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Date: Wed, 22 Jun 2016 14:16:05 +0300
From: "Kirill A. Shutemov" <kirill@...temov.name>
To: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
Cc: "linux-mm@...ck.org" <linux-mm@...ck.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Hugh Dickins <hughd@...gle.com>,
Mel Gorman <mgorman@...hsingularity.net>,
Michal Hocko <mhocko@...e.cz>,
Vlastimil Babka <vbabka@...e.cz>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
Naoya Horiguchi <nao.horiguchi@...il.com>
Subject: Re: [PATCH v1 1/2] mm: thp: move pmd check inside ptl for
freeze_page()
On Wed, Jun 22, 2016 at 02:42:25AM +0000, Naoya Horiguchi wrote:
> On Wed, Jun 22, 2016 at 01:37:00AM +0000, Horiguchi Naoya(堀口 直也) wrote:
> > On Tue, Jun 21, 2016 at 06:04:33PM +0300, Kirill A. Shutemov wrote:
> > > On Mon, Jun 20, 2016 at 12:32:01PM +0300, Kirill A. Shutemov wrote:
> > > > > +void split_huge_pmd_address_freeze(struct vm_area_struct *vma,
> > > > > + unsigned long address, struct page *page)
> > > > > +{
> > > > > + pgd_t *pgd;
> > > > > + pud_t *pud;
> > > > > + pmd_t *pmd;
> > > > > +
> > > > > + pgd = pgd_offset(vma->vm_mm, address);
> > > > > + if (!pgd_present(*pgd))
> > > > > + return;
> > > > > +
> > > > > + pud = pud_offset(pgd, address);
> > > > > + if (!pud_present(*pud))
> > > > > + return;
> > > > > +
> > > > > + pmd = pmd_offset(pud, address);
> > > > > + __split_huge_pmd(vma, pmd, address, page, true);
> > > > > }
> > > >
> > > > I don't see a reason to introduce new function. Just move the page
> > > > check under ptl from split_huge_pmd_address() and that should be enough.
> >
> > Sorry for my slow response (I was offline yesterday.)
> >
> > My point of separating function is to avoid checking pmd_present outside ptl
> > just for freeze=true case (I didn't want affect other path,
> > i.e. from vma_adjust_trans_huge().)
> > But I think that the new function is unnecessary if we move the following
> > part of split_huge_pmd_address() into ptl,
> >
> > if (!pmd_present(*pmd) || (!pmd_trans_huge(*pmd) && !pmd_devmap(*pmd)))
> > return;
> >
> > Does it make sense?
> >
> > > > Or am I missing something?
> > >
> > > I'm talking about something like patch below. Could you test it?
> >
> > Thanks, with this patch my 3-hour testing doesn't trigger the problem,
> > so it works. But I feel it's weird because I think that the source of the
> > race is "if (!pmd_present)" check in split_huge_pmd_address() called outside ptl.
> > Your patch doesn't change that part, so I'm not sure why this fix works.
>
> Hmm, after sending previous email, I found the bug triggered with your patch.
> So I updated the patch with removing the prechecks.
>
> Thanks,
> Naoya Horiguchi
> ---
> From: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
> Date: Wed, 22 Jun 2016 11:34:47 +0900
> Subject: [PATCH] mm: thp: move pmd check inside ptl for freeze_page()
>
> I found a race condition triggering VM_BUG_ON() in freeze_page(), when running
> a testcase with 3 processes:
> - process 1: keep writing thp,
> - process 2: keep clearing soft-dirty bits from virtual address of process 1
> - process 3: call migratepages for process 1,
>
> The kernel message is like this:
>
> kernel BUG at /src/linux-dev/mm/huge_memory.c:3096!
> invalid opcode: 0000 [#1] SMP
> Modules linked in: cfg80211 rfkill crc32c_intel ppdev serio_raw pcspkr virtio_balloon virtio_console parport_pc parport pvpanic acpi_cpufreq tpm_tis tpm i2c_piix4 virtio_blk virtio_net ata_generic pata_acpi floppy virtio_pci virtio_ring virtio
> CPU: 0 PID: 28863 Comm: migratepages Not tainted 4.6.0-v4.6-160602-0827-+ #2
> Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
> task: ffff880037320000 ti: ffff88007cdd0000 task.ti: ffff88007cdd0000
> RIP: 0010:[<ffffffff811f8e06>] [<ffffffff811f8e06>] split_huge_page_to_list+0x496/0x590
> RSP: 0018:ffff88007cdd3b70 EFLAGS: 00010202
> RAX: 0000000000000001 RBX: ffff88007c7b88c0 RCX: 0000000000000000
> RDX: 0000000000000000 RSI: 0000000700000200 RDI: ffffea0003188000
> RBP: ffff88007cdd3bb8 R08: 0000000000000001 R09: 00003ffffffff000
> R10: ffff880000000000 R11: ffffc000001fffff R12: ffffea0003188000
> R13: ffffea0003188000 R14: 0000000000000000 R15: 0400000000000080
> FS: 00007f8ec241d740(0000) GS:ffff88007dc00000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007f8ec1f3ed20 CR3: 000000003707b000 CR4: 00000000000006f0
> Stack:
> ffffffff8139ef6d ffffea00031c6280 ffff88011ffec000 0000000000000000
> 0000700000400000 0000700000200000 ffff88007cdd3d08 ffff8800dbbe3008
> 0400000000000080 ffff88007cdd3c20 ffffffff811dd0b1 ffff88007cdd3d68
> Call Trace:
> [<ffffffff8139ef6d>] ? list_del+0xd/0x30
> [<ffffffff811dd0b1>] queue_pages_pte_range+0x4d1/0x590
> [<ffffffff811ca1a4>] __walk_page_range+0x204/0x4e0
> [<ffffffff811ca4f1>] walk_page_range+0x71/0xf0
> [<ffffffff811db935>] queue_pages_range+0x75/0x90
> [<ffffffff811dcbe0>] ? queue_pages_hugetlb+0x190/0x190
> [<ffffffff811dca50>] ? new_node_page+0xc0/0xc0
> [<ffffffff811ddac0>] ? change_prot_numa+0x40/0x40
> [<ffffffff811dc001>] migrate_to_node+0x71/0xd0
> [<ffffffff811ddd73>] do_migrate_pages+0x1c3/0x210
> [<ffffffff811de0b1>] SyS_migrate_pages+0x261/0x290
> [<ffffffff816f53f2>] entry_SYSCALL_64_fastpath+0x1a/0xa4
> Code: e8 b0 87 fb ff 0f 0b 48 c7 c6 30 32 9f 81 e8 a2 87 fb ff 0f 0b 48 c7 c6 b8 46 9f 81 e8 94 87 fb ff 0f 0b 85 c0 0f 84 3e fd ff ff <0f> 0b 85 c0 0f 85 a6 00 00 00 48 8b 75 c0 4c 89 f7 41 be f0 ff
> RIP [<ffffffff811f8e06>] split_huge_page_to_list+0x496/0x590
> RSP <ffff88007cdd3b70>
>
> I'm not sure of the full scenario of the reproduction, but my debug showed that
> split_huge_pmd_address(freeze=true) returned without running main code of pmd
> splitting because pmd_present(*pmd) in precheck somehow returned 0.
> If this happens, the subsequent try_to_unmap() fails and returns non-zero
> (because page_mapcount() still > 0), and finally VM_BUG_ON() fires.
> This patch tries to fix it by prechecking pmd state inside ptl.
>
> Signed-off-by: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> ---
> Diff from v2:
> - don't use separate function
>
> Diff from v1:
> - passed page to __split_huge_pmd()
> - dropped unnecessary !pmd_present check
> - removed pmd_none check in split_huge_pmd_address_freeze because it's
> effectively done in __split_huge_pmd() with ptl.
> ---
> include/linux/huge_mm.h | 4 ++--
> mm/huge_memory.c | 25 ++++++++++++-------------
> 2 files changed, 14 insertions(+), 15 deletions(-)
>
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index d7b9e5346fba..a05ca41ae243 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -96,7 +96,7 @@ static inline int split_huge_page(struct page *page)
> void deferred_split_huge_page(struct page *page);
>
> void __split_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
> - unsigned long address, bool freeze);
> + unsigned long address, bool freeze, struct page *page);
>
> #define split_huge_pmd(__vma, __pmd, __address) \
> do { \
> @@ -104,7 +104,7 @@ void __split_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
> if (pmd_trans_huge(*____pmd) \
> || pmd_devmap(*____pmd)) \
> __split_huge_pmd(__vma, __pmd, __address, \
> - false); \
> + false, NULL); \
> } while (0)
>
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index b49ee126d4d1..44a3555c1d23 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2981,7 +2981,7 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
> }
>
> void __split_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
> - unsigned long address, bool freeze)
> + unsigned long address, bool freeze, struct page *page)
> {
> spinlock_t *ptl;
> struct mm_struct *mm = vma->vm_mm;
> @@ -2989,8 +2989,17 @@ void __split_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
>
> mmu_notifier_invalidate_range_start(mm, haddr, haddr + HPAGE_PMD_SIZE);
> ptl = pmd_lock(mm, pmd);
> +
> + /*
> + * If caller asks to setup a migration entries, we need a page to check
> + * pmd against. Otherwise we can end up replacing wrong page.
> + */
> + VM_BUG_ON(freeze && !page);
> + if (page && page != pmd_page(*pmd))
> + goto out;
> +
> if (pmd_trans_huge(*pmd)) {
> - struct page *page = pmd_page(*pmd);
> + page = pmd_page(*pmd);
> if (PageMlocked(page))
> clear_page_mlock(page);
> } else if (!pmd_devmap(*pmd))
> @@ -3017,22 +3026,12 @@ void split_huge_pmd_address(struct vm_area_struct *vma, unsigned long address,
> return;
>
> pmd = pmd_offset(pud, address);
> - if (!pmd_present(*pmd) || (!pmd_trans_huge(*pmd) && !pmd_devmap(*pmd)))
> - return;
> -
> - /*
> - * If caller asks to setup a migration entries, we need a page to check
> - * pmd against. Otherwise we can end up replacing wrong page.
> - */
> - VM_BUG_ON(freeze && !page);
> - if (page && page != pmd_page(*pmd))
> - return;
>
> /*
> * Caller holds the mmap_sem write mode, so a huge pmd cannot
> * materialize from under us.
> */
As you mentioned, the comment is no longer true. We should remove it (with
separate patch?).
Otherwise looks good to me.
> - __split_huge_pmd(vma, pmd, address, freeze);
> + __split_huge_pmd(vma, pmd, address, freeze, page);
> }
>
> void vma_adjust_trans_huge(struct vm_area_struct *vma,
> --
> 2.5.5
--
Kirill A. Shutemov
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