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

[Dan Williams]

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? 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?