Re: [PATCH -alternative] mm: hugetlbfs: Close race during teardown of hugetlbfs shared page tables V2 (resend)

[Michal Hocko <mhocko@suse.cz>]

On Tue 24-07-12 10:34:06, Mel Gorman wrote:
[...]
> ---8<---
> mm: hugetlbfs: Close race during teardown of hugetlbfs shared page tables
> 
> If a process creates a large hugetlbfs mapping that is eligible for page
> table sharing and forks heavily with children some of whom fault and
> others which destroy the mapping then it is possible for page tables to
> get corrupted. Some teardowns of the mapping encounter a "bad pmd" and
> output a message to the kernel log. The final teardown will trigger a
> BUG_ON in mm/filemap.c.
> 
> This was reproduced in 3.4 but is known to have existed for a long time
> and goes back at least as far as 2.6.37. It was probably was introduced in
> 2.6.20 by [39dde65c: shared page table for hugetlb page]. The messages
> look like this;
> 
> [  ..........] Lots of bad pmd messages followed by this
> [  127.164256] mm/memory.c:391: bad pmd ffff880412e04fe8(80000003de4000e7).
> [  127.164257] mm/memory.c:391: bad pmd ffff880412e04ff0(80000003de6000e7).
> [  127.164258] mm/memory.c:391: bad pmd ffff880412e04ff8(80000003de0000e7).
> [  127.186778] ------------[ cut here ]------------
> [  127.186781] kernel BUG at mm/filemap.c:134!
> [  127.186782] invalid opcode: 0000 [#1] SMP
> [  127.186783] CPU 7
> [  127.186784] Modules linked in: af_packet cpufreq_conservative cpufreq_userspace cpufreq_powersave acpi_cpufreq mperf ext3 jbd dm_mod coretemp crc32c_intel usb_storage ghash_clmulni_intel aesni_intel i2c_i801 r8169 mii uas sr_mod cdrom sg iTCO_wdt iTCO_vendor_support shpchp serio_raw cryptd aes_x86_64 e1000e pci_hotplug dcdbas aes_generic container microcode ext4 mbcache jbd2 crc16 sd_mod crc_t10dif i915 drm_kms_helper drm i2c_algo_bit ehci_hcd ahci libahci usbcore rtc_cmos usb_common button i2c_core intel_agp video intel_gtt fan processor thermal thermal_sys hwmon ata_generic pata_atiixp libata scsi_mod
> [  127.186801]
> [  127.186802] Pid: 9017, comm: hugetlbfs-test Not tainted 3.4.0-autobuild #53 Dell Inc. OptiPlex 990/06D7TR
> [  127.186804] RIP: 0010:[<ffffffff810ed6ce>]  [<ffffffff810ed6ce>] __delete_from_page_cache+0x15e/0x160
> [  127.186809] RSP: 0000:ffff8804144b5c08  EFLAGS: 00010002
> [  127.186810] RAX: 0000000000000001 RBX: ffffea000a5c9000 RCX: 00000000ffffffc0
> [  127.186811] RDX: 0000000000000000 RSI: 0000000000000009 RDI: ffff88042dfdad00
> [  127.186812] RBP: ffff8804144b5c18 R08: 0000000000000009 R09: 0000000000000003
> [  127.186813] R10: 0000000000000000 R11: 000000000000002d R12: ffff880412ff83d8
> [  127.186814] R13: ffff880412ff83d8 R14: 0000000000000000 R15: ffff880412ff83d8
> [  127.186815] FS:  00007fe18ed2c700(0000) GS:ffff88042dce0000(0000) knlGS:0000000000000000
> [  127.186816] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
> [  127.186817] CR2: 00007fe340000503 CR3: 0000000417a14000 CR4: 00000000000407e0
> [  127.186818] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> [  127.186819] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
> [  127.186820] Process hugetlbfs-test (pid: 9017, threadinfo ffff8804144b4000, task ffff880417f803c0)
> [  127.186821] Stack:
> [  127.186822]  ffffea000a5c9000 0000000000000000 ffff8804144b5c48 ffffffff810ed83b
> [  127.186824]  ffff8804144b5c48 000000000000138a 0000000000001387 ffff8804144b5c98
> [  127.186825]  ffff8804144b5d48 ffffffff811bc925 ffff8804144b5cb8 0000000000000000
> [  127.186827] Call Trace:
> [  127.186829]  [<ffffffff810ed83b>] delete_from_page_cache+0x3b/0x80
> [  127.186832]  [<ffffffff811bc925>] truncate_hugepages+0x115/0x220
> [  127.186834]  [<ffffffff811bca43>] hugetlbfs_evict_inode+0x13/0x30
> [  127.186837]  [<ffffffff811655c7>] evict+0xa7/0x1b0
> [  127.186839]  [<ffffffff811657a3>] iput_final+0xd3/0x1f0
> [  127.186840]  [<ffffffff811658f9>] iput+0x39/0x50
> [  127.186842]  [<ffffffff81162708>] d_kill+0xf8/0x130
> [  127.186843]  [<ffffffff81162812>] dput+0xd2/0x1a0
> [  127.186845]  [<ffffffff8114e2d0>] __fput+0x170/0x230
> [  127.186848]  [<ffffffff81236e0e>] ? rb_erase+0xce/0x150
> [  127.186849]  [<ffffffff8114e3ad>] fput+0x1d/0x30
> [  127.186851]  [<ffffffff81117db7>] remove_vma+0x37/0x80
> [  127.186853]  [<ffffffff81119182>] do_munmap+0x2d2/0x360
> [  127.186855]  [<ffffffff811cc639>] sys_shmdt+0xc9/0x170
> [  127.186857]  [<ffffffff81410a39>] system_call_fastpath+0x16/0x1b
> [  127.186858] Code: 0f 1f 44 00 00 48 8b 43 08 48 8b 00 48 8b 40 28 8b b0 40 03 00 00 85 f6 0f 88 df fe ff ff 48 89 df e8 e7 cb 05 00 e9 d2 fe ff ff <0f> 0b 55 83 e2 fd 48 89 e5 48 83 ec 30 48 89 5d d8 4c 89 65 e0
> [  127.186868] RIP  [<ffffffff810ed6ce>] __delete_from_page_cache+0x15e/0x160
> [  127.186870]  RSP <ffff8804144b5c08>
> [  127.186871] ---[ end trace 7cbac5d1db69f426 ]---
> 
> The bug is a race and not always easy to reproduce. To reproduce it I was
> doing the following on a single socket I7-based machine with 16G of RAM.
> 
> $ hugeadm --pool-pages-max DEFAULT:13G
> $ echo $((18*1048576*1024)) > /proc/sys/kernel/shmmax
> $ echo $((18*1048576*1024)) > /proc/sys/kernel/shmall
> $ for i in `seq 1 9000`; do ./hugetlbfs-test; done
> 
> On my particular machine, it usually triggers within 10 minutes but enabling
> debug options can change the timing such that it never hits. Once the bug is
> triggered, the machine is in trouble and needs to be rebooted. The machine
> will respond but processes accessing proc like "ps aux" will hang due to
> the BUG_ON. shutdown will also hang and needs a hard reset or a sysrq-b.
> 
> The basic problem is a race between page table sharing and teardown. For
> the most part page table sharing depends on i_mmap_mutex. In some cases,
> it is also taking the mm->page_table_lock for the PTE updates but with
> shared page tables, it is the i_mmap_mutex that is more important.
> 
> Unfortunately it appears to be also insufficient. Consider the following
> situation
> 
> Process A					Process B
> ---------					---------
> hugetlb_fault					shmdt
>   						LockWrite(mmap_sem)
>     						  do_munmap
> 						    unmap_region
> 						      unmap_vmas
> 						        unmap_single_vma
> 						          unmap_hugepage_range
>       						            Lock(i_mmap_mutex)
> 							    Lock(mm->page_table_lock)
> 							    huge_pmd_unshare/unmap tables <--- (1)
> 							    Unlock(mm->page_table_lock)
>       						            Unlock(i_mmap_mutex)
>   huge_pte_alloc				      ...
>     Lock(i_mmap_mutex)				      ...
>     vma_prio_walk, find svma, spte		      ...
>     Lock(mm->page_table_lock)			      ...
>     share spte					      ...
>     Unlock(mm->page_table_lock)			      ...
>     Unlock(i_mmap_mutex)			      ...
>   hugetlb_no_page									  <--- (2)
> 						      free_pgtables
> 						        unlink_file_vma
> 							hugetlb_free_pgd_range
> 						    remove_vma_list
> 
> In this scenario, it is possible for Process A to share page tables with
> Process B that is trying to tear them down.  The i_mmap_mutex on its own
> does not prevent Process A walking Process B's page tables. At (1) above,
> the page tables are not shared yet so it unmaps the PMDs. Process A sets
> up page table sharing and at (2) faults a new entry. Process B then trips
> up on it in free_pgtables.
> 
> This patch fixes the problem by clearing VM_MAYSHARE during
> unmap_hugepage_range() under the i_mmap_mutex. This makes the VMA
> ineligible for sharing and avoids the race. Superficially this looks
> like it would then be vunerable to truncate and madvise problems but
> this is avoided by the limitations of hugetlbfs.
> 
> madvise and trunctate would be problems if removing VM_MAYSHARE in
> __unmap_hugepage_range() but it is removed in unmap_hugepage_range().
> This is only called by unmap_single_vma(): which is called via unmap_vmas()
> by unmap_region() or exit_mmap() just before free_pgtables() (the problem
> cases); or by madvise_dontneed() via zap_page_range(), which is disallowed
> on VM_HUGETLB; or by zap_page_range_single().
> 
> zap_page_range_single() is called by zap_vma_ptes(), which is only allowed
> on VM_PFNMAP; or by unmap_mapping_range_vma(), which looked like it was
> going to deadlock on i_mmap_mutex (with or without my patch) but does
> not as hugetlbfs has its own hugetlbfs_setattr() and hugetlb_vmtruncate()
> which don't use unmap_mapping_range() at all.
> 
> invalidate_inode_pages2() (and _range()) do use unmap_mapping_range(),
> but hugetlbfs doesn't support direct_IO, and otherwise they're called by a
> filesystem directly on its own inodes, which hugetlbfs does not.  If there's
> a deadlock on i_mmap_mutex somewhere in there, it's not introduced by the
> proposed patch.
> 
> This should be treated as a -stable candidate if it is merged.
> 
> Test program is as follows. The test case was mostly written by Michal
> Hocko with a few minor changes to reproduce this bug.
> 
> ==== CUT HERE ====
> 
> static size_t huge_page_size = (2UL << 20);
> static size_t nr_huge_page_A = 512;
> static size_t nr_huge_page_B = 5632;
> 
> unsigned int get_random(unsigned int max)
> {
> 	struct timeval tv;
> 
> 	gettimeofday(&tv, NULL);
> 	srandom(tv.tv_usec);
> 	return random() % max;
> }
> 
> static void play(void *addr, size_t size)
> {
> 	unsigned char *start = addr,
> 		      *end = start + size,
> 		      *a;
> 	start += get_random(size/2);
> 
> 	/* we could itterate on huge pages but let's give it more time. */
> 	for (a = start; a < end; a += 4096)
> 		*a = 0;
> }
> 
> int main(int argc, char **argv)
> {
> 	key_t key = IPC_PRIVATE;
> 	size_t sizeA = nr_huge_page_A * huge_page_size;
> 	size_t sizeB = nr_huge_page_B * huge_page_size;
> 	int shmidA, shmidB;
> 	void *addrA = NULL, *addrB = NULL;
> 	int nr_children = 300, n = 0;
> 
> 	if ((shmidA = shmget(key, sizeA, IPC_CREAT|SHM_HUGETLB|0660)) == -1) {
> 		perror("shmget:");
> 		return 1;
> 	}
> 
> 	if ((addrA = shmat(shmidA, addrA, SHM_R|SHM_W)) == (void *)-1UL) {
> 		perror("shmat");
> 		return 1;
> 	}
> 	if ((shmidB = shmget(key, sizeB, IPC_CREAT|SHM_HUGETLB|0660)) == -1) {
> 		perror("shmget:");
> 		return 1;
> 	}
> 
> 	if ((addrB = shmat(shmidB, addrB, SHM_R|SHM_W)) == (void *)-1UL) {
> 		perror("shmat");
> 		return 1;
> 	}
> 
> fork_child:
> 	switch(fork()) {
> 		case 0:
> 			switch (n%3) {
> 			case 0:
> 				play(addrA, sizeA);
> 				break;
> 			case 1:
> 				play(addrB, sizeB);
> 				break;
> 			case 2:
> 				break;
> 			}
> 			break;
> 		case -1:
> 			perror("fork:");
> 			break;
> 		default:
> 			if (++n < nr_children)
> 				goto fork_child;
> 			play(addrA, sizeA);
> 			break;
> 	}
> 	shmdt(addrA);
> 	shmdt(addrB);
> 	do {
> 		wait(NULL);
> 	} while (--n > 0);
> 	shmctl(shmidA, IPC_RMID, NULL);
> 	shmctl(shmidB, IPC_RMID, NULL);
> 	return 0;
> }
> 
> Signed-off-by: Hugh Dickins <hughd@google.com>
> Reviewed-by: Michal Hocko <mhocko@suse.cz>
> Reviewed-by: Mel Gorman <mgorman@suse.de>

Thanks a lot! The patch and the description look good and really helpful!

> ---
>  mm/hugetlb.c |   25 +++++++++++++++++++++++--
>  1 file changed, 23 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index ae8f708..d488476 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -2383,6 +2383,22 @@ void unmap_hugepage_range(struct vm_area_struct *vma, unsigned long start,
>  {
>  	mutex_lock(&vma->vm_file->f_mapping->i_mmap_mutex);
>  	__unmap_hugepage_range(vma, start, end, ref_page);
> +	/*
> +	 * Clear this flag so that x86's huge_pmd_share page_table_shareable
> +	 * test will fail on a vma being torn down, and not grab a page table
> +	 * on its way out.  We're lucky that the flag has such an appropriate
> +	 * name, and can in fact be safely cleared here. We could clear it
> +	 * before the __unmap_hugepage_range above, but all that's necessary
> +	 * is to clear it before releasing the i_mmap_mutex below.
> +	 *
> +	 * This works because in the contexts this is called, the VMA is
> +	 * going to be destroyed. It is not vunerable to madvise(DONTNEED)
> +	 * because madvise is not supported on hugetlbfs. The same applies
> +	 * for direct IO. unmap_hugepage_range() is only being called just
> +	 * before free_pgtables() so clearing VM_MAYSHARE will not cause
> +	 * surprises later.
> +	 */
> +	vma->vm_flags &= ~VM_MAYSHARE;
>  	mutex_unlock(&vma->vm_file->f_mapping->i_mmap_mutex);
>  }
>  
> @@ -2949,9 +2965,14 @@ void hugetlb_change_protection(struct vm_area_struct *vma,
>  		}
>  	}
>  	spin_unlock(&mm->page_table_lock);
> -	mutex_unlock(&vma->vm_file->f_mapping->i_mmap_mutex);
> -
> +	/*
> +	 * Must flush TLB before releasing i_mmap_mutex: x86's huge_pmd_unshare
> +	 * may have cleared our pud entry and done put_page on the page table:
> +	 * once we release i_mmap_mutex, another task can do the final put_page
> +	 * and that page table be reused and filled with junk.
> +	 */
>  	flush_tlb_range(vma, start, end);
> +	mutex_unlock(&vma->vm_file->f_mapping->i_mmap_mutex);
>  }
>  
>  int hugetlb_reserve_pages(struct inode *inode,

-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic

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