Re: [PATCH 2/7] mm/thp: try_to_unmap() use TTU_SYNC for safe DEBUG_VM splitting

[Alistair Popple <apopple@nvidia.com>]

On Wednesday, 2 June 2021 7:07:53 AM AEST Hugh Dickins wrote:
> External email: Use caution opening links or attachments
> 
> 
> Stressing huge tmpfs often crashed on unmap_page()'s VM_BUG_ON_PAGE
> (!unmap_success): with dump_page() showing mapcount:1, but then its
> raw struct page output showing _mapcount ffffffff i.e. mapcount 0.
> 
> And even if that particular VM_BUG_ON_PAGE(!unmap_success) is removed,
> it is immediately followed by a VM_BUG_ON_PAGE(compound_mapcount(head)),
> and further down an IS_ENABLED(CONFIG_DEBUG_VM) total_mapcount BUG():
> all indicative of some mapcount difficulty in development here perhaps.
> But the !CONFIG_DEBUG_VM path handles the failures correctly and silently.
> 
> I believe the problem is that once a racing unmap has cleared pte or pmd,
> try_to_unmap_one() may skip taking the page table lock, and emerge from
> try_to_unmap() before the racing task has reached decrementing mapcount.
> 
> Instead of abandoning the unsafe VM_BUG_ON_PAGE(), and the ones that
> follow, use PVMW_SYNC in try_to_unmap_one() in this case: adding TTU_SYNC
> to the options, and passing that from unmap_page() when CONFIG_DEBUG_VM=y.
> It could be passed in the non-debug case too, but that would sometimes add
> a little overhead, whereas it's rare for this race to result in failure.
> 
> mm/memory-failure.c:hwpoison_user_mappings() should probably use the new
> TTU_SYNC option too, just in case this race coincides with its attempts to
> unmap a failing page (THP or not); but this commit does not add that.
> 
> Fixes: fec89c109f3a ("thp: rewrite freeze_page()/unfreeze_page() with
> generic rmap walkers") Signed-off-by: Hugh Dickins <hughd@google.com>
> Cc: <stable@vger.kernel.org>
> ---
>  include/linux/rmap.h |  3 ++-
>  mm/huge_memory.c     |  4 ++++
>  mm/page_vma_mapped.c |  8 ++++++++
>  mm/rmap.c            | 17 ++++++++++++++++-
>  4 files changed, 30 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/rmap.h b/include/linux/rmap.h
> index def5c62c93b3..891599a4cb8d 100644
> --- a/include/linux/rmap.h
> +++ b/include/linux/rmap.h
> @@ -97,7 +97,8 @@ enum ttu_flags {
>                                          * do a final flush if necessary */
>         TTU_RMAP_LOCKED         = 0x80, /* do not grab rmap lock:
>                                          * caller holds it */
> -       TTU_SPLIT_FREEZE        = 0x100,                /* freeze pte under
> splitting thp */ +       TTU_SPLIT_FREEZE        = 0x100, /* freeze pte
> under splitting thp */ +       TTU_SYNC                = 0x200, /* avoid
> racy checks with PVMW_SYNC */ };
> 
>  #ifdef CONFIG_MMU
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 9fb7b47da87e..305f709a7aca 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2357,6 +2357,10 @@ static void unmap_page(struct page *page)
>         if (PageAnon(page))
>                 ttu_flags |= TTU_SPLIT_FREEZE;
> 
> +       /* Make sure that the BUGs will not bite */
> +       if (IS_ENABLED(CONFIG_DEBUG_VM))
> +               ttu_flags |= TTU_SYNC;
> +
>         unmap_success = try_to_unmap(page, ttu_flags);
>         VM_BUG_ON_PAGE(!unmap_success, page);
>  }
> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
> index 2cf01d933f13..b45d22738b45 100644
> --- a/mm/page_vma_mapped.c
> +++ b/mm/page_vma_mapped.c
> @@ -212,6 +212,14 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk
> *pvmw) pvmw->ptl = NULL;
>                 }
>         } else if (!pmd_present(pmde)) {
> +               /*
> +                * If PVMW_SYNC, take and drop THP pmd lock so that we
> +                * cannot return prematurely, while zap_huge_pmd() has
> +                * cleared *pmd but not decremented compound_mapcount().
> +                */
> +               if ((pvmw->flags & PVMW_SYNC) &&
> +                   PageTransCompound(pvmw->page))
> +                       spin_unlock(pmd_lock(mm, pvmw->pmd));
>                 return false;
>         }
>         if (!map_pte(pvmw))
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 693a610e181d..07811b4ae793 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1405,6 +1405,15 @@ static bool try_to_unmap_one(struct page *page,
> struct vm_area_struct *vma, struct mmu_notifier_range range;
>         enum ttu_flags flags = (enum ttu_flags)(long)arg;
> 
> +       /*
> +        * When racing against e.g. zap_pte_range() on another cpu,
> +        * in between its ptep_get_and_clear_full() and page_remove_rmap(),
> +        * try_to_unmap() may return false when it is about to become true,
> +        * if page table locking is skipped: use TTU_SYNC to wait for that.
> +        */
> +       if (flags & TTU_SYNC)
> +               pvmw.flags = PVMW_SYNC;
> +

If this gets applied on top of my series then I think we would also need to 
add this to the start of try_to_migrate_one() as I assume you can hit this bug 
regardless of whether unmapping vs. installing swap migration entries.

We would also need to update the flag check at the start of try_to_migrate() 
to allow passing TTU_SYNC.

>         /* munlock has nothing to gain from examining un-locked vmas */
>         if ((flags & TTU_MUNLOCK) && !(vma->vm_flags & VM_LOCKED))
>                 return true;
> @@ -1777,7 +1786,13 @@ bool try_to_unmap(struct page *page, enum ttu_flags
> flags) else
>                 rmap_walk(page, &rwc);
> 
> -       return !page_mapcount(page) ? true : false;
> +       /*
> +        * When racing against e.g. zap_pte_range() on another cpu,
> +        * in between its ptep_get_and_clear_full() and page_remove_rmap(),
> +        * try_to_unmap() may return false when it is about to become true,
> +        * if page table locking is skipped: use TTU_SYNC to wait for that.
> +        */
> +       return !page_mapcount(page);
>  }
> 
>  /**
> --
> 2.32.0.rc0.204.g9fa02ecfa5-goog