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: <ZxLVSkEr_Zd2Ai3Q@casper.infradead.org>
Date: Fri, 18 Oct 2024 22:38:18 +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 02:00:16PM -0700, Yosry Ahmed wrote:
> On Fri, Oct 18, 2024 at 1:55 PM Matthew Wilcox <willy@...radead.org> wrote:
> >
> > 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.
> 
> I see, thanks for elaborating.
> 
> Could you confirm if the current patch with the xas_reset() added
> would be equivalent to just checking xa_load() before using
> xa_erase()?

Yes, I think it would, so it's probably a poor tradeoff.