On Tue, Dec 20, 2016 at 2:23 PM, Ross Zwisler
<ross.zwisler@xxxxxxxxxxxxxxx> wrote:
> Currently dax_mapping_entry_mkclean() fails to clean and write protect the
> pmd_t of a DAX PMD entry during an *sync operation. This can result in
> data loss in the following sequence:
>
> 1) mmap write to DAX PMD, dirtying PMD radix tree entry and making the
> pmd_t dirty and writeable
> 2) fsync, flushing out PMD data and cleaning the radix tree entry. We
> currently fail to mark the pmd_t as clean and write protected.
> 3) more mmap writes to the PMD. These don't cause any page faults since
> the pmd_t is dirty and writeable. The radix tree entry remains clean.
> 4) fsync, which fails to flush the dirty PMD data because the radix tree
> entry was clean.
> 5) crash - dirty data that should have been fsync'd as part of 4) could
> still have been in the processor cache, and is lost.
>
> Fix this by marking the pmd_t clean and write protected in
> dax_mapping_entry_mkclean(), which is called as part of the fsync
> operation 2). This will cause the writes in step 3) above to generate page
> faults where we'll re-dirty the PMD radix tree entry, resulting in flushes
> in the fsync that happens in step 4).
>
> Signed-off-by: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
> Cc: Jan Kara <jack@xxxxxxx>
> Fixes: 4b4bb46d00b3 ("dax: clear dirty entry tags on cache flush")
> ---
> fs/dax.c | 51 ++++++++++++++++++++++++++++++++++++---------------
> include/linux/mm.h | 2 --
> mm/memory.c | 4 ++--
> 3 files changed, 38 insertions(+), 19 deletions(-)
>
> diff --git a/fs/dax.c b/fs/dax.c
> index 5c74f60..ddcddfe 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -691,8 +691,8 @@ static void dax_mapping_entry_mkclean(struct address_space *mapping,
> pgoff_t index, unsigned long pfn)
> {
> struct vm_area_struct *vma;
> - pte_t *ptep;
> - pte_t pte;
> + pte_t pte, *ptep = NULL;
> + pmd_t *pmdp = NULL;
> spinlock_t *ptl;
> bool changed;
>
> @@ -707,21 +707,42 @@ static void dax_mapping_entry_mkclean(struct address_space *mapping,
>
> address = pgoff_address(index, vma);
> changed = false;
> - if (follow_pte(vma->vm_mm, address, &ptep, &ptl))
> + if (follow_pte_pmd(vma->vm_mm, address, &ptep, &pmdp, &ptl))
> continue;
> - if (pfn != pte_pfn(*ptep))
> - goto unlock;
> - if (!pte_dirty(*ptep) && !pte_write(*ptep))
> - goto unlock;
>
> - flush_cache_page(vma, address, pfn);
> - pte = ptep_clear_flush(vma, address, ptep);
> - pte = pte_wrprotect(pte);
> - pte = pte_mkclean(pte);
> - set_pte_at(vma->vm_mm, address, ptep, pte);
> - changed = true;
> -unlock:
> - pte_unmap_unlock(ptep, ptl);
> + if (pmdp) {
> +#ifdef CONFIG_FS_DAX_PMD
> + pmd_t pmd;
> +
> + if (pfn != pmd_pfn(*pmdp))
> + goto unlock_pmd;
> + if (!pmd_dirty(*pmdp) && !pmd_write(*pmdp))
> + goto unlock_pmd;
> +
> + flush_cache_page(vma, address, pfn);
> + pmd = pmdp_huge_clear_flush(vma, address, pmdp);
> + pmd = pmd_wrprotect(pmd);
> + pmd = pmd_mkclean(pmd);
> + set_pmd_at(vma->vm_mm, address, pmdp, pmd);
> + changed = true;
> +unlock_pmd:
> + spin_unlock(ptl);
> +#endif
Can we please kill this ifdef?
I know we've had problems with ARCH=um builds in the past with
undefined pmd helpers, but to me that simply means we now need to
extend the FS_DAX blacklist to include UML
diff --git a/fs/Kconfig b/fs/Kconfig
index c2a377cdda2b..661931fb0ce0 100644
--- a/fs/Kconfig
+++ b/fs/Kconfig
@@ -37,7 +37,7 @@ source "fs/f2fs/Kconfig"
config FS_DAX
bool "Direct Access (DAX) support"
depends on MMU
- depends on !(ARM || MIPS || SPARC)
+ depends on !(ARM || MIPS || SPARC || UML)
help
Direct Access (DAX) can be used on memory-backed block devices.
If the block device supports DAX and the filesystem supports DAX,
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