linux-kernel - Re: [PATCH 06/39] vfs: add f_op->pre

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Date:   Mon, 4 Jun 2018 01:48:27 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Miklos Szeredi <mszeredi@...hat.com>
Cc:     linux-unionfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 06/39] vfs: add f_op->pre_mmap()

On Tue, May 29, 2018 at 04:43:06PM +0200, Miklos Szeredi wrote:
> This is needed by overlayfs to be able to copy up a file from a read-only
> lower layer to a writable layer when being mapped shared.  When copying up,
> overlayfs takes VFS locks that would violate locking order when nested
> inside mmap_sem.
> 
> Add a new f_op->pre_mmap method, which is called before taking mmap_sem.

NAK.  We really should not add multiple methods for mmap, and everytime
this came up we found a better way to solve the problem instead.  Most
recent example was the socket zero copy receive code.