linux-kernel - Re: [PATCH v4 4/5] cramfs: add mmap support

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Date:   Tue, 3 Oct 2017 07:57:32 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Nicolas Pitre <nicolas.pitre@...aro.org>
Cc:     Richard Weinberger <richard.weinberger@...il.com>,
        Christoph Hellwig <hch@...radead.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        "linux-embedded@...r.kernel.org" <linux-embedded@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Chris Brandt <Chris.Brandt@...esas.com>
Subject: Re: [PATCH v4 4/5] cramfs: add mmap support

On Mon, Oct 02, 2017 at 07:33:29PM -0400, Nicolas Pitre wrote:
> On Tue, 3 Oct 2017, Richard Weinberger wrote:
> 
> > On Mon, Oct 2, 2017 at 12:29 AM, Nicolas Pitre <nicolas.pitre@...aro.org> wrote:
> > > On Sun, 1 Oct 2017, Christoph Hellwig wrote:
> > >
> > >> up_read(&mm->mmap_sem) in the fault path is a still a complete
> > >> no-go,
> > >>
> > >> NAK
> > >
> > > Care to elaborate?
> > >
> > > What about mm/filemap.c:__lock_page_or_retry() then?
> > 
> > As soon you up_read() in the page fault path other tasks will race
> > with you before
> > you're able to grab the write lock.
> 
> But I _know_ that.
> 
> Could you highlight an area in my code where this is not accounted for?

Existing users of lock_page_or_retry return VM_FAULT_RETRY right after
up()ing mmap_sem, and they must already have a reference to the page
which is the only thing touched until then.

Your patch instead goes for an exclusive mmap_sem if it can, and
even if there is nothing that breaks with that scheme right now
there s nothing documenting that this actually safe, and we are
way down in the complex page fault path.