This is a port on kernel 4.14 of the work done by Peter Zijlstra to handle page fault without holding the mm semaphore [1]. The idea is to try to handle user space page faults without holding the mmap_sem. This should allow better concurrency for massively threaded process since the page fault handler will not wait for other threads memory layout change to be done, assuming that this change is done in another part of the process's memory space. This type page fault is named speculative page fault. If the speculative page fault fails because of a concurrency is detected or because underlying PMD or PTE tables are not yet allocating, it is failing its processing and a classic page fault is then tried. The speculative page fault (SPF) has to look for the VMA matching the fault address without holding the mmap_sem, so the VMA list is now managed using SRCU allowing lockless walking. The only impact would be the deferred file derefencing in the case of a file mapping, since the file pointer is released once the SRCU cleaning is done. This patch relies on the change done recently by Paul McKenney in SRCU which now runs a callback per CPU instead of per SRCU structure [1]. The VMA's attributes checked during the speculative page fault processing have to be protected against parallel changes. This is done by using a per VMA sequence lock. This sequence lock allows the speculative page fault handler to fast check for parallel changes in progress and to abort the speculative page fault in that case. Once the VMA is found, the speculative page fault handler would check for the VMA's attributes to verify that the page fault has to be handled correctly or not. Thus the VMA is protected through a sequence lock which allows fast detection of concurrent VMA changes. If such a change is detected, the speculative page fault is aborted and a *classic* page fault is tried. VMA sequence locks are added when VMA attributes which are checked during the page fault are modified. When the PTE is fetched, the VMA is checked to see if it has been changed, so once the page table is locked, the VMA is valid, so any other changes leading to touching this PTE will need to lock the page table, so no parallel change is possible at this time. Compared to the Peter's initial work, this series introduces a spin_trylock when dealing with speculative page fault. This is required to avoid dead lock when handling a page fault while a TLB invalidate is requested by an other CPU holding the PTE. Another change due to a lock dependency issue with mapping->i_mmap_rwsem. In addition some VMA field values which are used once the PTE is unlocked at the end the page fault path are saved into the vm_fault structure to used the values matching the VMA at the time the PTE was locked. This series only support VMA with no vm_ops define, so huge page and mapped file are not managed with the speculative path. In addition transparent huge page are not supported. Once this series will be accepted upstream I'll extend the support to mapped files, and transparent huge pages. This series builds on top of v4.14-rc3-mmotm and is functional on x86 and PowerPC. Tests have been made using a large commercial in-memory database on a PowerPC system with 752 CPU using RFC v5 using a previous version of this series. The results are very encouraging since the loading of the 2TB database was faster by 14% with the speculative page fault. Using ebizzy test [3], which spreads a lot of threads, the result are good when running on both a large or a small system. When using kernbench, the result are quite similar which expected as not so much multithreaded processes are involved. But there is no performance degradation neither which is good. ------------------ Benchmarks results No change since v4. Please, see https://lkml.org/lkml/2017/10/9/180 for details. Impact on the text size for x86_64 (same as v4): UP SMP 4.14.0-rc3-mm1 0x008ed859 0x00966ea9 4.14.0-rc3-mm1-spf 0x008ed859 0x00968ea9 ------------------------ Changes since v4: - As requested by Andrew Morton, use CONFIG_SPF and define it earlier in the series to ease bisection. Changes since v3: - Don't build when CONFIG_SMP is not set - Fixed a lock dependency warning in __vma_adjust() - Use READ_ONCE to access p*d values in handle_speculative_fault() - Call memcp_oom() service in handle_speculative_fault() Changes since v2: - Perf event is renamed in PERF_COUNT_SW_SPF - On Power handle do_page_fault()'s cleaning - On Power if the VM_FAULT_ERROR is returned by handle_speculative_fault(), do not retry but jump to the error path - If VMA's flags are not matching the fault, directly returns VM_FAULT_SIGSEGV and not VM_FAULT_RETRY - Check for pud_trans_huge() to avoid speculative path - Handles _vm_normal_page()'s introduced by 6f16211df3bf ("mm/device-public-memory: device memory cache coherent with CPU") - add and review few comments in the code Changes since v1: - Remove PERF_COUNT_SW_SPF_FAILED perf event. - Add tracing events to details speculative page fault failures. - Cache VMA fields values which are used once the PTE is unlocked at the end of the page fault events. - Ensure that fields read during the speculative path are written and read using WRITE_ONCE and READ_ONCE. - Add checks at the beginning of the speculative path to abort it if the VMA is known to not be supported. Changes since RFC V5 [5] - Port to 4.13 kernel - Merging patch fixing lock dependency into the original patch - Replace the 2 parameters of vma_has_changed() with the vmf pointer - In patch 7, don't call __do_fault() in the speculative path as it may want to unlock the mmap_sem. - In patch 11-12, don't check for vma boundaries when page_add_new_anon_rmap() is called during the spf path and protect against anon_vma pointer's update. - In patch 13-16, add performance events to report number of successful and failed speculative events. [1] http://linux-kernel.2935.n7.nabble.com/RFC-PATCH-0-6-Another-go-at-speculative-page-faults-tt965642.html#none [2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=da915ad5cf25b5f5d358dd3670c3378d8ae8c03e [3] http://ebizzy.sourceforge.net/ [4] http://ck.kolivas.org/apps/kernbench/kernbench-0.50/ [5] https://lwn.net/Articles/725607/ Laurent Dufour (16): x86/mm: Define CONFIG_SPF powerpc/mm: Define CONFIG_SPF mm: Introduce pte_spinlock for FAULT_FLAG_SPECULATIVE mm: Protect VMA modifications using VMA sequence count mm: Cache some VMA fields in the vm_fault structure mm: Protect SPF handler against anon_vma changes mm/migrate: Pass vm_fault pointer to migrate_misplaced_page() mm: Introduce __lru_cache_add_active_or_unevictable mm: Introduce __maybe_mkwrite() mm: Introduce __vm_normal_page() mm: Introduce __page_add_new_anon_rmap() mm: Try spin lock in speculative path mm: Adding speculative page fault failure trace events perf: Add a speculative page fault sw event perf tools: Add support for the SPF perf event powerpc/mm: Add speculative page fault Peter Zijlstra (6): mm: Dont assume page-table invariance during faults mm: Prepare for FAULT_FLAG_SPECULATIVE mm: VMA sequence count mm: RCU free VMAs mm: Provide speculative fault infrastructure x86/mm: Add speculative pagefault handling arch/powerpc/Kconfig | 4 + arch/powerpc/mm/fault.c | 17 ++ arch/x86/Kconfig | 4 + arch/x86/mm/fault.c | 21 ++ fs/proc/task_mmu.c | 5 +- fs/userfaultfd.c | 17 +- include/linux/hugetlb_inline.h | 2 +- include/linux/migrate.h | 4 +- include/linux/mm.h | 69 ++++- include/linux/mm_types.h | 5 + include/linux/pagemap.h | 4 +- include/linux/rmap.h | 12 +- include/linux/swap.h | 11 +- include/trace/events/pagefault.h | 87 ++++++ include/uapi/linux/perf_event.h | 1 + kernel/fork.c | 1 + mm/hugetlb.c | 2 + mm/init-mm.c | 1 + mm/internal.h | 21 ++ mm/khugepaged.c | 5 + mm/madvise.c | 6 +- mm/memory.c | 496 +++++++++++++++++++++++++++++----- mm/mempolicy.c | 51 ++-- mm/migrate.c | 4 +- mm/mlock.c | 13 +- mm/mmap.c | 160 ++++++++--- mm/mprotect.c | 4 +- mm/mremap.c | 6 + mm/rmap.c | 5 +- mm/swap.c | 12 +- tools/include/uapi/linux/perf_event.h | 1 + tools/perf/util/evsel.c | 1 + tools/perf/util/parse-events.c | 4 + tools/perf/util/parse-events.l | 1 + tools/perf/util/python.c | 1 + 35 files changed, 906 insertions(+), 152 deletions(-) create mode 100644 include/trace/events/pagefault.h -- 2.7.4 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. 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