Re: [PATCH] mm, page_alloc: capture page in task context only

[Mel Gorman]

On Wed, Jun 10, 2020 at 01:48:59PM -0700, Hugh Dickins wrote:
> While stressing compaction, one run oopsed on NULL capc->cc in
> __free_one_page()'s task_capc(zone): compact_zone_order() had been
> interrupted, and a page was being freed in the return from interrupt.
> 
> Though you would not expect it from the source, both gccs I was using
> (a 4.8.1 and a 7.5.0) had chosen to compile compact_zone_order() with
> the ".cc = &cc" implemented by mov %rbx,-0xb0(%rbp) immediately before
> callq compact_zone - long after the "current->capture_control = &capc".
> An interrupt in between those finds capc->cc NULL (zeroed by an earlier
> rep stos).
> 
> This could presumably be fixed by a barrier() before setting
> current->capture_control in compact_zone_order(); but would also need
> more care on return from compact_zone(), in order not to risk leaking
> a page captured by interrupt just before capture_control is reset.
> 
> Maybe that is the preferable fix, but I felt safer for task_capc() to
> exclude the rather surprising possibility of capture at interrupt time.
> 
> Fixes: 5e1f0f098b46 ("mm, compaction: capture a page under direct compaction")
> Cc: stable@xxxxxxxxxxxxxxx # 5.1+
> Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>

Acked-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>

-- 
Mel Gorman
SUSE Labs