Re: [PATCH] mm/swap: fix race condition in direct swapin path

[SeongJae Park]

Hi Kairui,

On Mon,  5 Feb 2024 19:09:59 +0800 Kairui Song <ryncsn@xxxxxxxxx> wrote:

[...]
>  mm/memory.c   | 19 +++++++++++++++++++
>  mm/swap.h     |  5 +++++
>  mm/swapfile.c | 16 ++++++++++++++++
>  3 files changed, 40 insertions(+)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 7e1f4849463a..fd7c55a292f1 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3867,6 +3867,20 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>  	if (!folio) {
>  		if (data_race(si->flags & SWP_SYNCHRONOUS_IO) &&
>  		    __swap_count(entry) == 1) {
> +			/*
> +			 * With swap count == 1, after we read the entry,
> +			 * other threads could finish swapin first, free
> +			 * the entry, then swapout the modified page using
> +			 * the same entry. Now the content we just read is
> +			 * stalled, and it's undetectable as pte_same()
> +			 * returns true due to entry reuse.
> +			 *
> +			 * So pin the swap entry using the cache flag even
> +			 * cache is not used.
> +			 */
> +			if (swapcache_prepare(entry))
> +				goto out;
> +

I'm getting below build error after this patch.  I guess maybe the code need to
take care of CONFIG_SWAP unset case?

    .../mm/memory.c: In function 'do_swap_page':
    .../mm/memory.c:4004:8: error: implicit declaration of function 'swapcache_prepare'; did you mean 'swapcache_clear'? [-Werror=implicit-function-declaration]
     4004 |    if (swapcache_prepare(entry))
          |        ^~~~~~~~~~~~~~~~~
          |        swapcache_clear


Thanks,
SJ

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