On 7/2/18 5:33 AM, Kirill A. Shutemov wrote:
On Sat, Jun 30, 2018 at 06:39:44AM +0800, Yang Shi wrote:When running some mmap/munmap scalability tests with large memory (i.e.300GB), the below hung task issue may happen occasionally.INFO: task ps:14018 blocked for more than 120 seconds. Tainted: G E 4.9.79-009.ali3000.alios7.x86_64 #1 "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. ps D 0 14018 1 0x00000004 ffff885582f84000 ffff885e8682f000 ffff880972943000 ffff885ebf499bc0 ffff8828ee120000 ffffc900349bfca8 ffffffff817154d0 0000000000000040 00ffffff812f872a ffff885ebf499bc0 024000d000948300 ffff880972943000 Call Trace: [<ffffffff817154d0>] ? __schedule+0x250/0x730 [<ffffffff817159e6>] schedule+0x36/0x80 [<ffffffff81718560>] rwsem_down_read_failed+0xf0/0x150 [<ffffffff81390a28>] call_rwsem_down_read_failed+0x18/0x30 [<ffffffff81717db0>] down_read+0x20/0x40 [<ffffffff812b9439>] proc_pid_cmdline_read+0xd9/0x4e0 [<ffffffff81253c95>] ? do_filp_open+0xa5/0x100 [<ffffffff81241d87>] __vfs_read+0x37/0x150 [<ffffffff812f824b>] ? security_file_permission+0x9b/0xc0 [<ffffffff81242266>] vfs_read+0x96/0x130 [<ffffffff812437b5>] SyS_read+0x55/0xc0 [<ffffffff8171a6da>] entry_SYSCALL_64_fastpath+0x1a/0xc5 It is because munmap holds mmap_sem from very beginning to all the way down to the end, and doesn't release it in the middle. When unmapping large mapping, it may take long time (take ~18 seconds to unmap 320GB mapping with every single page mapped on an idle machine). It is because munmap holds mmap_sem from very beginning to all the way down to the end, and doesn't release it in the middle. When unmapping large mapping, it may take long time (take ~18 seconds to unmap 320GB mapping with every single page mapped on an idle machine). Zapping pages is the most time consuming part, according to the suggestion from Michal Hock [1], zapping pages can be done with holding read mmap_sem, like what MADV_DONTNEED does. Then re-acquire write mmap_sem to cleanup vmas. All zapped vmas will have VM_DEAD flag set, the page fault to VM_DEAD vma will trigger SIGSEGV. Define large mapping size thresh as PUD size or 1GB, just zap pages with read mmap_sem for mappings which are >= thresh value. If the vma has VM_LOCKED | VM_HUGETLB | VM_PFNMAP or uprobe, then just fallback to regular path since unmapping those mappings need acquire write mmap_sem. For the time being, just do this in munmap syscall path. Other vm_munmap() or do_munmap() call sites remain intact for stability reason. The below is some regression and performance data collected on a machine with 32 cores of E5-2680 @ 2.70GHz and 384GB memory. With the patched kernel, write mmap_sem hold time is dropped to us level from second. [1] https://lwn.net/Articles/753269/ Cc: Michal Hocko <mho...@kernel.org> Cc: Matthew Wilcox <wi...@infradead.org> Cc: Laurent Dufour <lduf...@linux.vnet.ibm.com> Cc: Andrew Morton <a...@linux-foundation.org> Signed-off-by: Yang Shi <yang....@linux.alibaba.com> --- mm/mmap.c | 136 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++- 1 file changed, 134 insertions(+), 2 deletions(-) diff --git a/mm/mmap.c b/mm/mmap.c index 87dcf83..d61e08b 100644 --- a/mm/mmap.c +++ b/mm/mmap.c @@ -2763,6 +2763,128 @@ static int munmap_lookup_vma(struct mm_struct *mm, struct vm_area_struct **vma, return 1; }+/* Consider PUD size or 1GB mapping as large mapping */+#ifdef HPAGE_PUD_SIZE +#define LARGE_MAP_THRESH HPAGE_PUD_SIZE +#else +#define LARGE_MAP_THRESH (1 * 1024 * 1024 * 1024) +#endifPUD_SIZE is defined everywhere.
If THP is defined, otherwise it is:
#define HPAGE_PUD_SIZE ({ BUILD_BUG(); 0; })
+ +/* Unmap large mapping early with acquiring read mmap_sem */ +static int do_munmap_zap_early(struct mm_struct *mm, unsigned long start, + size_t len, struct list_head *uf) +{ + unsigned long end = 0; + struct vm_area_struct *vma = NULL, *prev, *tmp; + bool success = false; + int ret = 0; + + if (!munmap_addr_sanity(start, len)) + return -EINVAL; + + len = PAGE_ALIGN(len); + + end = start + len; + + /* Just deal with uf in regular path */ + if (unlikely(uf)) + goto regular_path; + + if (len >= LARGE_MAP_THRESH) { + /* + * need write mmap_sem to split vma and set VM_DEAD flag + * splitting vma up-front to save PITA to clean if it is failedWhat errors do you talk about? ENOMEM on VMA split? Anything else?
Yes, ENOMEM on vma split.
+ */ + down_write(&mm->mmap_sem); + ret = munmap_lookup_vma(mm, &vma, &prev, start, end); + if (ret != 1) { + up_write(&mm->mmap_sem); + return ret; + } + /* This ret value might be returned, so reset it */ + ret = 0; + + /* + * Unmapping vmas, which has VM_LOCKED|VM_HUGETLB|VM_PFNMAP + * flag set or has uprobes set, need acquire write map_sem, + * so skip them in early zap. Just deal with such mapping in + * regular path. + * Borrow can_madv_dontneed_vma() to check the conditions. + */ + tmp = vma; + while (tmp && tmp->vm_start < end) { + if (!can_madv_dontneed_vma(tmp) || + vma_has_uprobes(tmp, start, end)) { + up_write(&mm->mmap_sem); + goto regular_path; + } + tmp = tmp->vm_next; + } + /* + * set VM_DEAD flag before tear down them. + * page fault on VM_DEAD vma will trigger SIGSEGV. + */ + tmp = vma; + for ( ; tmp && tmp->vm_start < end; tmp = tmp->vm_next) + tmp->vm_flags |= VM_DEAD;I probably miss the explanation somewhere, but what's wrong with allowing other thread to re-populate the VMA? I would rather allow the VMA to be re-populated by other thread while we are zapping the range. And later zap the range again under down_write. It should also lead to consolidated regular path: take mmap_sem for write and call do_munmap(). On the first path we just skip VMA we cannot deal with under down_read(mmap_sem), regular path will take care of them.+ up_write(&mm->mmap_sem); + + /* zap mappings with read mmap_sem */ + down_read(&mm->mmap_sem);Yeah. There's race between up_write() and down_read(). Use downgrade, as Andrew suggested.+ zap_page_range(vma, start, len); + /* indicates early zap is success */ + success = true; + up_read(&mm->mmap_sem);And here again. This race can be avoided if we wouldn't carry vma to regular_path, but just go directly to do_munmap().
Thanks, Kirill. Yes, I did think about re-validating vmas before. This sounds reasonable to avoid the race. Although we spend more time in re-looking up vmas, but it should be very short, and the duplicate zap should be very short too.
Yang
+ } + +regular_path: + /* hold write mmap_sem for vma manipulation or regular path */ + if (down_write_killable(&mm->mmap_sem)) + return -EINTR; + if (success) { + /* vmas have been zapped, here clean up pgtable and vmas */ + struct vm_area_struct *next = prev ? prev->vm_next : mm->mmap; + struct mmu_gather tlb; + tlb_gather_mmu(&tlb, mm, start, end); + free_pgtables(&tlb, vma, prev ? prev->vm_end : FIRST_USER_ADDRESS, + next ? next->vm_start : USER_PGTABLES_CEILING); + tlb_finish_mmu(&tlb, start, end); + + detach_vmas_to_be_unmapped(mm, vma, prev, end); + arch_unmap(mm, vma, start, end); + remove_vma_list(mm, vma); + } else { + /* vma is VM_LOCKED|VM_HUGETLB|VM_PFNMAP or has uprobe */ + if (vma) { + if (unlikely(uf)) { + int ret = userfaultfd_unmap_prep(vma, start, + end, uf); + if (ret) + goto out; + } + if (mm->locked_vm) { + tmp = vma; + while (tmp && tmp->vm_start < end) { + if (tmp->vm_flags & VM_LOCKED) { + mm->locked_vm -= vma_pages(tmp); + munlock_vma_pages_all(tmp); + } + tmp = tmp->vm_next; + } + } + detach_vmas_to_be_unmapped(mm, vma, prev, end); + unmap_region(mm, vma, prev, start, end); + remove_vma_list(mm, vma); + } else + /* When mapping size < LARGE_MAP_THRESH */ + ret = do_munmap(mm, start, len, uf); + } + +out: + up_write(&mm->mmap_sem); + return ret; +} + /* Munmap is split into 2 main parts -- this part which finds * what needs doing, and the areas themselves, which do the * work. This now handles partial unmappings. @@ -2829,6 +2951,17 @@ int do_munmap(struct mm_struct *mm, unsigned long start, size_t len, return 0; }+static int vm_munmap_zap_early(unsigned long start, size_t len)+{ + int ret; + struct mm_struct *mm = current->mm; + LIST_HEAD(uf); + + ret = do_munmap_zap_early(mm, start, len, &uf); + userfaultfd_unmap_complete(mm, &uf); + return ret; +} + int vm_munmap(unsigned long start, size_t len) { int ret; @@ -2848,10 +2981,9 @@ int vm_munmap(unsigned long start, size_t len) SYSCALL_DEFINE2(munmap, unsigned long, addr, size_t, len) { profile_munmap(addr); - return vm_munmap(addr, len); + return vm_munmap_zap_early(addr, len); }-/* * Emulation of deprecated remap_file_pages() syscall. */ -- 1.8.3.1
