Re: [RFC v3 PATCH 4/5] mm: mmap: zap pages with read mmap_sem for large mapping

[Yang Shi <yang.shi@linux.alibaba.com>]

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 <mhocko@kernel.org>
>> 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>
>> ---
>>   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)
>> +#endif
> PUD_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 failed
> What 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
>>