Re: [RFC v5 PATCH 2/2] mm: mmap: zap pages with read mmap_sem in munmap

[Laurent Dufour]

On 19/07/2018 01:21, 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 incuring significan penalty to small mappings.
                         ^       ^
                     incurring significant
> 
> 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@xxxxxxxxxx>
> Suggested-by: Kirill A. Shutemov <kirill@xxxxxxxxxxxxx>
> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> Cc: Laurent Dufour <ldufour@xxxxxxxxxxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
> ---
>  include/linux/mm.h |  2 +-
>  mm/memory.c        | 35 +++++++++++++------
>  mm/mmap.c          | 99 +++++++++++++++++++++++++++++++++++++++++++++++++-----
>  3 files changed, 117 insertions(+), 19 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index a0fbb9f..95a4e97 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_flags);
> 
>  /**
>   * mm_walk - callbacks for walk_page_range
> diff --git a/mm/memory.c b/mm/memory.c
> index 7206a63..00ecdae 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_flags)
>  {
>  	unsigned long start = max(vma->vm_start, start_addr);
>  	unsigned long end;
> @@ -1525,11 +1525,13 @@ static void unmap_single_vma(struct mmu_gather *tlb,
>  	if (end <= vma->vm_start)
>  		return;
> 
> -	if (vma->vm_file)
> -		uprobe_munmap(vma, start, end);
> +	if (!skip_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);
> +	}

I think a comment would be welcomed here to detail why it is safe to not call
uprobe_munmap() and untrack_pfn() here i.e this has already been done in
do_munmap_zap_rlock().

> 
>  	if (start != end) {
>  		if (unlikely(is_vm_hugetlb_page(vma))) {
> @@ -1546,7 +1548,19 @@ 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_flags)
> +					/*
> +					 * The vma is being unmapped with read
> +					 * mmap_sem.
> +					 * Can't update vm_flags, it will be
> +					 * updated later with exclusive lock
> +					 * 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
> @@ -1574,13 +1588,14 @@ static void unmap_single_vma(struct mmu_gather *tlb,
>   */
>  void unmap_vmas(struct mmu_gather *tlb,
>  		struct vm_area_struct *vma, unsigned long start_addr,
> -		unsigned long end_addr)
> +		unsigned long end_addr, bool skip_flags)
>  {
>  	struct mm_struct *mm = vma->vm_mm;
> 
>  	mmu_notifier_invalidate_range_start(mm, start_addr, end_addr);
>  	for ( ; vma && vma->vm_start < end_addr; vma = vma->vm_next)
> -		unmap_single_vma(tlb, vma, start_addr, end_addr, NULL);
> +		unmap_single_vma(tlb, vma, start_addr, end_addr, NULL,
> +				 skip_flags);
>  	mmu_notifier_invalidate_range_end(mm, start_addr, end_addr);
>  }
> 
> @@ -1604,7 +1619,7 @@ void zap_page_range(struct vm_area_struct *vma, unsigned long start,
>  	update_hiwater_rss(mm);
>  	mmu_notifier_invalidate_range_start(mm, start, end);
>  	for ( ; vma && vma->vm_start < end; vma = vma->vm_next) {
> -		unmap_single_vma(&tlb, vma, start, end, NULL);
> +		unmap_single_vma(&tlb, vma, start, end, NULL, false);
> 
>  		/*
>  		 * zap_page_range does not specify whether mmap_sem should be
> @@ -1641,7 +1656,7 @@ static void zap_page_range_single(struct vm_area_struct *vma, unsigned long addr
>  	tlb_gather_mmu(&tlb, mm, address, end);
>  	update_hiwater_rss(mm);
>  	mmu_notifier_invalidate_range_start(mm, address, end);
> -	unmap_single_vma(&tlb, vma, address, end, details);
> +	unmap_single_vma(&tlb, vma, address, end, details, false);
>  	mmu_notifier_invalidate_range_end(mm, address, end);
>  	tlb_finish_mmu(&tlb, address, end);
>  }
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 2504094..f5d5312 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -73,7 +73,7 @@
> 
>  static void unmap_region(struct mm_struct *mm,
>  		struct vm_area_struct *vma, struct vm_area_struct *prev,
> -		unsigned long start, unsigned long end);
> +		unsigned long start, unsigned long end, bool skip_flags);
> 
>  /* description of effects of mapping type and prot in current implementation.
>   * this is due to the limited x86 page protection hardware.  The expected
> @@ -1824,7 +1824,7 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
>  	fput(file);
> 
>  	/* Undo any partial mapping done by a device driver. */
> -	unmap_region(mm, vma, prev, vma->vm_start, vma->vm_end);
> +	unmap_region(mm, vma, prev, vma->vm_start, vma->vm_end, false);
>  	charged = 0;
>  	if (vm_flags & VM_SHARED)
>  		mapping_unmap_writable(file->f_mapping);
> @@ -2559,7 +2559,7 @@ static void remove_vma_list(struct mm_struct *mm, struct vm_area_struct *vma)
>   */
>  static void unmap_region(struct mm_struct *mm,
>  		struct vm_area_struct *vma, struct vm_area_struct *prev,
> -		unsigned long start, unsigned long end)
> +		unsigned long start, unsigned long end, bool skip_flags)
>  {
>  	struct vm_area_struct *next = prev ? prev->vm_next : mm->mmap;
>  	struct mmu_gather tlb;
> @@ -2567,7 +2567,7 @@ static void unmap_region(struct mm_struct *mm,
>  	lru_add_drain();
>  	tlb_gather_mmu(&tlb, mm, start, end);
>  	update_hiwater_rss(mm);
> -	unmap_vmas(&tlb, vma, start, end);
> +	unmap_vmas(&tlb, vma, start, end, skip_flags);
>  	free_pgtables(&tlb, vma, prev ? prev->vm_end : FIRST_USER_ADDRESS,
>  				 next ? next->vm_start : USER_PGTABLES_CEILING);
>  	tlb_finish_mmu(&tlb, start, end);
> @@ -2778,6 +2778,79 @@ static inline void munmap_mlock_vma(struct vm_area_struct *vma,
>  	}
>  }
> 
> +/*
> + * Zap pages with read mmap_sem held
> + *
> + * uf is the list for userfaultfd
> + */
> +static int do_munmap_zap_rlock(struct mm_struct *mm, unsigned long start,
> +			       size_t len, struct list_head *uf)
> +{
> +	unsigned long end = 0;
> +	struct vm_area_struct *start_vma = NULL, *prev, *vma;
> +	int ret = 0;
> +
> +	if (!munmap_addr_sanity(start, len))
> +		return -EINVAL;
> +
> +	len = PAGE_ALIGN(len);
> +
> +	end = start + len;
> +
> +	/*
> +	 * need write mmap_sem to split vmas and detach vmas
> +	 * splitting vma up-front to save PITA to clean if it is failed
> +	 */
> +	if (down_write_killable(&mm->mmap_sem))
> +		return -EINTR;
> +
> +	ret = munmap_lookup_vma(mm, &start_vma, &prev, start, end);
> +	if (ret != 1)
> +		goto out;
> +
> +	if (unlikely(uf)) {
> +		ret = userfaultfd_unmap_prep(start_vma, start, end, uf);
> +		if (ret)
> +			goto out;
> +	}
> +
> +	/* Handle mlocked vmas */
> +	if (mm->locked_vm)
> +		munmap_mlock_vma(start_vma, end);
> +
> +	/* Detach vmas from rbtree */
> +	detach_vmas_to_be_unmapped(mm, start_vma, prev, end);
> +
> +	/*
> +	 * Clear uprobe, VM_PFNMAP and hugetlb mapping in advance since they
> +	 * need update vm_flags with write mmap_sem
> +	 */
> +	vma = start_vma;
> +	for ( ; vma && vma->vm_start < end; vma = vma->vm_next) {
> +		if (vma->vm_file)
> +			uprobe_munmap(vma, vma->vm_start, vma->vm_end);
> +		if (unlikely(vma->vm_flags & VM_PFNMAP))
> +			untrack_pfn(vma, 0, 0);130680130680
> +		if (is_vm_hugetlb_page(vma))
> +			vma->vm_flags &= ~VM_MAYSHARE;
> +	}
> +
> +	downgrade_write(&mm->mmap_sem);
> +
> +	/* zap mappings with read mmap_sem */
> +	unmap_region(mm, start_vma, prev, start, end, true);
> +
> +	arch_unmap(mm, start_vma, start, end);
> +	remove_vma_list(mm, start_vma);
> +	up_read(&mm->mmap_sem);
> +
> +	return 0;
> +
> +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.
> @@ -2826,7 +2899,7 @@ int do_munmap(struct mm_struct *mm, unsigned long start, size_t len,
>  	 * Remove the vma's, and unmap the actual pages
>  	 */
>  	detach_vmas_to_be_unmapped(mm, vma, prev, end);
> -	unmap_region(mm, vma, prev, start, end);
> +	unmap_region(mm, vma, prev, start, end, false);
> 
>  	arch_unmap(mm, vma, start, end);
> 
> @@ -2836,6 +2909,17 @@ int do_munmap(struct mm_struct *mm, unsigned long start, size_t len,
>  	return 0;
>  }
> 
> +static int vm_munmap_zap_rlock(unsigned long start, size_t len)
> +{
> +	int ret;
> +	struct mm_struct *mm = current->mm;
> +	LIST_HEAD(uf);
> +
> +	ret = do_munmap_zap_rlock(mm, start, len, &uf);
> +	userfaultfd_unmap_complete(mm, &uf);
> +	return ret;
> +}
> +
>  int vm_munmap(unsigned long start, size_t len)
>  {
>  	int ret;

A stupid question, since the overhead of vm_munmap_zap_rlock() compared to
vm_munmap() is not significant, why not putting that in vm_munmap() instead of
introducing a new vm_munmap_zap_rlock() ?

> @@ -2855,10 +2939,9 @@ 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_rlock(addr, len);
>  }
> 
> -
>  /*
>   * Emulation of deprecated remap_file_pages() syscall.
>   */
> @@ -3146,7 +3229,7 @@ void exit_mmap(struct mm_struct *mm)
>  	tlb_gather_mmu(&tlb, mm, 0, -1);
>  	/* update_hiwater_rss(mm) here? but nobody should be looking */
>  	/* Use -1 here to ensure all VMAs in the mm are unmapped */
> -	unmap_vmas(&tlb, vma, 0, -1);
> +	unmap_vmas(&tlb, vma, 0, -1, false);
>  	free_pgtables(&tlb, vma, FIRST_USER_ADDRESS, USER_PGTABLES_CEILING);
>  	tlb_finish_mmu(&tlb, 0, -1);
>