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SubjectRe: [PATCH v2 00/20] Speculative page faults
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On 21/08/2017 04:26, Sergey Senozhatsky wrote:
> Hello,
>
> On (08/18/17 00:04), Laurent Dufour wrote:
>> This is a port on kernel 4.13 of the work done by Peter Zijlstra to
>> handle page fault without holding the mm semaphore [1].
>>
>> The idea is to try to handle user space page faults without holding the
>> mmap_sem. This should allow better concurrency for massively threaded
>> process since the page fault handler will not wait for other threads memory
>> layout change to be done, assuming that this change is done in another part
>> of the process's memory space. This type page fault is named speculative
>> page fault. If the speculative page fault fails because of a concurrency is
>> detected or because underlying PMD or PTE tables are not yet allocating, it
>> is failing its processing and a classic page fault is then tried.
>>
>> The speculative page fault (SPF) has to look for the VMA matching the fault
>> address without holding the mmap_sem, so the VMA list is now managed using
>> SRCU allowing lockless walking. The only impact would be the deferred file
>> derefencing in the case of a file mapping, since the file pointer is
>> released once the SRCU cleaning is done. This patch relies on the change
>> done recently by Paul McKenney in SRCU which now runs a callback per CPU
>> instead of per SRCU structure [1].
>>
>> The VMA's attributes checked during the speculative page fault processing
>> have to be protected against parallel changes. This is done by using a per
>> VMA sequence lock. This sequence lock allows the speculative page fault
>> handler to fast check for parallel changes in progress and to abort the
>> speculative page fault in that case.
>>
>> Once the VMA is found, the speculative page fault handler would check for
>> the VMA's attributes to verify that the page fault has to be handled
>> correctly or not. Thus the VMA is protected through a sequence lock which
>> allows fast detection of concurrent VMA changes. If such a change is
>> detected, the speculative page fault is aborted and a *classic* page fault
>> is tried. VMA sequence locks are added when VMA attributes which are
>> checked during the page fault are modified.
>>
>> When the PTE is fetched, the VMA is checked to see if it has been changed,
>> so once the page table is locked, the VMA is valid, so any other changes
>> leading to touching this PTE will need to lock the page table, so no
>> parallel change is possible at this time.
>
> [ 2311.315400] ======================================================
> [ 2311.315401] WARNING: possible circular locking dependency detected
> [ 2311.315403] 4.13.0-rc5-next-20170817-dbg-00039-gaf11d7500492-dirty #1743 Not tainted
> [ 2311.315404] ------------------------------------------------------
> [ 2311.315406] khugepaged/43 is trying to acquire lock:
> [ 2311.315407] (&mapping->i_mmap_rwsem){++++}, at: [<ffffffff8111b339>] rmap_walk_file+0x5a/0x147
> [ 2311.315415]
> but task is already holding lock:
> [ 2311.315416] (fs_reclaim){+.+.}, at: [<ffffffff810ebd80>] fs_reclaim_acquire+0x12/0x35
> [ 2311.315420]
> which lock already depends on the new lock.
>
> [ 2311.315422]
> the existing dependency chain (in reverse order) is:
> [ 2311.315423]
> -> #3 (fs_reclaim){+.+.}:
> [ 2311.315427] fs_reclaim_acquire+0x32/0x35
> [ 2311.315429] __alloc_pages_nodemask+0x8d/0x217
> [ 2311.315432] pte_alloc_one+0x13/0x5e
> [ 2311.315434] __pte_alloc+0x1f/0x83
> [ 2311.315436] move_page_tables+0x2c9/0x5ac
> [ 2311.315438] move_vma.isra.25+0xff/0x2a2
> [ 2311.315439] SyS_mremap+0x41b/0x49e
> [ 2311.315442] entry_SYSCALL_64_fastpath+0x18/0xad
> [ 2311.315443]
> -> #2 (&vma->vm_sequence/1){+.+.}:
> [ 2311.315449] write_seqcount_begin_nested+0x1b/0x1d
> [ 2311.315451] __vma_adjust+0x1b7/0x5d6
> [ 2311.315453] __split_vma+0x142/0x1a3
> [ 2311.315454] do_munmap+0x128/0x2af
> [ 2311.315455] vm_munmap+0x5a/0x73
> [ 2311.315458] elf_map+0xb1/0xce
> [ 2311.315459] load_elf_binary+0x8e0/0x1348
> [ 2311.315462] search_binary_handler+0x70/0x1f3
> [ 2311.315464] load_script+0x1a6/0x1b5
> [ 2311.315466] search_binary_handler+0x70/0x1f3
> [ 2311.315468] do_execveat_common+0x461/0x691
> [ 2311.315471] kernel_init+0x5a/0xf0
> [ 2311.315472] ret_from_fork+0x27/0x40
> [ 2311.315473]
> -> #1 (&vma->vm_sequence){+.+.}:
> [ 2311.315478] write_seqcount_begin_nested+0x1b/0x1d
> [ 2311.315480] __vma_adjust+0x19c/0x5d6
> [ 2311.315481] __split_vma+0x142/0x1a3
> [ 2311.315482] do_munmap+0x128/0x2af
> [ 2311.315484] vm_munmap+0x5a/0x73
> [ 2311.315485] elf_map+0xb1/0xce
> [ 2311.315487] load_elf_binary+0x8e0/0x1348
> [ 2311.315489] search_binary_handler+0x70/0x1f3
> [ 2311.315490] load_script+0x1a6/0x1b5
> [ 2311.315492] search_binary_handler+0x70/0x1f3
> [ 2311.315494] do_execveat_common+0x461/0x691
> [ 2311.315496] kernel_init+0x5a/0xf0
> [ 2311.315497] ret_from_fork+0x27/0x40
> [ 2311.315498]
> -> #0 (&mapping->i_mmap_rwsem){++++}:
> [ 2311.315503] lock_acquire+0x176/0x19e
> [ 2311.315505] down_read+0x3b/0x55
> [ 2311.315507] rmap_walk_file+0x5a/0x147
> [ 2311.315508] page_referenced+0x11c/0x134
> [ 2311.315511] shrink_page_list+0x36b/0xb80
> [ 2311.315512] shrink_inactive_list+0x1d9/0x437
> [ 2311.315514] shrink_node_memcg.constprop.71+0x3e7/0x571
> [ 2311.315515] shrink_node+0x3f/0x149
> [ 2311.315517] try_to_free_pages+0x270/0x45f
> [ 2311.315518] __alloc_pages_slowpath+0x34a/0xaa2
> [ 2311.315520] __alloc_pages_nodemask+0x111/0x217
> [ 2311.315523] khugepaged_alloc_page+0x17/0x45
> [ 2311.315524] khugepaged+0xa29/0x16b5
> [ 2311.315527] kthread+0xfb/0x103
> [ 2311.315529] ret_from_fork+0x27/0x40
> [ 2311.315530]
> other info that might help us debug this:
>
> [ 2311.315531] Chain exists of:
> &mapping->i_mmap_rwsem --> &vma->vm_sequence/1 --> fs_reclaim

Hi Sergey,

I can't see where such a chain could happen.

I tried to recreate it on top of the latest mm tree, to latest stack output
but I can't get it.
How did you raised this one ?

Thanks,
Laurent.


>
> [ 2311.315537] Possible unsafe locking scenario:
>
> [ 2311.315538] CPU0 CPU1
> [ 2311.315539] ---- ----
> [ 2311.315540] lock(fs_reclaim);
> [ 2311.315542] lock(&vma->vm_sequence/1);
> [ 2311.315545] lock(fs_reclaim);
> [ 2311.315547] lock(&mapping->i_mmap_rwsem);
> [ 2311.315549]
> *** DEADLOCK ***
>
> [ 2311.315551] 1 lock held by khugepaged/43:
> [ 2311.315552] #0: (fs_reclaim){+.+.}, at: [<ffffffff810ebd80>] fs_reclaim_acquire+0x12/0x35
> [ 2311.315556]
> stack backtrace:
> [ 2311.315559] CPU: 0 PID: 43 Comm: khugepaged Not tainted 4.13.0-rc5-next-20170817-dbg-00039-gaf11d7500492-dirty #1743
> [ 2311.315560] Call Trace:
> [ 2311.315564] dump_stack+0x67/0x8e
> [ 2311.315568] print_circular_bug.isra.39+0x1c7/0x1d4
> [ 2311.315570] __lock_acquire+0xb1a/0xe06
> [ 2311.315572] ? graph_unlock+0x69/0x69
> [ 2311.315575] lock_acquire+0x176/0x19e
> [ 2311.315577] ? rmap_walk_file+0x5a/0x147
> [ 2311.315579] down_read+0x3b/0x55
> [ 2311.315581] ? rmap_walk_file+0x5a/0x147
> [ 2311.315583] rmap_walk_file+0x5a/0x147
> [ 2311.315585] page_referenced+0x11c/0x134
> [ 2311.315587] ? page_vma_mapped_walk_done.isra.15+0xb/0xb
> [ 2311.315589] ? page_get_anon_vma+0x6d/0x6d
> [ 2311.315591] shrink_page_list+0x36b/0xb80
> [ 2311.315593] ? _raw_spin_unlock_irq+0x29/0x46
> [ 2311.315595] shrink_inactive_list+0x1d9/0x437
> [ 2311.315597] shrink_node_memcg.constprop.71+0x3e7/0x571
> [ 2311.315600] shrink_node+0x3f/0x149
> [ 2311.315602] try_to_free_pages+0x270/0x45f
> [ 2311.315604] __alloc_pages_slowpath+0x34a/0xaa2
> [ 2311.315608] ? ___might_sleep+0xd5/0x234
> [ 2311.315609] __alloc_pages_nodemask+0x111/0x217
> [ 2311.315612] khugepaged_alloc_page+0x17/0x45
> [ 2311.315613] khugepaged+0xa29/0x16b5
> [ 2311.315616] ? remove_wait_queue+0x47/0x47
> [ 2311.315618] ? collapse_shmem.isra.43+0x882/0x882
> [ 2311.315620] kthread+0xfb/0x103
> [ 2311.315622] ? __list_del_entry+0x1d/0x1d
> [ 2311.315624] ret_from_fork+0x27/0x40
>
> -ss
>

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