linux-kernel - Re: [PATCH v4] mm/swap: fix race when skipping swapcache

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Date: Mon, 19 Feb 2024 17:31:47 -0800
From: Andrew Morton <akpm@...ux-foundation.org>
To: Kairui Song <kasong@...cent.com>
Cc: Kairui Song <ryncsn@...il.com>, linux-mm@...ck.org, "Huang, Ying"
 <ying.huang@...el.com>, Chris Li <chrisl@...nel.org>, Minchan Kim
 <minchan@...nel.org>, Barry Song <v-songbaohua@...o.com>, Yu Zhao
 <yuzhao@...gle.com>, SeongJae Park <sj@...nel.org>, David Hildenbrand
 <david@...hat.com>, Hugh Dickins <hughd@...gle.com>, Johannes Weiner
 <hannes@...xchg.org>, Matthew Wilcox <willy@...radead.org>, Michal Hocko
 <mhocko@...e.com>, Yosry Ahmed <yosryahmed@...gle.com>, Aaron Lu
 <aaron.lu@...el.com>, stable@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4] mm/swap: fix race when skipping swapcache

On Mon, 19 Feb 2024 16:20:40 +0800 Kairui Song <ryncsn@...il.com> wrote:

> From: Kairui Song <kasong@...cent.com>
> 
> 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);