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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, 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: <541a75fe-3635-49ab-b61f-d86afc96df98@lucifer.local> References: <20251022183717.70829-1-npache@redhat.com> <20251022183717.70829-14-npache@redhat.com> Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: 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 +rBFHJgk 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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 The stack is a DFS, so this isn't correct, you may have pushed a bunch of higher order candidate mTHPs (I don't like plain 'region') which you will also true. > + * - break: stop all attempts (system-wide failure) > + */ This comment isn't hugely helpful, just put the relevant comments next to each of the goto, continue, break (soon to be return re: review below) statements please. > + switch (ret) { > + /* Cases were we should continue to the next region */ > + case SCAN_SUCCEED: > collapsed += 1UL << order; > + fallthrough; > + case SCAN_PTE_MAPPED_HUGEPAGE: > continue; Would we not run into trouble potentially in the previous patch's implementation of this examing candidate mTHPs that are part of an already existing huge page, or would a folio check in the collapse just make this wasted work? > + /* Cases were lower orders might still succeed */ > + case SCAN_LACK_REFERENCED_PAGE: > + case SCAN_EXCEED_NONE_PTE: How can we, having checked the max_pte_none, still fail due to this? > + 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 */ I don't love us having a catch-all, plase spell out all cases. > + default: > + break; > } > + break; _Hate_ this double break. Just return collapsed please. > } > > next_order: > -- > 2.51.0 >