[RFC PATCH 3/7] dax: replace mmap entry in case of CoW

Shiyang Ruan
2019-07-31T11:49:59Z

From: Goldwyn Rodrigues <rgoldwyn@suse.com>

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.

btrfs does not support hugepages so we don't handle PMD.

Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
---
fs/dax.c | 36 ++++++++++++++++++++++++++++--------
1 file changed, 28 insertions(+), 8 deletions(-)

diff --git a/fs/dax.c b/fs/dax.c
index 084cc21d47a4..8eb065a1ec51 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -700,6 +700,9 @@ static int copy_user_dax(struct block_device *bdev, struct dax_device *dax_dev,
return 0;
}

+#define DAX_IF_DIRTY (1ULL << 0)
+#define DAX_IF_COW (1ULL << 1)
+
/*
* 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
@@ -709,14 +712,17 @@ static int copy_user_dax(struct block_device *bdev, struct dax_device *dax_dev,
*/
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,
+ unsigned int insert_flags)
{
void *new_entry = dax_make_entry(pfn, flags);
+ bool dirty = insert_flags & DAX_IF_DIRTY;
+ bool cow = insert_flags & DAX_IF_COW;

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))
@@ -728,12 +734,12 @@ static void *dax_insert_entry(struct xa_state *xas,

xas_reset(xas);
xas_lock_irq(xas);
- if (dax_entry_size(entry) != dax_entry_size(new_entry)) {
+ if (cow || (dax_entry_size(entry) != dax_entry_size(new_entry))) {
dax_disassociate_entry(entry, mapping, false);
dax_associate_entry(new_entry, mapping, vmf->vma, vmf->address);
}

- if (dax_is_zero_entry(entry) || dax_is_empty_entry(entry)) {
+ if (cow || dax_is_zero_entry(entry) || dax_is_empty_entry(entry)) {
/*
* Only swap our new entry into the page cache if the current
* entry is a zero page or an empty entry. If a normal PTE or
@@ -753,6 +759,9 @@ 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;
}
@@ -1040,7 +1049,7 @@ static vm_fault_t dax_load_hole(struct xa_state *xas,
vm_fault_t ret;

*entry = dax_insert_entry(xas, mapping, vmf, *entry, pfn,
- DAX_ZERO_PAGE, false);
+ DAX_ZERO_PAGE, 0);

ret = vmf_insert_mixed(vmf->vma, vaddr, pfn);
trace_dax_load_hole(inode, vmf, ret);
@@ -1310,6 +1319,7 @@ 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);
/*
@@ -1371,6 +1381,8 @@ static vm_fault_t dax_iomap_pte_fault(struct vm_fault *vmf, pfn_t *pfnp,
error = copy_user_dax(iomap.bdev, iomap.dax_dev,
sector, PAGE_SIZE, vmf->cow_page, vaddr);
break;
+ case IOMAP_COW:
+ /* Should not be setting this - fallthrough */
default:
WARN_ON_ONCE(1);
error = -EIO;
@@ -1391,6 +1403,8 @@ static vm_fault_t dax_iomap_pte_fault(struct vm_fault *vmf, pfn_t *pfnp,

switch (iomap.type) {
case IOMAP_COW:
+ insert_flags |= DAX_IF_COW;
+ /* fallthrough */
case IOMAP_MAPPED:
if (iomap.flags & IOMAP_F_NEW) {
count_vm_event(PGMAJFAULT);
@@ -1401,8 +1415,10 @@ static vm_fault_t dax_iomap_pte_fault(struct vm_fault *vmf, pfn_t *pfnp,
if (error < 0)
goto error_finish_iomap;

+ if (write && !sync)
+ insert_flags |= DAX_IF_DIRTY;
entry = dax_insert_entry(&xas, mapping, vmf, entry, pfn,
- 0, write && !sync);
+ 0, insert_flags);

if (iomap.type == IOMAP_COW) {
error = dax_copy_edges(inode, pos, PAGE_SIZE, &srcmap,
@@ -1490,7 +1506,7 @@ static vm_fault_t dax_pmd_load_hole(struct xa_state *xas, struct vm_fault *vmf,

pfn = page_to_pfn_t(zero_page);
*entry = dax_insert_entry(xas, mapping, vmf, *entry, pfn,
- DAX_PMD | DAX_ZERO_PAGE, false);
+ DAX_PMD | DAX_ZERO_PAGE, 0);

if (arch_needs_pgtable_deposit()) {
pgtable = pte_alloc_one(vma->vm_mm);
@@ -1542,6 +1558,7 @@ 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
@@ -1627,8 +1644,11 @@ static vm_fault_t dax_iomap_pmd_fault(struct vm_fault *vmf, pfn_t *pfnp,
if (error < 0)
goto finish_iomap;

+ if (write && !sync)
+ insert_flags |= DAX_IF_DIRTY;
+
entry = dax_insert_entry(&xas, mapping, vmf, entry, pfn,
- DAX_PMD, write && !sync);
+ DAX_PMD, insert_flags);

if (iomap.type == IOMAP_COW) {
error = dax_copy_edges(inode, pos, PMD_SIZE, &srcmap,
--
2.17.0