On Mon, 19 Feb 2024 16:20:40 +0800 Kairui Song <ryncsn@xxxxxxxxx> wrote:
> From: Kairui Song <kasong@xxxxxxxxxxx>
>
> When skipping swapcache for SWP_SYNCHRONOUS_IO, if two or more threads
> swapin the same entry at the same time, they get different pages (A, B).
> Before one thread (T0) finishes the swapin and installs page (A)
> to the PTE, another thread (T1) could finish swapin of page (B),
> swap_free the entry, then swap out the possibly modified page
> reusing the same entry. It breaks the pte_same check in (T0) because
> PTE value is unchanged, causing ABA problem. Thread (T0) will
> install a stalled page (A) into the PTE and cause data corruption.
>
> @@ -3867,6 +3868,20 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> if (!folio) {
> if (data_race(si->flags & SWP_SYNCHRONOUS_IO) &&
> __swap_count(entry) == 1) {
> + /*
> + * Prevent parallel swapin from proceeding with
> + * the cache flag. Otherwise, another thread may
> + * finish swapin first, free the entry, and swapout
> + * reusing the same entry. It's undetectable as
> + * pte_same() returns true due to entry reuse.
> + */
> + if (swapcache_prepare(entry)) {
> + /* Relax a bit to prevent rapid repeated page faults */
> + schedule_timeout_uninterruptible(1);
Well this is unpleasant. How often can we expect this to occur?
> + goto out;
> + }
> + need_clear_cache = true;
> +
> /* skip swapcache */
> folio = vma_alloc_folio(GFP_HIGHUSER_MOVABLE, 0,
> vma, vmf->address, false);
