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: <CAJD7tkaycBYvtTCoGsuNUekSDNnE5SZb-XyS5a8j83JsKQw4FQ@mail.gmail.com>
Date: Fri, 18 Oct 2024 14:00:16 -0700
From: Yosry Ahmed <yosryahmed@...gle.com>
To: Matthew Wilcox <willy@...radead.org>
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 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()?