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Message-ID: <Y0lSChlclGPkwTeA@casper.infradead.org>
Date: Fri, 14 Oct 2022 13:11:54 +0100
From: Matthew Wilcox <willy@...radead.org>
To: "zhaoyang.huang" <zhaoyang.huang@...soc.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Zhaoyang Huang <huangzhaoyang@...il.com>, linux-mm@...ck.org,
linux-kernel@...r.kernel.org, ke.wang@...soc.com,
steve.kang@...soc.com, baocong.liu@...soc.com,
linux-fsdevel@...r.kernel.org
Subject: Re: [RFC PATCH] mm: move xa forward when run across zombie page
On Fri, Oct 14, 2022 at 01:30:48PM +0800, zhaoyang.huang wrote:
> From: Zhaoyang Huang <zhaoyang.huang@...soc.com>
>
> Bellowing RCU stall is reported where kswapd traps in a live lock when shrink
> superblock's inode list. The direct reason is zombie page keeps staying on the
> xarray's slot and make the check and retry loop permanently. The root cause is unknown yet
> and supposed could be an xa update without synchronize_rcu etc. I would like to
> suggest skip this page to break the live lock as a workaround.
No, the underlying bug should be fixed.
> if (!folio || xa_is_value(folio))
> return folio;
>
> - if (!folio_try_get_rcu(folio))
> + if (!folio_try_get_rcu(folio)) {
> + xas_advance(xas, folio->index + folio_nr_pages(folio) - 1);
> goto reset;
> + }
You can't do this anyway. To call folio_nr_pages() and to look at
folio->index, you must have a refcount on the page, and this is the
path where we failed to get the refcount.
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