linux-kernel - Re: [PATCH v2 1/2] mm, thp: check page mapping when truncating page cache

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Message-Id: <20210923194343.ca0f29e1c4d361170343a6f2@linux-foundation.org>
Date:   Thu, 23 Sep 2021 19:43:43 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Rongwei Wang <rongwei.wang@...ux.alibaba.com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Linux MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        song@...nel.org, william.kucharski@...cle.com,
        Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH v2 1/2] mm, thp: check page mapping when truncating page
 cache

On Thu, 23 Sep 2021 01:04:54 +0800 Rongwei Wang <rongwei.wang@...ux.alibaba.com> wrote:

> 
> 
> > On Sep 22, 2021, at 7:37 PM, Matthew Wilcox <willy@...radead.org> wrote:
> > 
> > On Wed, Sep 22, 2021 at 03:06:44PM +0800, Rongwei Wang wrote:
> >> Transparent huge page has supported read-only non-shmem files. The file-
> >> backed THP is collapsed by khugepaged and truncated when written (for
> >> shared libraries).
> >> 
> >> However, there is race in two possible places.
> >> 
> >> 1) multiple writers truncate the same page cache concurrently;
> >> 2) collapse_file rolls back when writer truncates the page cache;
> > 
> > As I've said before, the bug here is that somehow there is a writable fd
> > to a file with THPs.  That's what we need to track down and fix.
> Hi, Matthew
> I am not sure get your means. We know “mm, thp: relax the VM_DENYWRITE constraint on file-backed THPs"
> Introduced file-backed THPs for DSO. It is possible {very rarely} for DSO to be opened in writeable way.
>
> ...
>
> > https://lore.kernel.org/linux-mm/YUdL3lFLFHzC80Wt@casper.infradead.org/
> All in all, what you mean is that we should solve this race at the source?

Matthew is being pretty clear here: we shouldn't be permitting
userspace to get a writeable fd for a thp-backed file.

Why are we permitting the DSO to be opened writeably?  If there's a
legitimate case for doing this then presumably "mm, thp: relax the
VM_DENYWRITE constraint on file-backed THPs: should be fixed or
reverted.

If there is no legitimate use case for returning a writeable fd for a
thp-backed file then we should fail such an attempt at open().  This
approach has back-compatibility issues which need to be thought about. 
Perhaps we should permit the open-writeably attempt to appear to
succeed, but to really return a read-only fd?