Re: [PATCH v2 03/10] mm/thp: try_to_unmap() use TTU_SYNC for safe splitting (fwd)

[Kirill A. Shutemov]

On Tue, Jun 08, 2021 at 09:12:20PM -0700, Hugh Dickins wrote:
> 
> 
> ---------- Forwarded message ----------
> Date: Tue, 8 Jun 2021 21:10:19 -0700 (PDT)
> From: Hugh Dickins <hughd@xxxxxxxxxx>
> To: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Hugh Dickins <hughd@xxxxxxxxxx>,
>     Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>,
>     Yang Shi <shy828301@xxxxxxxxx>, Wang Yugui <wangyugui@xxxxxxxxxxxx>,
>     Matthew Wilcox <willy@xxxxxxxxxxxxx>,
>     Naoya Horiguchi <naoya.horiguchi@xxxxxxx>,
>     Alistair Popple <apopple@xxxxxxxxxx>, Ralph Campbell <rcampbell@xxxxxxxxxx>,
>     Zi Yan <ziy@xxxxxxxxxx>, Miaohe Lin <linmiaohe@xxxxxxxxxx>,
>     Minchan Kim <minchan@xxxxxxxxxx>, Jue Wang <juew@xxxxxxxxxx>,
>     Peter Xu <peterx@xxxxxxxxxx>, Jan Kara <jack@xxxxxxx>,
>     Shakeel Butt <shakeelb@xxxxxxxxxx>, Oscar Salvador <osalvador@xxxxxxx>
> Subject: [PATCH v2 03/10] mm/thp: try_to_unmap() use TTU_SYNC for safe splitting
> 
> 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, or for non-debug too?  Consensus is to do the same
> for both: the slight overhead added should rarely matter, except perhaps
> if splitting sparsely-populated multiply-mapped shmem.  Once confident
> that bugs are fixed, TTU_SYNC here can be removed, and the race tolerated.
> 
> Fixes: fec89c109f3a ("thp: rewrite freeze_page()/unfreeze_page() with generic rmap walkers")
> Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx>

Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>

-- 
 Kirill A. Shutemov