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From: "Huang, Ying" <ying.huang@intel.com>
To: Minchan Kim <minchan@kernel.org>
Cc: Kairui Song <ryncsn@gmail.com>, linux-mm@kvack.org,  Kairui Song
 <kasong@tencent.com>,  Andrew Morton <akpm@linux-foundation.org>,  Chris
 Li <chrisl@kernel.org>,  Hugh Dickins <hughd@google.com>,  Johannes Weiner
 <hannes@cmpxchg.org>,  Matthew Wilcox <willy@infradead.org>,  Michal Hocko
 <mhocko@suse.com>,  Yosry Ahmed <yosryahmed@google.com>,  David
 Hildenbrand <david@redhat.com>,  linux-kernel@vger.kernel.org, Yu Zhao
 <yuzhao@google.com>
Subject: Whether is the race for SWP_SYNCHRONOUS_IO possible? (was Re:
 [PATCH v3 6/7] mm/swap, shmem: use unified swapin helper for shmem)
In-Reply-To: <20240129175423.1987-7-ryncsn@gmail.com> (Kairui Song's message
	of "Tue, 30 Jan 2024 01:54:21 +0800")
References: <20240129175423.1987-1-ryncsn@gmail.com>
	<20240129175423.1987-7-ryncsn@gmail.com>
Date: Wed, 31 Jan 2024 10:51:38 +0800
Message-ID: <87bk92gqpx.fsf_-_@yhuang6-desk2.ccr.corp.intel.com>
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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()


Do I miss anything?

--
Best Regards,
Huang, Ying