linux-kernel - Re: [PATCH] mm: fix sleeping copy_huge

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Message-ID: <YXKdRDKp+l6lis/R@casper.infradead.org>
Date:   Fri, 22 Oct 2021 12:15:16 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Andrea Righi <andrea.righi@...onical.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Yang Shi <yang.shi@...ux.alibaba.com>,
        Minchan Kim <minchan@...nel.org>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH] mm: fix sleeping copy_huge_page called from atomic
 context

On Fri, Oct 22, 2021 at 09:46:19AM +0200, Andrea Righi wrote:
> copy_huge_page() can be called with mapping->private_lock held from
> __buffer_migrate_page() -> migrate_page_copy(), so it is not safe to
> do a cond_resched() in this context.
> 
> Introduce migrate_page_copy_nowait() and copy_huge_page_nowait()
> variants that can be used from an atomic context.

I think this is a consequence of THPs being created when they should not
be.  This is the wrong way to fix this problem; and I suspect it may
already be fixed at least in -mm.  We should have taken this path:

        if (!page_has_buffers(page))
                return migrate_page(mapping, newpage, page, mode);

but since we didn't, we can infer that there's a THP which has buffers
(this should never occur).  It's the same root cause as the invalidatepage
problem, just with a very different signature.