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Date: Wed, 12 Dec 2018 13:15:40 +0800
From: Peter Xu <peterx@...hat.com>
To: Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
Cc: 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 <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 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? 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:
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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