside of the registered range. >+ * So, just be simple. >+ */ >+ *scan_result = SCAN_PTE_UFFD_WP; >+ return PTE_CHECK_FAIL; >+ } >+ >+ folio = vm_normal_folio(vma, addr, pte); >+ if (unlikely(!folio) || unlikely(folio_is_zone_device(folio))) { >+ *scan_result = SCAN_PAGE_NULL; >+ return PTE_CHECK_FAIL; >+ } >+ >+ if (!folio_test_anon(folio)) { >+ VM_WARN_ON_FOLIO(true, folio); >+ *scan_result = SCAN_PAGE_ANON; >+ return PTE_CHECK_FAIL; >+ } >+ >+ /* >+ * We treat a single page as shared if any part of the THP >+ * is shared. >+ */ >+ if (folio_maybe_mapped_shared(folio)) { >+ (*shared)++; >+ if (cc->is_khugepaged && *shared > khugepaged_max_ptes_shared) { >+ *scan_result = SCAN_EXCEED_SHARED_PTE; >+ count_vm_event(THP_SCAN_EXCEED_SHARED_PTE); >+ return PTE_CHECK_FAIL; >+ } >+ } >+ >+ *foliop = folio; >+ >+ return PTE_CHECK_SUCCEED; >+} >+ This one looks much better. While my personal feeling is this is not a complete work to merge the scanning logic. We still have folio_expected_ref_count() and pte_young() check present both in __collapse_huge_page_isolate() and huge_collapse_scan_pmd(). -- Wei Yang Help you, Help me