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Date: Wed, 12 Dec 2018 16:51:38 +0300
From: Konstantin Khlebnikov <koct9i@...il.com>
To: peterx@...hat.com
Cc: Константин Хлебников
<khlebnikov@...dex-team.ru>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
Andrea Arcangeli <aarcange@...hat.com>,
Andrew Morton <akpm@...ux-foundation.org>,
"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
Matthew Wilcox <willy@...radead.org>,
Michal Hocko <mhocko@...e.com>, dave.jiang@...el.com,
"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
Souptick Joarder <jrdr.linux@...il.com>, linux-mm@...ck.org
Subject: Re: [PATCH v2] mm: thp: fix flags for pmd migration when split
On Wed, Dec 12, 2018 at 8:15 AM Peter Xu <peterx@...hat.com> wrote:
>
> On Tue, Dec 11, 2018 at 11:21:44AM +0300, Konstantin Khlebnikov wrote:
> > On 11.12.2018 8:12, Peter Xu wrote:
> > > When splitting a huge migrating PMD, we'll transfer all the existing
> > > PMD bits and apply them again onto the small PTEs. However we are
> > > fetching the bits unconditionally via pmd_soft_dirty(), pmd_write()
> > > or pmd_yound() while actually they don't make sense at all when it's
> > > a migration entry. Fix them up by make it conditional.
> > >
> > > Note that if my understanding is correct about the problem then if
> > > without the patch there is chance to lose some of the dirty bits in
> > > the migrating pmd pages (on x86_64 we're fetching bit 11 which is part
> > > of swap offset instead of bit 2) and it could potentially corrupt the
> > > memory of an userspace program which depends on the dirty bit.
> > >
> > > CC: Andrea Arcangeli <aarcange@...hat.com>
> > > CC: Andrew Morton <akpm@...ux-foundation.org>
> > > CC: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
> > > CC: Matthew Wilcox <willy@...radead.org>
> > > CC: Michal Hocko <mhocko@...e.com>
> > > CC: Dave Jiang <dave.jiang@...el.com>
> > > CC: "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
> > > CC: Souptick Joarder <jrdr.linux@...il.com>
> > > CC: Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
> > > CC: linux-mm@...ck.org
> > > CC: linux-kernel@...r.kernel.org
> > > Signed-off-by: Peter Xu <peterx@...hat.com>
> > > ---
> > > v2:
> > > - fix it up for young/write/dirty bits too [Konstantin]
> > > ---
> > > mm/huge_memory.c | 15 ++++++++++-----
> > > 1 file changed, 10 insertions(+), 5 deletions(-)
> > >
> > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > > index f2d19e4fe854..b00941b3d342 100644
> > > --- a/mm/huge_memory.c
> > > +++ b/mm/huge_memory.c
> > > @@ -2157,11 +2157,16 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
> > > page = pmd_page(old_pmd);
> > > VM_BUG_ON_PAGE(!page_count(page), page);
> > > page_ref_add(page, HPAGE_PMD_NR - 1);
> > > - if (pmd_dirty(old_pmd))
> > > - SetPageDirty(page);
> > > - write = pmd_write(old_pmd);
> > > - young = pmd_young(old_pmd);
> > > - soft_dirty = pmd_soft_dirty(old_pmd);
> > > + if (unlikely(pmd_migration)) {
> > > + soft_dirty = pmd_swp_soft_dirty(old_pmd);
> > > + young = write = false;
> > > + } else {
> > > + if (pmd_dirty(old_pmd))
> > > + SetPageDirty(page);
> > > + write = pmd_write(old_pmd);
> > > + young = pmd_young(old_pmd);
> > > + soft_dirty = pmd_soft_dirty(old_pmd);
> > > + }
> >
> > Write/read-only is encoded into migration entry.
> > I suppose there should be something like this:
> >
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -2151,16 +2151,21 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
> >
> > entry = pmd_to_swp_entry(old_pmd);
> > page = pfn_to_page(swp_offset(entry));
> > + write = is_write_migration_entry(entry);
> > + young = false;
> > + soft_dirty = pmd_swp_soft_dirty(old_pmd);
> > } else
> > #endif
> > + {
> > page = pmd_page(old_pmd);
> > + if (pmd_dirty(old_pmd))
> > + SetPageDirty(page);
> > + write = pmd_write(old_pmd);
> > + young = pmd_young(old_pmd);
> > + soft_dirty = pmd_soft_dirty(old_pmd);
> > + }
> > VM_BUG_ON_PAGE(!page_count(page), page);
> > page_ref_add(page, HPAGE_PMD_NR - 1);
> > - if (pmd_dirty(old_pmd))
> > - SetPageDirty(page);
> > - write = pmd_write(old_pmd);
> > - young = pmd_young(old_pmd);
> > - soft_dirty = pmd_soft_dirty(old_pmd);
> >
> > /*
> > * Withdraw the table only after we mark the pmd entry invalid.
> >
>
> Oops yes, I missed the write bit. Thanks for pointing it out.
>
> Should I repost with your authorship and your sign-off?
Feel free to use this piece for your own patch.
> Or even I'll
> consider to directly drop the CONFIG_ARCH_ENABLE_THP_MIGRATION if with
> that since I don't see much gain to keep it:
Yep, this ifdef could be removed.
Without CONFIG_ARCH_ENABLE_THP_MIGRATION
is_pmd_migration_entry() is constant 0 so compiler should eliminate "if" branch.
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index f2d19e4fe854..aebade83cec9 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2145,23 +2145,25 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
> */
> old_pmd = pmdp_invalidate(vma, haddr, pmd);
>
> -#ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION
> pmd_migration = is_pmd_migration_entry(old_pmd);
> - if (pmd_migration) {
> + if (unlikely(pmd_migration)) {
> swp_entry_t entry;
>
> entry = pmd_to_swp_entry(old_pmd);
> page = pfn_to_page(swp_offset(entry));
> - } else
> -#endif
> + write = is_write_migration_entry(entry);
> + young = false;
> + soft_dirty = pmd_swp_soft_dirty(old_pmd);
> + } else {
> page = pmd_page(old_pmd);
> + if (pmd_dirty(old_pmd))
> + SetPageDirty(page);
> + write = pmd_write(old_pmd);
> + young = pmd_young(old_pmd);
> + soft_dirty = pmd_soft_dirty(old_pmd);
> + }
> VM_BUG_ON_PAGE(!page_count(page), page);
> page_ref_add(page, HPAGE_PMD_NR - 1);
> - if (pmd_dirty(old_pmd))
> - SetPageDirty(page);
> - write = pmd_write(old_pmd);
> - young = pmd_young(old_pmd);
> - soft_dirty = pmd_soft_dirty(old_pmd);
>
> /*
> * Withdraw the table only after we mark the pmd entry invalid.
>
> Thanks,
>
> --
> Peter Xu
>
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