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?
