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Date:   Mon, 15 Mar 2021 23:46:05 +0000
From:   Matthew Wilcox <willy@infradead.org>
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     Chris Goldsworthy <cgoldswo@codeaurora.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>, linux-mm@kvack.org,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        Laura Abbott <lauraa@codeaurora.org>
Subject: Re: [PATCH v3] fs/buffer.c: Revoke LRU when trying to drop buffers
Message-ID: <20210315234605.GD2577561@casper.infradead.org>
References: <cover.1610572007.git.cgoldswo@codeaurora.org>
 <2f13c006ad12b047e9e4d5de008e5d5c41322754.1610572007.git.cgoldswo@codeaurora.org>
 <20210315164138.c15727adeb184313f5e7e9f6@linux-foundation.org>
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On Mon, Mar 15, 2021 at 04:41:38PM -0700, Andrew Morton wrote:
> > When a buffer is added to the LRU list, a reference is taken which is
> > not dropped until the buffer is evicted from the LRU list. This is the
> > correct behavior, however this LRU reference will prevent the buffer
> > from being dropped. This means that the buffer can't actually be dropped
> > until it is selected for eviction. There's no bound on the time spent
> > on the LRU list, which means that the buffer may be undroppable for
> > very long periods of time. Given that migration involves dropping
> > buffers, the associated page is now unmigratible for long periods of
> > time as well. CMA relies on being able to migrate a specific range
> > of pages, so these types of failures make CMA significantly
> > less reliable, especially under high filesystem usage.
>
> It looks like patch this turns drop_buffers() into a very expensive
> operation.  And that expensive operation occurs under the
> address_space-wide private_lock, which is more ouch.

This patch set is obsoleted by Minchan Kim's more recent patch-set.