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wangkefeng.wang@huawei.com, usamaarif642@gmail.com, sunnanyong@huawei.com, vishal.moola@gmail.com, thomas.hellstrom@linux.intel.com, yang@os.amperecomputing.com, kirill.shutemov@linux.intel.com, 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 Subject: Re: [PATCH v10 08/13] khugepaged: avoid unnecessary mTHP collapse attempts Message-ID: <300be96e-6029-42eb-b284-e659d815ee8a@lucifer.local> References: <20250819134205.622806-1-npache@redhat.com> <20250819134205.622806-9-npache@redhat.com> Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20250819134205.622806-9-npache@redhat.com> X-ClientProxiedBy: LO4P265CA0235.GBRP265.PROD.OUTLOOK.COM (2603:10a6:600:315::12) To 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List-Subscribe: List-Unsubscribe: On Tue, Aug 19, 2025 at 07:42:00AM -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 6a4cf7e4a7cc..7d9b5100bea1 100644 > --- a/mm/khugepaged.c > +++ b/mm/khugepaged.c > @@ -1389,10 +1389,39 @@ 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 * KHUGEPAGED_MIN_MTHP_NR); > - 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 += (1 << order); Yeah as bot noticed (and clang locally) This needs a break or fallthrough. > + case SCAN_PAGE_RO: > + 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; Wouldn't it be better to not have this so the compiler asserts that you have all cases listed here? > } > + break; > } > > next_order: > -- > 2.50.1 >