[PATCH v7 01/11] mm/mremap: Fix race between MOVE_PMD mremap and pageout

[Kirill A. Shutemov]

On Tue, Jun 08, 2021 at 01:22:23PM +0530, Aneesh Kumar K.V wrote:
> 
> Hi Hugh,
> 
> Hugh Dickins <hughd at google.com> writes:
> 
> > On Mon, 7 Jun 2021, Aneesh Kumar K.V wrote:
> >
> >> CPU 1				CPU 2					CPU 3
> >> 
> >> mremap(old_addr, new_addr)      page_shrinker/try_to_unmap_one
> >> 
> >> mmap_write_lock_killable()
> >> 
> >> 				addr = old_addr
> >> 				lock(pte_ptl)
> >> lock(pmd_ptl)
> >> pmd = *old_pmd
> >> pmd_clear(old_pmd)
> >> flush_tlb_range(old_addr)
> >> 
> >> *new_pmd = pmd
> >> 									*new_addr = 10; and fills
> >> 									TLB with new addr
> >> 									and old pfn
> >> 
> >> unlock(pmd_ptl)
> >> 				ptep_clear_flush()
> >> 				old pfn is free.
> >> 									Stale TLB entry
> >> 
> >> Fix this race by holding pmd lock in pageout. This still doesn't handle the race
> >> between MOVE_PUD and pageout.
> >> 
> >> Fixes: 2c91bd4a4e2e ("mm: speed up mremap by 20x on large regions")
> >> Link: https://lore.kernel.org/linux-mm/CAHk-=wgXVR04eBNtxQfevontWnP6FDm+oj5vauQXP3S-huwbPw@mail.gmail.com
> >> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar at linux.ibm.com>
> >
> > This seems very wrong to me, to require another level of locking in the
> > rmap lookup, just to fix some new pagetable games in mremap.
> >
> > But Linus asked "Am I missing something?": neither of you have mentioned
> > mremap's take_rmap_locks(), so I hope that already meets your need.  And
> > if it needs to be called more often than before (see "need_rmap_locks"),
> > that's probably okay.
> >
> > Hugh
> >
> 
> Thanks for reviewing the change. I missed the rmap lock in the code
> path. How about the below change?
> 
>     mm/mremap: hold the rmap lock in write mode when moving page table entries.
>     
>     To avoid a race between rmap walk and mremap, mremap does take_rmap_locks().
>     The lock was taken to ensure that rmap walk don't miss a page table entry due to
>     PTE moves via move_pagetables(). The kernel does further optimization of
>     this lock such that if we are going to find the newly added vma after the
>     old vma, the rmap lock is not taken. This is because rmap walk would find the
>     vmas in the same order and if we don't find the page table attached to
>     older vma we would find it with the new vma which we would iterate later.
>     The actual lifetime of the page is still controlled by the PTE lock.
>     
>     This patch updates the locking requirement to handle another race condition
>     explained below with optimized mremap::
>     
>     Optmized PMD move
>     
>         CPU 1                           CPU 2                                   CPU 3
>     
>         mremap(old_addr, new_addr)      page_shrinker/try_to_unmap_one
>     
>         mmap_write_lock_killable()
>     
>                                         addr = old_addr
>                                         lock(pte_ptl)
>         lock(pmd_ptl)
>         pmd = *old_pmd
>         pmd_clear(old_pmd)
>         flush_tlb_range(old_addr)
>     
>         *new_pmd = pmd
>                                                                                 *new_addr = 10; and fills
>                                                                                 TLB with new addr
>                                                                                 and old pfn
>     
>         unlock(pmd_ptl)
>                                         ptep_clear_flush()
>                                         old pfn is free.
>                                                                                 Stale TLB entry
>     
>     Optmized PUD move:
>     
>         CPU 1                           CPU 2                                   CPU 3
>     
>         mremap(old_addr, new_addr)      page_shrinker/try_to_unmap_one
>     
>         mmap_write_lock_killable()
>     
>                                         addr = old_addr
>                                         lock(pte_ptl)
>         lock(pud_ptl)
>         pud = *old_pud
>         pud_clear(old_pud)
>         flush_tlb_range(old_addr)
>     
>         *new_pud = pud
>                                                                                 *new_addr = 10; and fills
>                                                                                 TLB with new addr
>                                                                                 and old pfn
>     
>         unlock(pud_ptl)
>                                         ptep_clear_flush()
>                                         old pfn is free.
>                                                                                 Stale TLB entry
>     
>     Both the above race condition can be fixed if we force mremap path to take rmap lock.
>     
>     Signed-off-by: Aneesh Kumar K.V <aneesh.kumar at linux.ibm.com>

Looks like it should be enough to address the race.

It would be nice to understand what is performance overhead of the
additional locking. Is it still faster to move single PMD page table under
these locks comparing to moving PTE page table entries without the locks?

-- 
 Kirill A. Shutemov