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From:   Shakeel Butt <shakeelb@google.com>
Date:   Fri, 4 Jun 2021 17:41:03 -0700
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Subject: Re: [PATCH v9 03/10] mm/rmap: Split try_to_munlock from try_to_unmap
To:     Liam Howlett <liam.howlett@oracle.com>
Cc:     Alistair Popple <apopple@nvidia.com>,
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        Christoph Hellwig <hch@lst.de>
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On Fri, Jun 4, 2021 at 1:49 PM Liam Howlett <liam.howlett@oracle.com> wrote:
>
> * Shakeel Butt <shakeelb@google.com> [210525 19:45]:
> > On Tue, May 25, 2021 at 11:40 AM Liam Howlett <liam.howlett@oracle.com> wrote:
> > >
> > [...]
> > > >
> > > > +/*
> > > > + * Walks the vma's mapping a page and mlocks the page if any locked vma's are
> > > > + * found. Once one is found the page is locked and the scan can be terminated.
> > > > + */
> > >
> > > Can you please add that this requires the mmap_sem() lock to the
> > > comments?
> > >
> >
> > Why does this require mmap_sem() lock? Also mmap_sem() lock of which mm_struct?
>
>
> Doesn't the mlock_vma_page() require the mmap_sem() for reading?  The
> mm_struct in vma->vm_mm;
>

We are traversing all the vmas where this page is mapped of possibly
different mm_structs. I don't think we want to take mmap_sem() of all
those mm_structs. The commit b87537d9e2fe ("mm: rmap use pte lock not
mmap_sem to set PageMlocked") removed exactly that.

>
> From what I can see, at least the following paths have mmap_lock held
> for writing:
>
> munlock_vma_pages_range() from __do_munmap()
> munlokc_vma_pages_range() from remap_file_pages()
>

The following path does not hold mmap_sem:

exit_mmap() -> munlock_vma_pages_all() -> munlock_vma_pages_range().

I would really suggest all to carefully read the commit message of
b87537d9e2fe ("mm: rmap use pte lock not mmap_sem to set
PageMlocked").

Particularly the following paragraph:
...
    Vlastimil Babka points out another race which this patch protects against.
     try_to_unmap_one() might reach its mlock_vma_page() TestSetPageMlocked a
    moment after munlock_vma_pages_all() did its Phase 1 TestClearPageMlocked:
    leaving PageMlocked and unevictable when it should be evictable.  mmap_sem
    is ineffective because exit_mmap() does not hold it; page lock ineffective
    because __munlock_pagevec() only takes it afterwards, in Phase 2; pte lock
    is effective because __munlock_pagevec_fill() takes it to get the page,
    after VM_LOCKED was cleared from vm_flags, so visible to try_to_unmap_one.
...

Alistair, please bring back the VM_LOCKED check with pte lock held and
the comment "Holding pte lock, we do *not* need mmap_lock here".

One positive outcome of this cleanup patch is the removal of
unnecessary invalidation (unmapping for kvm case) of secondary mmus.