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References: <20220302082718.32268-1-songmuchun@bytedance.com> <20220302082718.32268-6-songmuchun@bytedance.com>
In-Reply-To: <20220302082718.32268-6-songmuchun@bytedance.com>
From:   Dan Williams <dan.j.williams@intel.com>
Date:   Wed, 9 Mar 2022 16:59:02 -0800
Message-ID: <CAPcyv4hsMWe1AreVVhGJD-St3FGtGBMeA-BX7XbA_kVX97tw4Q@mail.gmail.com>
Subject: Re: [PATCH v4 5/6] dax: fix missing writeprotect the pte entry
To:     Muchun Song <songmuchun@bytedance.com>
Cc:     Matthew Wilcox <willy@infradead.org>, Jan Kara <jack@suse.cz>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Andrew Morton <akpm@linux-foundation.org>,
        Alistair Popple <apopple@nvidia.com>,
        Yang Shi <shy828301@gmail.com>,
        Ralph Campbell <rcampbell@nvidia.com>,
        Hugh Dickins <hughd@google.com>, xiyuyang19@fudan.edu.cn,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Ross Zwisler <zwisler@kernel.org>,
        Christoph Hellwig <hch@infradead.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Linux NVDIMM <nvdimm@lists.linux.dev>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux MM <linux-mm@kvack.org>, duanxiongchun@bytedance.com,
        Muchun Song <smuchun@gmail.com>
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On Wed, Mar 2, 2022 at 12:30 AM Muchun Song <songmuchun@bytedance.com> wrote:
>
> Currently dax_mapping_entry_mkclean() fails to clean and write protect
> the pte entry within a DAX PMD entry during an *sync operation. This
> can result in data loss in the following sequence:
>
>   1) process A mmap write to DAX PMD, dirtying PMD radix tree entry and
>      making the pmd entry dirty and writeable.
>   2) process B mmap with the @offset (e.g. 4K) and @length (e.g. 4K)
>      write to the same file, dirtying PMD radix tree entry (already
>      done in 1)) and making the pte entry dirty and writeable.
>   3) fsync, flushing out PMD data and cleaning the radix tree entry. We
>      currently fail to mark the pte entry as clean and write protected
>      since the vma of process B is not covered in dax_entry_mkclean().
>   4) process B writes to the pte. These don't cause any page faults since
>      the pte entry is dirty and writeable. The radix tree entry remains
>      clean.
>   5) fsync, which fails to flush the dirty PMD data because the radix tree
>      entry was clean.
>   6) crash - dirty data that should have been fsync'd as part of 5) could
>      still have been in the processor cache, and is lost.

Excellent description.

>
> Just to use pfn_mkclean_range() to clean the pfns to fix this issue.

So the original motivation for CONFIG_FS_DAX_LIMITED was for archs
that do not have spare PTE bits to indicate pmd_devmap(). So this fix
can only work in the CONFIG_FS_DAX_LIMITED=n case and in that case it
seems you can use the current page_mkclean_one(), right? So perhaps
the fix is to skip patch 3, keep patch 4 and make this patch use
page_mkclean_one() along with this:

diff --git a/fs/Kconfig b/fs/Kconfig
index 7a2b11c0b803..42108adb7a78 100644
--- a/fs/Kconfig
+++ b/fs/Kconfig
@@ -83,6 +83,7 @@ config FS_DAX_PMD
        depends on FS_DAX
        depends on ZONE_DEVICE
        depends on TRANSPARENT_HUGEPAGE
+       depends on !FS_DAX_LIMITED

 # Selected by DAX drivers that do not expect filesystem DAX to support
 # get_user_pages() of DAX mappings. I.e. "limited" indicates no support

...to preclude the pmd conflict in that case?