On 6/29/20 1:07 PM, gregkh@xxxxxxxxxxxxxxxxxxx wrote:
>
> The patch below does not apply to the 5.4-stable tree.
> If someone wants it applied there, or to any other stable or longterm
> tree, then please email the backport, including the original git commit
> id to <stable@xxxxxxxxxxxxxxx>.
>
> thanks,
>
> greg k-h
Hi, please simply cherrypick commit 6467552ca64c ("mm, compaction: fully assume
capture is not NULL in compact_zone_order()") first and then cherrypicking
b9e20f0da1f5c9c68689450a8cb436c9486434c8 will apply cleanly.
Thanks,
Vlastimil
> ------------------ original commit in Linus's tree ------------------
>
> From b9e20f0da1f5c9c68689450a8cb436c9486434c8 Mon Sep 17 00:00:00 2001
> From: Vlastimil Babka <vbabka@xxxxxxx>
> Date: Thu, 25 Jun 2020 20:29:24 -0700
> Subject: [PATCH] mm, compaction: make capture control handling safe wrt
> interrupts
>
> Hugh reports:
>
> "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
> (4.8.1 and 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"
>
> I have checked that gcc10 also behaves the same.
>
> The advantage of fix in compact_zone_order() is that we don't add
> another test in the page freeing hot path, and that it might prevent
> future problems if we stop exposing pointers to uninitialized structures
> in current task.
>
> So this patch implements the suggestion for compact_zone_order() with
> barrier() (and WRITE_ONCE() to prevent store tearing) for setting
> current->capture_control, and prevents page leaking with
> WRITE_ONCE/READ_ONCE in the proper order.
>
> Link: http://lkml.kernel.org/r/20200616082649.27173-1-vbabka@xxxxxxx
> Fixes: 5e1f0f098b46 ("mm, compaction: capture a page under direct compaction")
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
> Reported-by: Hugh Dickins <hughd@xxxxxxxxxx>
> Suggested-by: Hugh Dickins <hughd@xxxxxxxxxx>
> Acked-by: Hugh Dickins <hughd@xxxxxxxxxx>
> Cc: Alex Shi <alex.shi@xxxxxxxxxxxxxxxxx>
> Cc: Li Wang <liwang@xxxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx> [5.1+]
> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
>
> diff --git a/mm/compaction.c b/mm/compaction.c
> index fd988b7e5f2b..86375605faa9 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -2316,15 +2316,26 @@ static enum compact_result compact_zone_order(struct zone *zone, int order,
> .page = NULL,
> };
>
> - current->capture_control = &capc;
> + /*
> + * Make sure the structs are really initialized before we expose the
> + * capture control, in case we are interrupted and the interrupt handler
> + * frees a page.
> + */
> + barrier();
> + WRITE_ONCE(current->capture_control, &capc);
>
> ret = compact_zone(&cc, &capc);
>
> VM_BUG_ON(!list_empty(&cc.freepages));
> VM_BUG_ON(!list_empty(&cc.migratepages));
>
> - *capture = capc.page;
> - current->capture_control = NULL;
> + /*
> + * Make sure we hide capture control first before we read the captured
> + * page pointer, otherwise an interrupt could free and capture a page
> + * and we would leak it.
> + */
> + WRITE_ONCE(current->capture_control, NULL);
> + *capture = READ_ONCE(capc.page);
>
> return ret;
> }
>
