On Thu, Jul 27, 2017 at 03:12:43PM +0200, Jan Kara wrote:
> Add a flag to iomap interface informing the caller that inode needs
> fdstasync(2) for returned extent to become persistent and use it in DAX
> fault code so that we map such extents only read only. We propagate the
> information that the page table entry has been inserted write-protected
> from dax_iomap_fault() with a new VM_FAULT_RO flag. Filesystem fault
> handler is then responsible for calling fdatasync(2) and updating page
> tables to map pfns read-write. dax_iomap_fault() also takes care of
> updating vmf->orig_pte to match the PTE that was inserted so that we can
> safely recheck that PTE did not change while write-enabling it.
>
> Signed-off-by: Jan Kara <j...@suse.cz>
<>
> @@ -1385,9 +1409,13 @@ static int dax_iomap_pmd_fault(struct vm_fault *vmf,
> bool sync,
> if (iomap.offset + iomap.length < pos + PMD_SIZE)
> goto finish_iomap;
>
> + force_ro = (vmf->flags & FAULT_FLAG_WRITE) && sync &&
> + (iomap.flags & IOMAP_F_NEEDDSYNC);
I already mentioned this in my response to your cover letter, but I think that
we can just lean really heavily on IOMAP_F_NEEDDSYNC and have that let us know
that we are doing sync faults and that the fault is a write. That simplifies
a few things in this patch, with the above just becoming:
force_ro = (iomap.flags & IOMAP_F_NEEDDSYNC);
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index fa036093e76c..5085647d9f2f 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1142,6 +1142,8 @@ static inline void clear_page_pfmemalloc(struct page
> *page)
> #define VM_FAULT_RETRY 0x0400 /* ->fault blocked, must retry */
> #define VM_FAULT_FALLBACK 0x0800 /* huge page fault failed, fall back to
> small */
> #define VM_FAULT_DONE_COW 0x1000 /* ->fault has fully handled COW */
> +#define VM_FAULT_RO 0x2000 /* Write fault was handled just by
> + * inserting RO page table entry for
> DAX */
I wonder if we should name this flag something a little stronger and more
specific to its usage with respect to DAX and sync faults? Maybe
"VM_FAULT_NEEDDSYNC" for consistency with the iomap flag?
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