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

[Dan Williams]

On Fri, Mar 11, 2022 at 1:06 AM Muchun Song <songmuchun@xxxxxxxxxxxxx> wrote:
>
> On Thu, Mar 10, 2022 at 8:59 AM Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
> >
> > On Wed, Mar 2, 2022 at 12:30 AM Muchun Song <songmuchun@xxxxxxxxxxxxx> 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?

CONFIG_FS_DAX_LIMITED was created to preserve some DAX use for S390
which does not have CONFIG_ARCH_HAS_PTE_DEVMAP. Without PTE_DEVMAP
then get_user_pages() for DAX mappings fails.

To your question, no, there are no pages at all in the
CONFIG_FS_DAX_LIMITED=y case. So page_mkclean_one() could only be
deployed for PMD mappings, but I think it is reasonable to just
disable PMD mappings for the CONFIG_FS_DAX_LIMITED=y case.

Going forward the hope is to remove the ARCH_HAS_PTE_DEVMAP
requirement for DAX, and use PTE_SPECIAL for the S390 case. However,
that still wants to have 'struct page' availability as an across the
board requirement.

> 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?

Perhaps I missed a 'struct page' dependency? I thought the bug you are
fixing only triggers in the presence of PMDs. The
CONFIG_FS_DAX_LIMITED=y case can still use the current "page-less"
mkclean path for PTEs.