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SubjectRe: [RFC v6 PATCH 2/2] mm: mmap: zap pages with read mmap_sem in munmap
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On 7/26/18 11:34 AM, Mika Penttilä wrote:
>
> On 26.07.2018 21:10, 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 exclusively 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 Hocko [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.
>>
>> But, some part may need write mmap_sem, for example, vma splitting. So,
>> the design is as follows:
>> acquire write mmap_sem
>> lookup vmas (find and split vmas)
>> detach vmas
>> deal with special mappings
>> downgrade_write
>>
>> zap pages
>> free page tables
>> release mmap_sem
>>
>> The vm events with read mmap_sem may come in during page zapping, but
>> since vmas have been detached before, they, i.e. page fault, gup, etc,
>> will not be able to find valid vma, then just return SIGSEGV or -EFAULT
>> as expected.
>>
>> If the vma has VM_LOCKED | VM_HUGETLB | VM_PFNMAP or uprobe, they are
>> considered as special mappings. They will be dealt with before zapping
>> pages with write mmap_sem held. Basically, just update vm_flags.
>>
>> And, since they are also manipulated by unmap_single_vma() which is
>> called by unmap_vma() with read mmap_sem held in this case, to
>> prevent from updating vm_flags in read critical section, a new
>> parameter, called "skip_flags" is added to unmap_region(), unmap_vmas()
>> and unmap_single_vma(). If it is true, then just skip unmap those
>> special mappings. Currently, the only place which pass true to this
>> parameter is us.
>>
>> With this approach we don't have to re-acquire mmap_sem again to clean
>> up vmas to avoid race window which might get the address space changed.
>>
>> And, since the lock acquire/release cost is managed to the minimum and
>> almost as same as before, the optimization could be extended to any size
>> of mapping without incurring significant penalty to small mappings.
>>
>> For the time being, just do this in munmap syscall path. Other
>> vm_munmap() or do_munmap() call sites (i.e mmap, mremap, etc) remain
>> intact for stability reason.
>>
>> With the patches, exclusive mmap_sem hold time when munmap a 80GB
>> address space on a machine with 32 cores of E5-2680 @ 2.70GHz dropped to
>> us level from second.
>>
>> munmap_test-15002 [008] 594.380138: funcgraph_entry: | vm_munmap_zap_rlock() {
>> munmap_test-15002 [008] 594.380146: funcgraph_entry: !2485684 us | unmap_region();
>> munmap_test-15002 [008] 596.865836: funcgraph_exit: !2485692 us | }
>>
>> Here the excution time of unmap_region() is used to evaluate the time of
>> holding read mmap_sem, then the remaining time is used with holding
>> exclusive lock.
>>
>> [1] https://lwn.net/Articles/753269/
>>
>> Suggested-by: Michal Hocko <mhocko@kernel.org>
>> Suggested-by: Kirill A. Shutemov <kirill@shutemov.name>
>> Cc: Matthew Wilcox <willy@infradead.org>
>> Cc: Laurent Dufour <ldufour@linux.vnet.ibm.com>
>> Cc: Andrew Morton <akpm@linux-foundation.org>
>> Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>
>> ---
>> include/linux/mm.h | 2 +-
>> mm/memory.c | 41 ++++++++++++++++------
>> mm/mmap.c | 99 +++++++++++++++++++++++++++++++++++++++++++++++++-----
>> 3 files changed, 123 insertions(+), 19 deletions(-)
>>
>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>> index a0fbb9f..e4480d8 100644
>> --- a/include/linux/mm.h
>> +++ b/include/linux/mm.h
>> @@ -1321,7 +1321,7 @@ void zap_vma_ptes(struct vm_area_struct *vma, unsigned long address,
>> void zap_page_range(struct vm_area_struct *vma, unsigned long address,
>> unsigned long size);
>> void unmap_vmas(struct mmu_gather *tlb, struct vm_area_struct *start_vma,
>> - unsigned long start, unsigned long end);
>> + unsigned long start, unsigned long end, bool skip_vm_flags);
>>
>> /**
>> * mm_walk - callbacks for walk_page_range
>> diff --git a/mm/memory.c b/mm/memory.c
>> index 7206a63..6a772bd 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -1514,7 +1514,7 @@ void unmap_page_range(struct mmu_gather *tlb,
>> static void unmap_single_vma(struct mmu_gather *tlb,
>> struct vm_area_struct *vma, unsigned long start_addr,
>> unsigned long end_addr,
>> - struct zap_details *details)
>> + struct zap_details *details, bool skip_vm_flags)
>> {
>> unsigned long start = max(vma->vm_start, start_addr);
>> unsigned long end;
>> @@ -1525,11 +1525,19 @@ static void unmap_single_vma(struct mmu_gather *tlb,
>> if (end <= vma->vm_start)
>> return;
>>
>> - if (vma->vm_file)
>> - uprobe_munmap(vma, start, end);
>> + /*
>> + * Since unmap_single_vma might be called with read mmap_sem held
>> + * in munmap optimization, so vm_flags can't be updated in this case.
>> + * They have been updated before this call with write mmap_sem held.
>> + * Here if skip_vm_flags is true, just skip the update.
>> + */
>> + if (!skip_vm_flags) {
>> + if (vma->vm_file)
>> + uprobe_munmap(vma, start, end);
>>
>> - if (unlikely(vma->vm_flags & VM_PFNMAP))
>> - untrack_pfn(vma, 0, 0);
>> + if (unlikely(vma->vm_flags & VM_PFNMAP))
>> + untrack_pfn(vma, 0, 0);
>> + }
>>
>> if (start != end) {
>> if (unlikely(is_vm_hugetlb_page(vma))) {
>> @@ -1546,7 +1554,19 @@ static void unmap_single_vma(struct mmu_gather *tlb,
>> */
>> if (vma->vm_file) {
>> i_mmap_lock_write(vma->vm_file->f_mapping);
>> - __unmap_hugepage_range_final(tlb, vma, start, end, NULL);
>> + if (!skip_vm_flags) {
> Should that be :
> if (skip_vm_flags) {
> instead?

Oh, yes. Thanks for catching this.

Yang

>
>
>> + /*
>> + * The vma is being unmapped with read
>> + * mmap_sem.
>> + * Can't update vm_flags here, it has
>> + * been updated before this call with
>> + * write mmap_sem held.
>> + */
>> + __unmap_hugepage_range(tlb, vma, start,
>> + end, NULL);
>> + } else
>> + __unmap_hugepage_range_final(tlb, vma,
>> + start, end, NULL);
>> i_mmap_unlock_write(vma->vm_file->f_mapping);
>> }
>> } else
>>
> --Mika

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