linux-kernel - Re: Whether is the race for SWP_SYNCHRONOUS_IO possible? (was Re: [PATCH v3 6/7] mm/swap, shmem: use unified swapin helper for shmem)

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Date: Wed, 31 Jan 2024 15:38:02 -0800
From: Chris Li <chrisl@...nel.org>
To: "Huang, Ying" <ying.huang@...el.com>
Cc: Minchan Kim <minchan@...nel.org>, Kairui Song <ryncsn@...il.com>, linux-mm@...ck.org, 
	Kairui Song <kasong@...cent.com>, Andrew Morton <akpm@...ux-foundation.org>, 
	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>, David Hildenbrand <david@...hat.com>, linux-kernel@...r.kernel.org, 
	Yu Zhao <yuzhao@...gle.com>
Subject: Re: Whether is the race for SWP_SYNCHRONOUS_IO possible? (was Re:
 [PATCH v3 6/7] mm/swap, shmem: use unified swapin helper for shmem)

On Tue, Jan 30, 2024 at 6:53 PM Huang, Ying <ying.huang@...el.com> wrote:
>
> Hi, Minchan,
>
> When I review the patchset from Kairui, I checked the code to skip swap
> cache in do_swap_page() for swap device with SWP_SYNCHRONOUS_IO.  Is the
> following race possible?  Where a page is swapped out to a swap device
> with SWP_SYNCHRONOUS_IO and the swap count is 1.  Then 2 threads of the
> process runs on CPU0 and CPU1 as below.  CPU0 is running do_swap_page().
>
> CPU0                            CPU1
> ----                            ----
> swap_cache_get_folio()
> check sync io and swap count
> alloc folio
> swap_readpage()
> folio_lock_or_retry()
>                                 swap in the swap entry
>                                 write page
>                                 swap out to same swap entry
> pte_offset_map_lock()
> check pte_same()
> swap_free()   <-- new content lost!
> set_pte_at()  <-- stale page!
> folio_unlock()
> pte_unmap_unlock()

Yes, that path looks possible but hard to hit due to the requirement
of swap in and swap out in a short window.
I have the similar question on the previous zswap rb tree to xarray
discussion regarding deleting an entry where the entry might change
due to swap in then swap out.

Chris