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Sat, 08 Nov 2025 18:40:13 -0800 (PST) Date: Sun, 9 Nov 2025 02:40:13 +0000 From: Wei Yang To: Nico Pache Cc: linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org, linux-mm@kvack.org, linux-doc@vger.kernel.org, david@redhat.com, ziy@nvidia.com, baolin.wang@linux.alibaba.com, lorenzo.stoakes@oracle.com, Liam.Howlett@oracle.com, ryan.roberts@arm.com, dev.jain@arm.com, corbet@lwn.net, rostedt@goodmis.org, mhiramat@kernel.org, mathieu.desnoyers@efficios.com, akpm@linux-foundation.org, baohua@kernel.org, willy@infradead.org, peterx@redhat.com, wangkefeng.wang@huawei.com, usamaarif642@gmail.com, sunnanyong@huawei.com, vishal.moola@gmail.com, thomas.hellstrom@linux.intel.com, yang@os.amperecomputing.com, kas@kernel.org, aarcange@redhat.com, raquini@redhat.com, anshuman.khandual@arm.com, catalin.marinas@arm.com, tiwai@suse.de, will@kernel.org, dave.hansen@linux.intel.com, jack@suse.cz, cl@gentwo.org, jglisse@google.com, surenb@google.com, zokeefe@google.com, hannes@cmpxchg.org, rientjes@google.com, mhocko@suse.com, rdunlap@infradead.org, hughd@google.com, richard.weiyang@gmail.com, lance.yang@linux.dev, vbabka@suse.cz, rppt@kernel.org, jannh@google.com, pfalcato@suse.de Subject: Re: [PATCH v12 mm-new 13/15] khugepaged: avoid unnecessary mTHP collapse attempts Message-ID: <20251109024013.fzt7xxpmxwi75xgr@master> Reply-To: Wei Yang References: <20251022183717.70829-1-npache@redhat.com> <20251022183717.70829-14-npache@redhat.com> 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: <20251022183717.70829-14-npache@redhat.com> User-Agent: NeoMutt/20170113 (1.7.2) On Wed, Oct 22, 2025 at 12:37:15PM -0600, Nico Pache wrote: >There are cases where, if an attempted collapse fails, all subsequent >orders are guaranteed to also fail. Avoid these collapse attempts by >bailing out early. > >Signed-off-by: Nico Pache >--- > mm/khugepaged.c | 31 ++++++++++++++++++++++++++++++- > 1 file changed, 30 insertions(+), 1 deletion(-) > >diff --git a/mm/khugepaged.c b/mm/khugepaged.c >index e2319bfd0065..54f5c7888e46 100644 >--- a/mm/khugepaged.c >+++ b/mm/khugepaged.c >@@ -1431,10 +1431,39 @@ static int collapse_scan_bitmap(struct mm_struct *mm, unsigned long address, > ret = collapse_huge_page(mm, address, referenced, > unmapped, cc, mmap_locked, > order, offset); >- if (ret == SCAN_SUCCEED) { >+ >+ /* >+ * Analyze failure reason to determine next action: >+ * - goto next_order: try smaller orders in same region >+ * - continue: try other regions at same order >+ * - break: stop all attempts (system-wide failure) >+ */ >+ switch (ret) { >+ /* Cases were we should continue to the next region */ >+ case SCAN_SUCCEED: > collapsed += 1UL << order; >+ fallthrough; >+ case SCAN_PTE_MAPPED_HUGEPAGE: > continue; >+ /* Cases were lower orders might still succeed */ >+ case SCAN_LACK_REFERENCED_PAGE: >+ case SCAN_EXCEED_NONE_PTE: >+ case SCAN_EXCEED_SWAP_PTE: >+ case SCAN_EXCEED_SHARED_PTE: >+ case SCAN_PAGE_LOCK: >+ case SCAN_PAGE_COUNT: >+ case SCAN_PAGE_LRU: >+ case SCAN_PAGE_NULL: >+ case SCAN_DEL_PAGE_LRU: >+ case SCAN_PTE_NON_PRESENT: >+ case SCAN_PTE_UFFD_WP: >+ case SCAN_ALLOC_HUGE_PAGE_FAIL: >+ goto next_order; >+ /* All other cases should stop collapse attempts */ >+ default: >+ break; > } >+ break; One question here: Suppose we have iterated several orders and not collapse successfully yet. So the mthp_bitmap_stack[] would look like this: [8 7 6 6] ^ | Now we found this one pass the threshold check, but it fails with other result. Current code looks it would give up at all, but we may still have a chance to collapse the above 3 range? > } > > next_order: >-- >2.51.0 -- Wei Yang Help you, Help me