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From: Andrew Morton <akpm@linux-foundation.org>
To: Rongwei Wang <rongwei.wang@linux.alibaba.com>
Cc: Matthew Wilcox <willy@infradead.org>,
	Linux MM <linux-mm@kvack.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	song@kernel.org, william.kucharski@oracle.com,
	Hugh Dickins <hughd@google.com>
Subject: Re: [PATCH v2 1/2] mm, thp: check page mapping when truncating page cache
Date: Thu, 23 Sep 2021 19:43:43 -0700	[thread overview]
Message-ID: <20210923194343.ca0f29e1c4d361170343a6f2@linux-foundation.org> (raw)
In-Reply-To: <BC145393-93AC-4DF4-9CF4-2FB1C736B70C@linux.alibaba.com>

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

> 
> 
> > On Sep 22, 2021, at 7:37 PM, Matthew Wilcox <willy@infradead.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?



  reply	other threads:[~2021-09-24  2:43 UTC|newest]

Thread overview: 65+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-06 12:11 [PATCH 0/2] mm, thp: fix file-backed THP race in collapse_file Rongwei Wang
2021-09-06 12:11 ` [PATCH 1/2] mm, thp: check page mapping when truncating page cache Rongwei Wang
2021-09-07  2:49   ` Yu Xu
2021-09-07 18:08   ` Yang Shi
2021-09-08  2:35     ` Rongwei Wang
2021-09-08 21:48       ` Yang Shi
2021-09-09  1:25         ` Rongwei Wang
2021-09-13 14:49   ` [mm, thp] 20753096b6: BUG:unable_to_handle_page_fault_for_address kernel test robot
2021-09-06 12:12 ` [PATCH 2/2] mm, thp: bail out early in collapse_file for writeback page Rongwei Wang
2021-09-07 16:56   ` Yang Shi
2021-09-08  2:16     ` Rongwei Wang
2021-09-08 21:51       ` Yang Shi
2021-09-09  1:33         ` Rongwei Wang
2021-09-22  7:06 ` [PATCH v2 0/2] mm, thp: fix file-backed THP race in collapse_file and truncate pagecache Rongwei Wang
2021-09-22  7:06 ` [PATCH v2 1/2] mm, thp: check page mapping when truncating page cache Rongwei Wang
2021-09-22 11:37   ` Matthew Wilcox
2021-09-22 17:04     ` Rongwei Wang
2021-09-24  2:43       ` Andrew Morton [this message]
2021-09-24  3:08         ` Yang Shi
2021-09-24  3:35         ` Rongwei Wang
2021-09-24  7:12         ` Rongwei Wang
2021-09-27 22:24           ` Song Liu
2021-09-28 12:06             ` Matthew Wilcox
2021-09-28 16:59               ` Song Liu
2021-09-28 16:20             ` Rongwei Wang
2021-09-29  7:14               ` Song Liu
2021-09-29  7:50                 ` Rongwei Wang
2021-09-29 16:59                   ` Song Liu
2021-09-29 17:55                     ` Matthew Wilcox
2021-09-29 23:41                       ` Song Liu
2021-09-30  0:00                         ` Matthew Wilcox
2021-09-30  0:41                           ` Song Liu
2021-09-30  2:14                             ` Rongwei Wang
2021-10-04 17:26                             ` Rongwei Wang
2021-10-04 19:05                               ` Matthew Wilcox
2021-10-05  1:58                                 ` Rongwei Wang
2021-10-04 20:26                               ` Song Liu
2021-10-05  2:58                               ` Hugh Dickins
2021-10-05  3:07                                 ` Matthew Wilcox
2021-10-05  9:03                                 ` Rongwei Wang
2021-09-30  1:54                         ` Rongwei Wang
2021-09-30  3:26                           ` Song Liu
2021-09-30  5:24                             ` Hugh Dickins
2021-09-30 15:28                               ` Matthew Wilcox
2021-09-30 16:49                                 ` Hugh Dickins
2021-09-30 17:39                                   ` Yang Shi
2021-10-02 17:08                                     ` Matthew Wilcox
2021-10-04 18:28                                       ` Yang Shi
2021-10-04 19:31                                         ` Matthew Wilcox
2021-10-05  2:26                                           ` Hugh Dickins
2021-10-02  2:22                                   ` Rongwei Wang
2021-09-22  7:06 ` [PATCH v2 2/2] mm, thp: bail out early in collapse_file for writeback page Rongwei Wang
2021-10-06  2:18 ` [PATCH v3 v3 0/2] mm, thp: fix file-backed THP race in collapse_file and truncate pagecache Rongwei Wang
2021-10-06  2:18   ` [PATCH v3 v3 1/2] mm, thp: lock filemap when truncating page cache Rongwei Wang
2021-10-06  2:18   ` [PATCH v3 v3 2/2] mm, thp: bail out early in collapse_file for writeback page Rongwei Wang
2021-10-06  2:41     ` Matthew Wilcox
2021-10-06  8:39       ` Rongwei Wang
2021-10-06 17:58     ` Yang Shi
2021-10-11  2:22 ` [PATCH v4 0/2] mm, thp: fix file-backed THP race in collapse_file and truncate pagecache Rongwei Wang
2021-10-11  2:22   ` [PATCH v4 1/2] mm, thp: lock filemap when truncating page cache Rongwei Wang
2021-10-13  7:55     ` Rongwei Wang
2021-10-11  2:22   ` [PATCH v4 2/2] mm, thp: bail out early in collapse_file for writeback page Rongwei Wang
2021-10-11  3:08     ` Matthew Wilcox
2021-10-11  3:22       ` Rongwei Wang
2021-10-11  5:08     ` [PATCH v4 RESEND " Rongwei Wang

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