linux-kernel - Re: [PATCH] mm, zswap: don't touch the XArray lock if there is no entry to free

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Message-ID: <ZxLLNfbLifp-b3W7@casper.infradead.org>
Date: Fri, 18 Oct 2024 21:55:17 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Yosry Ahmed <yosryahmed@...gle.com>
Cc: Kairui Song <ryncsn@...il.com>, linux-mm@...ck.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>, Nhat Pham <nphamcs@...il.com>,
	Chengming Zhou <chengming.zhou@...ux.dev>,
	Chris Li <chrisl@...nel.org>, Barry Song <v-songbaohua@...o.com>,
	"Huang, Ying" <ying.huang@...el.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm, zswap: don't touch the XArray lock if there is no
 entry to free

On Fri, Oct 18, 2024 at 01:40:18PM -0700, Yosry Ahmed wrote:
> Oh I thought xas_reload() is enough here to check that the entry is
> still there after the lock is acquired. Do we have to start the walk
> over after holding the lock?

Yes.  The entry is guaranteed to still be valid, but the node you're
looking in might have been freed, so you can't modify the node without
making sure the node is still in the tree.  We could make that cheaper
than a rewalk, but you're going to need to write that code since you're
the first to want to do something like this.