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Subject[RFC PATCH 5/8] fs/dax: replace mmap entry in case of CoW
Date
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 | 37 ++++++++++++++++++++++++++++---------
1 file changed, 28 insertions(+), 9 deletions(-)

diff --git a/fs/dax.c b/fs/dax.c
index 13a6a1d3c3b3..12096edb2569 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -835,6 +835,9 @@ 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
@@ -844,14 +847,17 @@ 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))
@@ -863,7 +869,7 @@ 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, xas->xa_index, false);
@@ -887,6 +893,9 @@ 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;
}
@@ -1430,6 +1439,7 @@ 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);
/*
@@ -1522,8 +1532,10 @@ static vm_fault_t dax_iomap_pte_fault(struct vm_fault *vmf, pfn_t *pfnp,
if (error < 0)
goto error_finish_iomap;

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

if (srcmap.type != IOMAP_HOLE) {
error = dax_copy_edges(pos, PAGE_SIZE, &srcmap, kaddr,
@@ -1555,8 +1567,10 @@ static vm_fault_t dax_iomap_pte_fault(struct vm_fault *vmf, pfn_t *pfnp,

goto finish_iomap;
case IOMAP_UNWRITTEN:
- if (srcmap.type != IOMAP_HOLE)
+ if (srcmap.type != IOMAP_HOLE) {
+ insert_flags |= DAX_IF_COW;
goto cow;
+ }
/*FALLTHRU*/
case IOMAP_HOLE:
if (!write) {
@@ -1614,7 +1628,7 @@ 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);
@@ -1666,6 +1680,7 @@ 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
@@ -1753,8 +1768,10 @@ 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 (srcmap.type != IOMAP_HOLE) {
error = dax_copy_edges(pos, PMD_SIZE, &srcmap, kaddr,
@@ -1781,8 +1798,10 @@ 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 (srcmap.type != IOMAP_HOLE)
+ if (srcmap.type != IOMAP_HOLE) {
+ insert_flags |= DAX_IF_COW;
goto cow;
+ }
/*FALLTHRU*/
case IOMAP_HOLE:
if (WARN_ON_ONCE(write))
--
2.26.2


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