Nouveau - Jun 2021 - [PATCH v9 03/10] mm/rmap: Split try_to_munlock from try_to_unmap

* Shakeel Butt <shakeelb at google.com> [210525
19:45]:
> On Tue, May 25, 2021 at 11:40 AM Liam Howlett <liam.howlett at
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;

>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()
> 
> > > +static bool page_mlock_one(struct page *page, struct
vm_area_struct *vma,
> > > +                              unsigned long address, void
*unused)
> > > +{
> > > +     struct page_vma_mapped_walk pvmw = {
> > > +             .page = page,
> > > +             .vma = vma,
> > > +             .address = address,
> > > +     };
> > > +
> > > +     /* An un-locked vma doesn't have any pages to lock,
continue the scan */
> > > +     if (!(vma->vm_flags & VM_LOCKED))
> > > +             return true;
> > > +
> > > +     while (page_vma_mapped_walk(&pvmw)) {
> > > +             /* PTE-mapped THP are never mlocked */
> > > +             if (!PageTransCompound(page))
> > > +                     mlock_vma_page(page);
> > > +             page_vma_mapped_walk_done(&pvmw);
> > > +
> > > +             /*
> > > +              * no need to continue scanning other vma's if
the page has
> > > +              * been locked.
> > > +              */
> > > +             return false;
> > > +     }
> > > +
> > > +     return true;
> > > +}
munlock_vma_pages_range() comments still references try_to_{munlock|unmap}

On Fri, Jun 4, 2021 at 1:49 PM Liam Howlett <liam.howlett at oracle.com>
wrote:
>
> * Shakeel Butt <shakeelb at google.com> [210525 19:45]:
> > On Tue, May 25, 2021 at 11:40 AM Liam Howlett <liam.howlett at
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.