Re: [PATCH 4/7] fsdax: Replace mmap entry in case of CoW

David Sterba
2021-02-16T13:14:00Z

On Mon, Feb 08, 2021 at 01:09:21AM +0800, Shiyang Ruan wrote:
> We replace the existing entry to the newly allocated one
> in case of CoW. Also, we mark the entry as PAGECACHE_TAG_TOWRITE
> so writeback marks this entry as writeprotected. This
> helps us snapshots so new write pagefaults after snapshots
> trigger a CoW.
>
> Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
> Signed-off-by: Shiyang Ruan <ruansy.fnst@cn.fujitsu.com>
> ---
> fs/dax.c | 31 +++++++++++++++++++++++--------
> 1 file changed, 23 insertions(+), 8 deletions(-)
>
> diff --git a/fs/dax.c b/fs/dax.c
> index b2195cbdf2dc..29698a3d2e37 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -722,6 +722,9 @@ static int copy_cow_page_dax(struct block_device *bdev, struct dax_device *dax_d
> return 0;
> }
>
> +#define DAX_IF_DIRTY (1ULL << 0)
> +#define DAX_IF_COW (1ULL << 1)

The constants are ULL, but I see other flags only 'unsigned long'

> +
> /*
> * By this point grab_mapping_entry() has ensured that we have a locked entry
> * of the appropriate size so we don't have to worry about downgrading PMDs to
> @@ -731,14 +734,16 @@ static int copy_cow_page_dax(struct block_device *bdev, struct dax_device *dax_d
> */
> static void *dax_insert_entry(struct xa_state *xas,
> struct address_space *mapping, struct vm_fault *vmf,
> - void *entry, pfn_t pfn, unsigned long flags, bool dirty)
> + void *entry, pfn_t pfn, unsigned long flags, bool insert_flags)

insert_flags is bool

> {
> void *new_entry = dax_make_entry(pfn, flags);
> + bool dirty = insert_flags & DAX_IF_DIRTY;

"insert_flags & DAX_IF_DIRTY" is "bool & ULL", this can't be right

> + bool cow = insert_flags & DAX_IF_COW;

Same

>
> if (dirty)
> __mark_inode_dirty(mapping->host, I_DIRTY_PAGES);
>
> - if (dax_is_zero_entry(entry) && !(flags & DAX_ZERO_PAGE)) {
> + if (cow || (dax_is_zero_entry(entry) && !(flags & DAX_ZERO_PAGE))) {
> unsigned long index = xas->xa_index;
> /* we are replacing a zero page with block mapping */
> if (dax_is_pmd_entry(entry))
> @@ -750,7 +755,7 @@ static void *dax_insert_entry(struct xa_state *xas,
>
> xas_reset(xas);
> xas_lock_irq(xas);
> - if (dax_is_zero_entry(entry) || dax_is_empty_entry(entry)) {
> + if (cow || dax_is_zero_entry(entry) || dax_is_empty_entry(entry)) {
> void *old;
>
> dax_disassociate_entry(entry, mapping, false);
> @@ -774,6 +779,9 @@ static void *dax_insert_entry(struct xa_state *xas,
> if (dirty)
> xas_set_mark(xas, PAGECACHE_TAG_DIRTY);
>
> + if (cow)
> + xas_set_mark(xas, PAGECACHE_TAG_TOWRITE);
> +
> xas_unlock_irq(xas);
> return entry;
> }
> @@ -1319,6 +1327,7 @@ static vm_fault_t dax_iomap_pte_fault(struct vm_fault *vmf, pfn_t *pfnp,
> void *entry;
> pfn_t pfn;
> void *kaddr;
> + unsigned long insert_flags = 0;
>
> trace_dax_pte_fault(inode, vmf, ret);
> /*
> @@ -1444,8 +1453,10 @@ static vm_fault_t dax_iomap_pte_fault(struct vm_fault *vmf, pfn_t *pfnp,
>
> goto finish_iomap;
> case IOMAP_UNWRITTEN:
> - if (write && iomap.flags & IOMAP_F_SHARED)
> + if (write && (iomap.flags & IOMAP_F_SHARED)) {
> + insert_flags |= DAX_IF_COW;

Here's an example of 'unsigned long = unsigned long long', though it'll
work, it would be better to unify all the types.

> goto cow;
> + }
> fallthrough;
> case IOMAP_HOLE:
> if (!write) {
> @@ -1555,6 +1566,7 @@ static vm_fault_t dax_iomap_pmd_fault(struct vm_fault *vmf, pfn_t *pfnp,
> int error;
> pfn_t pfn;
> void *kaddr;
> + unsigned long insert_flags = 0;
>
> /*
> * Check whether offset isn't beyond end of file now. Caller is
> @@ -1670,14 +1682,17 @@ static vm_fault_t dax_iomap_pmd_fault(struct vm_fault *vmf, pfn_t *pfnp,
> result = vmf_insert_pfn_pmd(vmf, pfn, write);
> break;
> case IOMAP_UNWRITTEN:
> - if (write && iomap.flags & IOMAP_F_SHARED)
> + if (write && (iomap.flags & IOMAP_F_SHARED)) {
> + insert_flags |= DAX_IF_COW;
> goto cow;
> + }
> fallthrough;
> case IOMAP_HOLE:
> - if (WARN_ON_ONCE(write))
> + if (!write) {
> + result = dax_pmd_load_hole(&xas, vmf, &iomap, &entry);
> break;
> - result = dax_pmd_load_hole(&xas, vmf, &iomap, &entry);
> - break;
> + }
> + fallthrough;
> default:
> WARN_ON_ONCE(1);
> break;
> --
> 2.30.0