From 6375e95f381e3dc85065b6f74263a61522736203 Mon Sep 17 00:00:00 2001 From: Qi Zheng Date: Wed, 4 Dec 2024 19:09:49 +0800 Subject: mm: pgtable: reclaim empty PTE page in madvise(MADV_DONTNEED) Now in order to pursue high performance, applications mostly use some high-performance user-mode memory allocators, such as jemalloc or tcmalloc. These memory allocators use madvise(MADV_DONTNEED or MADV_FREE) to release physical memory, but neither MADV_DONTNEED nor MADV_FREE will release page table memory, which may cause huge page table memory usage. The following are a memory usage snapshot of one process which actually happened on our server: VIRT: 55t RES: 590g VmPTE: 110g In this case, most of the page table entries are empty. For such a PTE page where all entries are empty, we can actually free it back to the system for others to use. As a first step, this commit aims to synchronously free the empty PTE pages in madvise(MADV_DONTNEED) case. We will detect and free empty PTE pages in zap_pte_range(), and will add zap_details.reclaim_pt to exclude cases other than madvise(MADV_DONTNEED). Once an empty PTE is detected, we first try to hold the pmd lock within the pte lock. If successful, we clear the pmd entry directly (fast path). Otherwise, we wait until the pte lock is released, then re-hold the pmd and pte locks and loop PTRS_PER_PTE times to check pte_none() to re-detect whether the PTE page is empty and free it (slow path). For other cases such as madvise(MADV_FREE), consider scanning and freeing empty PTE pages asynchronously in the future. The following code snippet can show the effect of optimization: mmap 50G while (1) { for (; i < 1024 * 25; i++) { touch 2M memory madvise MADV_DONTNEED 2M } } As we can see, the memory usage of VmPTE is reduced: before after VIRT 50.0 GB 50.0 GB RES 3.1 MB 3.1 MB VmPTE 102640 KB 240 KB [zhengqi.arch@bytedance.com: fix uninitialized symbol 'ptl'] Link: https://lkml.kernel.org/r/20241206112348.51570-1-zhengqi.arch@bytedance.com Link: https://lore.kernel.org/linux-mm/224e6a4e-43b5-4080-bdd8-b0a6fb2f0853@stanley.mountain/ Link: https://lkml.kernel.org/r/92aba2b319a734913f18ba41e7d86a265f0b84e2.1733305182.git.zhengqi.arch@bytedance.com Signed-off-by: Qi Zheng Cc: Andy Lutomirski Cc: Catalin Marinas Cc: Dave Hansen Cc: David Hildenbrand Cc: David Rientjes Cc: Hugh Dickins Cc: Jann Horn Cc: Lorenzo Stoakes Cc: Matthew Wilcox Cc: Mel Gorman Cc: Muchun Song Cc: Peter Xu Cc: Peter Zijlstra Cc: Will Deacon Cc: Zach O'Keefe Cc: Dan Carpenter Signed-off-by: Andrew Morton --- include/linux/mm.h | 1 + 1 file changed, 1 insertion(+) (limited to 'include/linux') diff --git a/include/linux/mm.h b/include/linux/mm.h index 2e5ef71b8629..9372bc058b43 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -2320,6 +2320,7 @@ extern void pagefault_out_of_memory(void); struct zap_details { struct folio *single_folio; /* Locked folio to be unmapped */ bool even_cows; /* Zap COWed private pages too? */ + bool reclaim_pt; /* Need reclaim page tables? */ zap_flags_t zap_flags; /* Extra flags for zapping */ }; -- cgit v1.2.3