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[PATCH 3.16 001/212] mm/huge_memory.c: fix up "mm/huge_memory.c: respect FOLL_FORCE/FOLL_COW for thp" backport

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3.16.44-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Michal Hocko <mhocko@suse.com>

This is a stable follow up fix for an incorrect backport. The issue is
not present in the upstream kernel.

Miroslav has noticed the following splat when testing my 3.2 forward
port of 8310d48b125d ("mm/huge_memory.c: respect FOLL_FORCE/FOLL_COW for
thp") to 3.12:

BUG: Bad page state in process a.out  pfn:26400
page:ffffea000085e000 count:0 mapcount:1 mapping:          (null) index:0x7f049d600
page flags: 0x1fffff80108018(uptodate|dirty|head|swapbacked)
page dumped because: nonzero mapcount
[iii]
CPU: 2 PID: 5926 Comm: a.out Tainted: G            E    3.12.61-0-default #1
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.0.0-prebuilt.qemu-project.org 04/01/2014
 0000000000000000 ffffffff81515830 ffffea000085e000 ffffffff81800ad7
 ffffffff815118a5 ffffea000085e000 0000000000000000 000fffff80000000
 ffffffff81140f18 fff000007c000000 ffffea000085e000 0000000000000009
Call Trace:
 [<ffffffff8100475d>] dump_trace+0x7d/0x2d0
 [<ffffffff81004a44>] show_stack_log_lvl+0x94/0x170
 [<ffffffff81005ce1>] show_stack+0x21/0x50
 [<ffffffff81515830>] dump_stack+0x5d/0x78
 [<ffffffff815118a5>] bad_page.part.67+0xe8/0x102
 [<ffffffff81140f18>] free_pages_prepare+0x198/0x1b0
 [<ffffffff81141275>] __free_pages_ok+0x15/0xd0
 [<ffffffff8116444c>] __access_remote_vm+0x7c/0x1e0
 [<ffffffff81205afb>] mem_rw.isra.13+0x14b/0x1a0
 [<ffffffff811a3b18>] vfs_write+0xb8/0x1e0
 [<ffffffff811a469b>] SyS_pwrite64+0x6b/0xa0
 [<ffffffff81523b49>] system_call_fastpath+0x16/0x1b
 [<00007f049da18573>] 0x7f049da18572

The problem is that the original 3.2 backport didn't return NULL page on
the FOLL_COW page and so the page got reused.

Reported-and-tested-by: Miroslav Beneš <mbenes@suse.com>
Signed-off-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
 mm/huge_memory.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1226,7 +1226,7 @@ struct page *follow_trans_huge_pmd(struc
 	VM_BUG_ON_PAGE(!PageHead(page), page);
 
 	if (flags & FOLL_WRITE && !can_follow_write_pmd(*pmd, page, flags))
-		goto out;
+		return NULL;
 
 	if (flags & FOLL_TOUCH) {
 		pmd_t _pmd;

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[PATCH 3.16 000/212] 3.16.44-rc1 review Ben Hutchings <ben@decadent.org.uk> - 2017-06-01 18:50 +0200
  [PATCH 3.16 003/212] adm80211: return an error if adm8211_alloc_rings()  fails Ben Hutchings <ben@decadent.org.uk> - 2017-06-01 18:50 +0200
  [PATCH 3.16 006/212] MIPS: Clear ISA bit correctly in  get_frame_info() Ben Hutchings <ben@decadent.org.uk> - 2017-06-01 18:50 +0200
  [PATCH 3.16 023/212] Drivers: hv: vmbus: Raise retry/wait limits  in vmbus_post_msg() Ben Hutchings <ben@decadent.org.uk> - 2017-06-01 18:50 +0200
  [PATCH 3.16 008/212] MIPS: Fix get_frame_info() handling of  microMIPS function size Ben Hutchings <ben@decadent.org.uk> - 2017-06-01 18:50 +0200
  [PATCH 3.16 011/212] MIPS: Handle microMIPS jumps in the same way  as MIPS32/MIPS64 jumps Ben Hutchings <ben@decadent.org.uk> - 2017-06-01 18:50 +0200
  [PATCH 3.16 031/212] ASoC: rt5640: use msleep() for long delays Ben Hutchings <ben@decadent.org.uk> - 2017-06-01 18:50 +0200
  [PATCH 3.16 027/212] ext4: avoid deadlock when expanding inode size Ben Hutchings <ben@decadent.org.uk> - 2017-06-01 18:50 +0200
  [PATCH 3.16 004/212] iio: st_pressure: Fix data sign Ben Hutchings <ben@decadent.org.uk> - 2017-06-01 18:50 +0200
  [PATCH 3.16 001/212] mm/huge_memory.c: fix up "mm/huge_memory.c:  respect FOLL_FORCE/FOLL_COW for thp" backport Ben Hutchings <ben@decadent.org.uk> - 2017-06-01 18:50 +0200
  Re: [PATCH 3.16 000/212] 3.16.44-rc1 review Guenter Roeck <linux@roeck-us.net> - 2017-06-01 23:20 +0200

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