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From: Andy Lutomirski <luto@kernel.org>
To: Joerg Roedel <jroedel@suse.de>
Cc: Andy Lutomirski <luto@kernel.org>, Joerg Roedel <joro@8bytes.org>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Andrew Morton <akpm@linux-foundation.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Linux-MM <linux-mm@kvack.org>
Subject: Re: [PATCH 3/3] mm/vmalloc: Sync unmappings in vunmap_page_range()
Date: Thu, 18 Jul 2019 12:04:49 -0700	[thread overview]
Message-ID: <CALCETrXJYuHN872F74kVTuw4dYOc5saKqoUFbgJ5X0EuGEhXcA@mail.gmail.com> (raw)
In-Reply-To: <20190718091745.GG13091@suse.de>

On Thu, Jul 18, 2019 at 2:17 AM Joerg Roedel <jroedel@suse.de> wrote:
>
> Hi Andy,
>
> On Wed, Jul 17, 2019 at 02:24:09PM -0700, Andy Lutomirski wrote:
> > On Wed, Jul 17, 2019 at 12:14 AM Joerg Roedel <joro@8bytes.org> wrote:
> > > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > > index 4fa8d84599b0..322b11a374fd 100644
> > > --- a/mm/vmalloc.c
> > > +++ b/mm/vmalloc.c
> > > @@ -132,6 +132,8 @@ static void vunmap_page_range(unsigned long addr, unsigned long end)
> > >                         continue;
> > >                 vunmap_p4d_range(pgd, addr, next);
> > >         } while (pgd++, addr = next, addr != end);
> > > +
> > > +       vmalloc_sync_all();
> > >  }
> >
> > I'm confused.  Shouldn't the code in _vm_unmap_aliases handle this?
> > As it stands, won't your patch hurt performance on x86_64?  If x86_32
> > is a special snowflake here, maybe flush_tlb_kernel_range() should
> > handle this?
>
> Imo this is the logical place to handle this. The code first unmaps the
> area from the init_mm page-table and then syncs that page-table to all
> other page-tables in the system, so one place to update the page-tables.


I find it problematic that there is no meaningful documentation as to
what vmalloc_sync_all() is supposed to do.  The closest I can find is
this comment by following the x86_64 code, which calls
sync_global_pgds(), which says:

/*
 * When memory was added make sure all the processes MM have
 * suitable PGD entries in the local PGD level page.
 */
void sync_global_pgds(unsigned long start, unsigned long end)
{

Which is obviously entirely inapplicable.  If I'm understanding
correctly, the underlying issue here is that the vmalloc fault
mechanism can propagate PGD entry *addition*, but nothing (not even
flush_tlb_kernel_range()) propagates PGD entry *removal*.

I find it suspicious that only x86 has this.  How do other
architectures handle this?

At the very least, I think this series needs a comment in
vmalloc_sync_all() explaining exactly what the function promises to
do.  But maybe a better fix is to add code to flush_tlb_kernel_range()
to sync the vmalloc area if the flushed range overlaps the vmalloc
area.  Or, even better, improve x86_32 the way we did x86_64: adjust
the memory mapping code such that top-level paging entries are never
deleted in the first place.

  reply	other threads:[~2019-07-18 19:05 UTC|newest]

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-07-17  7:14 [PATCH 0/3 v2] Sync unmappings in vmalloc/ioremap areas Joerg Roedel
2019-07-17  7:14 ` [PATCH 1/3] x86/mm: Check for pfn instead of page in vmalloc_sync_one() Joerg Roedel
2019-07-17  7:14 ` [PATCH 2/3] x86/mm: Sync also unmappings " Joerg Roedel
2019-07-17 21:06   ` Dave Hansen
2019-07-18  8:44     ` Joerg Roedel
2019-07-17 21:43   ` Thomas Gleixner
2019-07-18  8:46     ` Joerg Roedel
2019-07-18  9:04       ` Thomas Gleixner
2019-07-18  9:25         ` Joerg Roedel
2019-07-19 14:01         ` Joerg Roedel
2019-07-19 21:10           ` Thomas Gleixner
2019-07-17  7:14 ` [PATCH 3/3] mm/vmalloc: Sync unmappings in vunmap_page_range() Joerg Roedel
2019-07-17 21:24   ` Andy Lutomirski
2019-07-18  9:17     ` Joerg Roedel
2019-07-18 19:04       ` Andy Lutomirski [this message]
2019-07-19 12:21         ` Joerg Roedel
2019-07-19 12:24           ` Andy Lutomirski
2019-07-19 13:00             ` Joerg Roedel
  -- strict thread matches above, loose matches on Subject: below --
2019-07-19 18:46 [PATCH 0/3 v3] Sync unmappings in vmalloc/ioremap areas Joerg Roedel
2019-07-19 18:46 ` [PATCH 3/3] mm/vmalloc: Sync unmappings in vunmap_page_range() Joerg Roedel
2019-07-22  8:11   ` Joerg Roedel
2019-07-22  8:19     ` Thomas Gleixner
2019-07-22  8:29       ` Joerg Roedel
2019-07-15 11:02 [PATCH 0/3] Sync unmappings in vmalloc/ioremap areas Joerg Roedel
2019-07-15 11:02 ` [PATCH 3/3] mm/vmalloc: Sync unmappings in vunmap_page_range() Joerg Roedel

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