On Saturday, 5 June 2021 10:41:03 AM AEST Shakeel Butt wrote:
> External email: Use caution opening links or attachments
>
>
> On Fri, Jun 4, 2021 at 1:49 PM Liam Howlett <liam.howlett@xxxxxxxxxx> wrote:
> >
> > * Shakeel Butt <shakeelb@xxxxxxxxxx> [210525 19:45]:
> > > On Tue, May 25, 2021 at 11:40 AM Liam Howlett <liam.howlett@xxxxxxxxxx>
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".
Actually thanks for highlighting that paragraph. I have gone back through the
code again in munlock_vma_pages_range() and think I have a better
understanding of it now. So now I agree - the check of VM_LOCKED under the PTL
is important to ensure mlock_vma_page() does not run after VM_LOCKED has been
cleared and __munlock_pagevec_fill() has run.
Will post v10 to fix this and the try_to_munlock reference pointed out by Liam
which I missed for v9. Thanks Shakeel for taking the time to point this out.
> One positive outcome of this cleanup patch is the removal of
> unnecessary invalidation (unmapping for kvm case) of secondary mmus.
