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

[Andrew Morton]

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

> 
> 
> > On Sep 22, 2021, at 7:37 PM, Matthew Wilcox <willy@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxxx/
> 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?