From: Oleg Nesterov <oleg@redhat.com>
To: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Song Liu <songliubraving@fb.com>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Linux MM <linux-mm@kvack.org>,
Andrew Morton <akpm@linux-foundation.org>,
Matthew Wilcox <matthew.wilcox@oracle.com>,
"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
Kernel Team <Kernel-team@fb.com>,
William Kucharski <william.kucharski@oracle.com>,
"srikar@linux.vnet.ibm.com" <srikar@linux.vnet.ibm.com>
Subject: Re: [PATCH v12 5/6] khugepaged: enable collapse pmd for pte-mapped THP
Date: Tue, 13 Aug 2019 18:24:51 +0200 [thread overview]
Message-ID: <20190813162451.GD6971@redhat.com> (raw)
In-Reply-To: <20190813150539.ciai477wk2cratvc@box>
On 08/13, Kirill A. Shutemov wrote:
>
> On Tue, Aug 13, 2019 at 04:05:53PM +0200, Oleg Nesterov wrote:
> > >
> > > I thought that retract_page_tables() checks vma->anon_vma to ensure that
> > > this vma doesn't have a cow'ed PageAnon() page. And I still can't understand
> > > why can't it race with __handle_mm_fault() paths.
>
> vma->anon_vma check is a cheap way to exclude MAP_PRIVATE mappings that
> got written from userspace.
Yes, and this is how I understood it from the very beginning, but then
I was confused.
> vma->anon_vma can be set up after the check but before taking mmap_sem.
> But page lock would prevent establishing any new ptes of the page, so we
> are safe.
And this is what was not clear to me until I noticed unmap_mapping_pages()
in collapse_shmem().
Plus I was confused by the comment above down_write_trylock(mmap_sem).
To me it looks as if we _could_ do down_write(), but we do not want to
disturb the system.
But iiuc we simply can't do down_write(), exactly because handle_mm_fault()
can wait for page lock with mmap_sem held.
> > > Suppose that shmem_file was mmaped with PROT_READ|WRITE, MAP_PRIVATE.
> > > To simplify, suppose that a non-THP page was already faulted in,
> > > pte_present() == T.
> > >
> > > Userspace writes to this page.
> > >
> > > Why __handle_mm_fault()->handle_pte_fault()->do_wp_page()->wp_page_copy()
> > > can not cow this page and update pte after the vma->anon_vma chech and
> > > before down_write_trylock(mmap_sem) ?
> >
> > OK, probably this is impossible, collapse_shmem() does unmap_mapping_pages(),
> > so handle_pte_fault() will call shmem_fault() which iiuc should block in
> > find_lock_entry() because new_page is locked, and thus down_write_trylock()
> > can't succeed.
>
> You've got it right.
Great, thanks.
> > Nevermind, I am sure I missed something. Perhaps you can update the comments
> > to make this more clear.
>
> Let me see first that my explanation makes sense :P
It does ;)
Oleg.
next prev parent reply other threads:[~2019-08-13 16:24 UTC|newest]
Thread overview: 30+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-07 23:37 [PATCH v12 0/6] THP aware uprobe Song Liu
2019-08-07 23:37 ` [PATCH v12 1/6] mm: move memcmp_pages() and pages_identical() Song Liu
2019-08-07 23:37 ` [PATCH v12 2/6] uprobe: use original page when all uprobes are removed Song Liu
2019-08-07 23:37 ` [PATCH v12 3/6] mm, thp: introduce FOLL_SPLIT_PMD Song Liu
2019-08-08 16:37 ` Oleg Nesterov
2019-08-08 17:16 ` Song Liu
2019-08-09 16:35 ` Oleg Nesterov
2019-08-09 16:50 ` Song Liu
2019-08-07 23:37 ` [PATCH v12 4/6] uprobe: use FOLL_SPLIT_PMD instead of FOLL_SPLIT Song Liu
2019-08-07 23:37 ` [PATCH v12 5/6] khugepaged: enable collapse pmd for pte-mapped THP Song Liu
2019-08-08 16:33 ` Oleg Nesterov
2019-08-08 17:05 ` Song Liu
2019-08-09 15:24 ` Oleg Nesterov
2019-08-09 16:30 ` Song Liu
2019-08-09 18:01 ` Song Liu
2019-08-12 12:11 ` Kirill A. Shutemov
2019-08-12 13:22 ` Oleg Nesterov
2019-08-12 14:40 ` Kirill A. Shutemov
2019-08-12 21:04 ` Song Liu
2019-08-13 14:44 ` Song Liu
2019-08-15 10:16 ` Oleg Nesterov
2019-08-15 16:27 ` Song Liu
2019-08-13 13:30 ` Oleg Nesterov
2019-08-13 14:05 ` Oleg Nesterov
2019-08-13 15:05 ` Kirill A. Shutemov
2019-08-13 16:24 ` Oleg Nesterov [this message]
2019-08-16 14:54 ` Kirill A. Shutemov
2019-08-12 13:06 ` Oleg Nesterov
2019-08-12 14:36 ` Song Liu
2019-08-07 23:37 ` [PATCH v12 6/6] uprobe: collapse THP pmd after removing all uprobes Song Liu
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