Re: [PATCH] mm: hugetlbfs: Skip shared VMAs when unmapping private pages to satisfy a fault

[Michal Hocko]

On Tue 22-09-15 16:49:38, Mel Gorman wrote:
[...]
> mm: hugetlbfs: Skip shared VMAs when unmapping private pages to satisfy a fault
> 
> SunDong reported the following on https://bugzilla.kernel.org/show_bug.cgi?id=103841
> 
> 	I think I find a linux bug, I have the test cases is constructed. I
> 	can stable recurring problems in fedora22(4.0.4) kernel version,
> 	arch for x86_64.  I construct transparent huge page, when the parent
> 	and child process with MAP_SHARE, MAP_PRIVATE way to access the same
> 	huge page area, it has the opportunity to lead to huge page copy on
> 	write failure, and then it will munmap the child corresponding mmap
> 	area, but then the child mmap area with VM_MAYSHARE attributes, child
> 	process munmap this area can trigger VM_BUG_ON in set_vma_resv_flags
> 	functions (vma - > vm_flags & VM_MAYSHARE).
> 
> There were a number of problems with the report (e.g. it's hugetlbfs that
> triggers this, not transparent huge pages) but it was fundamentally correct
> in that a VM_BUG_ON in set_vma_resv_flags() can be triggered that looks
> like this
> 
> 	 vma ffff8804651fd0d0 start 00007fc474e00000 end 00007fc475e00000
> 	 next ffff8804651fd018 prev ffff8804651fd188 mm ffff88046b1b1800
> 	 prot 8000000000000027 anon_vma           (null) vm_ops ffffffff8182a7a0
> 	 pgoff 0 file ffff88106bdb9800 private_data           (null)
> 	 flags: 0x84400fb(read|write|shared|mayread|maywrite|mayexec|mayshare|dontexpand|hugetlb)
> 	 ------------
> 	 kernel BUG at mm/hugetlb.c:462!
> 	 SMP
> 	 Modules linked in: xt_pkttype xt_LOG xt_limit iscsi_ibft iscsi_boot_sysfs af_packet ip6t_REJECT nf_reject_ipv6
> xt_tcpudp nf_conntrack_ipv6 nf_defrag_ipv6 ip6table_raw ipt_REJECT nf_reject_ipv4 iptable_raw xt_CT iptable_filter ip6table_mangle
> nf_conntrack_netbios_ns nf_conntrack_broadcast nf_conntrack_ipv4 nf_defrag_ipv4 ip_tables xt_conntrack nf_conntrack ip6table_filter
> ip6_tables x_tables intel_powerclamp coretemp kvm_intel kvm mgag200 ttm drm_kms_helper drm crct10dif_pclmul crc32_pclmul crc32c_intel
> ghash_clmulni_intel aesni_intel aes_x86_64 lrw ipmi_devintf gf128mul iTCO_wdt gpio_ich iTCO_vendor_support glue_helper ablk_helper
> dcdbas i7core_edac cryptd syscopyarea sysfillrect bnx2 sysimgblt lpc_ich serio_raw edac_core i2c_algo_bit shpchp mfd_core ipmi_si
> tpm_tis tpm ipmi_msghandler wmi acpi_power_meter button acpi_cpufreq processor dm_mod sr_mod cdrom ata_generic hid_generic usbhid hid
> uhci_hcd ehci_pci ehci_hcd usbcore ata_piix usb_common megaraid_sas sg
> 	 CPU: 38 PID: 26839 Comm: map Not tainted 4.0.4-default #1
> 	 Hardware name: Dell Inc. PowerEdge R810/0TT6JF, BIOS 2.7.4 04/26/2012
> 	 task: ffff88085ed10490 ti: ffff88085ed14000 task.ti: ffff88085ed14000
> 	 set_vma_resv_flags+0x2d/0x30
> 
> The VM_BUG_ON is correct because private and shared mappings have different
> reservation accounting but the warning clearly shows that the VMA is shared.
> 
> When a private COW fails to allocate a new page then only the process that
> created the VMA gets the page -- all the children unmap the page. If the
> children access that data in the future then they get killed.
> 
> The problem is that the same file is mapped shared and private. During
> the COW, the allocation fails, the VMAs are traversed to unmap the other
> private pages but a shared VMA is found and the bug is triggered. This
> patch identifies such VMAs and skips them.
> 
> Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> Reported-by: SunDong <sund_sky@xxxxxxx>

Reviewed-by: Michal Hocko <mhocko@xxxxxxxx>

I guess you wanted to add Cc: stable back. I believe this goes very long
way back.
 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 999fb0aef8f1..9cc773483624 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -3202,6 +3202,14 @@ static void unmap_ref_private(struct mm_struct *mm, struct vm_area_struct *vma,
>  			continue;
>  
>  		/*
> +		 * Shared VMAs have their own reserves and do not affect
> +		 * MAP_PRIVATE accounting but it is possible that a shared
> +		 * VMA is using the same page so check and skip such VMAs.
> +		 */
> +		if (iter_vma->vm_flags & VM_MAYSHARE)
> +			continue;
> +
> +		/*
>  		 * Unmap the page from other VMAs without their own reserves.
>  		 * They get marked to be SIGKILLed if they fault in these
>  		 * areas. This is because a future no-page fault on this VMA

-- 
Michal Hocko
SUSE Labs
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