On 2023/11/10 13:04, Aneesh Kumar K.V wrote:
"zhangpeng (AS)" <zhangpeng362@xxxxxxxxxx> writes:Hi everyone, There is a performance issue that has been bothering us recently. This problem can reproduce in the latest mainline version (Linux 6.6). We use mlockall(MCL_CURRENT | MCL_FUTURE) in the user mode process to avoid performance problems caused by major fault. There is a stage in numa fault which will set pte as 0 in do_numa_page() : ptep_modify_prot_start() will clear the vmf->pte, until ptep_modify_prot_commit() assign a value to the vmf->pte. For the data segment of the user-mode program, the global variable area is a private mapping. After the pagecache is loaded, the private anonymous page is generated after the COW is triggered. Mlockall can lock COW pages (anonymous pages), but the original file pages cannot be locked and may be reclaimed. If the global variable (private anon page) is accessed when vmf->pte is zero which is concurrently set by numa fault, a file page fault will be triggered. At this time, the original private file page may have been reclaimed. If the page cache is not available at this time, a major fault will be triggered and the file will be read, causing additional overhead. Our problem scenario is as follows: task 1 task 2 ------ ------ /* scan global variables */ do_numa_page() spin_lock(vmf->ptl) ptep_modify_prot_start() /* set vmf->pte as null */ /* Access global variables */ handle_pte_fault() /* no pte lock */ do_pte_missing() do_fault() do_read_fault() ptep_modify_prot_commit() /* ptep update done */ pte_unmap_unlock(vmf->pte, vmf->ptl) do_fault_around() __do_fault() filemap_fault() /* page cache is not available and a major fault is triggered */ do_sync_mmap_readahead() /* page_not_uptodate and goto out_retry. */ Is there any way to avoid such a major fault?This is also true w.r.t change_pte_range() in addition to do_numa_page() ? -aneesh
It seems that change_pte_range() should have similar problems. However, the trigger frequency of do numa fault is generally relatively high, and it is more likely to occur in actual scenarios. -- Best Regards, Peng
