[BISECTED] Linux 3.12.7 introduces page map handling regression

[Steven Noonan]

A user reported a problem starting vsftpd on a Xen paravirtualized
guest, with this in dmesg:

[   60.654862] BUG: Bad page map in process vsftpd  pte:8000000493b88165 pmd:e9cc01067
[   60.654876] page:ffffea00124ee200 count:0 mapcount:-1 mapping:     (null) index:0x0
[   60.654879] page flags: 0x2ffc0000000014(referenced|dirty)
[   60.654885] addr:00007f97eea74000 vm_flags:00100071 anon_vma:ffff880e98f80380 mapping:          (null) index:7f97eea74
[   60.654890] CPU: 4 PID: 587 Comm: vsftpd Not tainted 3.12.7-1-ec2 #1
[   60.654893]  ffff880e9cc6ec38 ffff880e9cc61ca0 ffffffff814c763b 00007f97eea74000
[   60.654900]  ffff880e9cc61ce8 ffffffff8116784e 0000000000000000 0000000000000000
[   60.654906]  ffff880e9cc013a0 ffffea00124ee200 00007f97eea75000 ffff880e9cc61e10
[   60.654912] Call Trace:
[   60.654921]  [<ffffffff814c763b>] dump_stack+0x45/0x56
[   60.654928]  [<ffffffff8116784e>] print_bad_pte+0x22e/0x250
[   60.654933]  [<ffffffff81169073>] unmap_single_vma+0x583/0x890
[   60.654938]  [<ffffffff8116a405>] unmap_vmas+0x65/0x90
[   60.654942]  [<ffffffff81173795>] exit_mmap+0xc5/0x170
[   60.654948]  [<ffffffff8105d295>] mmput+0x65/0x100
[   60.654952]  [<ffffffff81062983>] do_exit+0x393/0x9e0
[   60.654955]  [<ffffffff810630dc>] do_group_exit+0xcc/0x140
[   60.654959]  [<ffffffff81063164>] SyS_exit_group+0x14/0x20
[   60.654965]  [<ffffffff814d602d>] system_call_fastpath+0x1a/0x1f
[   60.654968] Disabling lock debugging due to kernel taint
[   60.655191] BUG: Bad rss-counter state mm:ffff880e9ca60580 idx:0 val:-1
[   60.655196] BUG: Bad rss-counter state mm:ffff880e9ca60580 idx:1 val:1


The issue could not be reproduced under an HVM instance with the same
kernel, so it appears to be exclusive to paravirtual Xen guests.

I noted that it wasn't present in 3.10.27, but was present in 3.12.7 and
3.12.8. I ran through a bisection to find the root cause:

 # start: 'v3.12.7' 'v3.10.27'
 # bad:  [4301b7a8] Linux 3.12.7
 # good: [1071ea6e] Linux 3.10.27
 # good: [8bb495e3] Linux 3.10
 # good: [8fe73691] staging: comedi: comedi_bond: change return value
 # good: [22e04f6b] Merge branch 'for-linus' of git://git.kernel.org/p
 # good: [b7c09ad4] Merge branch 'for-linus' of git://git.kernel.org/p
 # good: [13caa8ed] Merge git://git.kernel.org/pub/scm/linux/kernel/gi
 # good: [13caa8ed] Merge git://git.kernel.org/pub/scm/linux/kernel/gi
 # good: [f5fa9283] ipv6: reset dst.expires value when clearing expire
 # good: [4af9d888] bridge: flush br's address entry in fdb when remov
 # good: [8c13daf6] dm delay: fix a possible deadlock due to shared wo
 # good: [93c02d70] firewire: sbp2: bring back WRITE SAME support
 # good: [18065245] ACPI / PCI / hotplug: Avoid warning when _ADR not
 # bad:  [8807a436] mm/memory-failure.c: transfer page count from head
 # bad:  [fd5df800] mm: numa: avoid unnecessary disruption of NUMA hin
 # good: [c18e3316] mm: numa: do not clear PMD during PTE update scan
 # good: [f3b578d9] mm: numa: avoid unnecessary work on the failure pa
 # bad:  [3d792d61] mm: numa: clear numa hinting information on mprote
 # good: [cefeb279] sched: numa: skip inaccessible VMAs
 # first bad:  [3d792d61] mm: numa: clear numa hinting information on mprote

If only I'd tested v3.12.0, that bisection would have been a lot shorter!


It looks like this is the change implicated (introduced in v3.12.7):

    commit 3d792d616ba408ab55a54c1bb75a9367d997acfa
    Author: Mel Gorman <mgorman@xxxxxxx>
    Date:   Tue Jan 7 14:00:44 2014 +0000
    
        mm: numa: clear numa hinting information on mprotect
    
        commit 1667918b6483b12a6496bf54151b827b8235d7b1 upstream.
    
        On a protection change it is no longer clear if the page should be still
        accessible.  This patch clears the NUMA hinting fault bits on a
        protection change.
    
        Signed-off-by: Mel Gorman <mgorman@xxxxxxx>
        Reviewed-by: Rik van Riel <riel@xxxxxxxxxx>
        Cc: Alex Thorlton <athorlton@xxxxxxx>
        Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
        Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
        Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>


This clearly points to breakage of mprotect() in particular. Checking
what vsftpd was doing via strace, I was able to come up with a simple
test case which triggers the issue:

    #include <errno.h>
    #include <stdio.h>
    #include <stdlib.h>
    #include <sys/mman.h>
    
    void die(const char *what)
    {
    	perror(what);
    	exit(1);
    }
    
    int main(int arg, char **argv)
    {
    	void *p = mmap(NULL, 4096, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
    
    	if (p == MAP_FAILED)
    		die("mmap");
    
    	/* Tickle the page. */
    	((char *)p)[0] = 0;
    
    	if (mprotect(p, 4096, PROT_NONE) != 0)
    		die("mprotect");
    
    	if (mprotect(p, 4096, PROT_READ) != 0)
    		die("mprotect");
    
    	if (munmap(p, 4096) != 0)
    		die("munmap");
    
    	return 0;
    }

This could probably be reduced further. I didn't spend much time on it.

Adding people cited in the patch to CC, as well as Konrad since this is
a Xen issue (I haven't been able to repro on HVM or bare metal so far).

Any ideas what's causing the BUG, and how we can fix it?

- Steven
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