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

Ruan Shiyang
2021-02-17T03:07:14Z

On 2021/2/16 下午9:11, David Sterba wrote:
> 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

This is a mistake caused by rebasing my old version patchset. Thanks
for pointing out. I'll fix this in next version.
>
>> + 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.

Yes, I'll fix it.

--
Thanks,
Ruan Shiyang.
>
>> 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
>>
>>