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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> 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) X-Rspamd-Server: rspam12 X-Rspam-User: X-Rspamd-Queue-Id: 9AD4540006 X-Stat-Signature: 4mxah1ym8me6arj4us9yd3r8s5dxqybp X-HE-Tag: 1762656016-551337 X-HE-Meta: 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 PPgpzxMe 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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