Re: [PATCH v4 5/6] dax: fix missing writeprotect the pte entry

[Muchun Song <songmuchun@bytedance.com>]

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.