Received-SPF: softfail (google.com: domain of transitioning linux-kernel-owner@vger.kernel.org does not designate 23.128.96.19 as permitted sender) client-ip=23.128.96.19;
MIME-Version: 1.0
References: <20220302082718.32268-1-songmuchun@bytedance.com>
 <20220302082718.32268-6-songmuchun@bytedance.com> <CAPcyv4hsMWe1AreVVhGJD-St3FGtGBMeA-BX7XbA_kVX97tw4Q@mail.gmail.com>
In-Reply-To: <CAPcyv4hsMWe1AreVVhGJD-St3FGtGBMeA-BX7XbA_kVX97tw4Q@mail.gmail.com>
From:   Muchun Song <songmuchun@bytedance.com>
Date:   Fri, 11 Mar 2022 17:04:06 +0800
Message-ID: <CAMZfGtUmhcryboPdRC7ZhWVuV3TX0rLcKUxhvamAGbHUoATaow@mail.gmail.com>
Subject: Re: [PATCH v4 5/6] dax: fix missing writeprotect the pte entry
To:     Dan Williams <dan.j.williams@intel.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>,
        Xiyu Yang <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>,
        Xiongchun duan <duanxiongchun@bytedance.com>,
        Muchun Song <smuchun@gmail.com>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Thu, Mar 10, 2022 at 8:59 AM Dan Williams <dan.j.williams@intel.com> wrote:
>
> 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?

I don't know the history of CONFIG_FS_DAX_LIMITED.
page_mkclean_one() need a struct page associated with
the pfn,  do the struct pages exist when CONFIG_FS_DAX_LIMITED
and ! FS_DAX_PMD? If yes, I think you are right. But I don't
see this guarantee. I am not familiar with DAX code, so what am
I missing here?

Thanks.