RE: [PATCH v3 05/10] fsdax: Replace mmap entry in case of CoW

["ruansy.fnst@fujitsu.com" <ruansy.fnst@fujitsu.com>]

> -----Original Message-----
> From: Ritesh Harjani <ritesh.list@gmail.com>
> Sent: Thursday, April 1, 2021 2:40 PM
> Subject: Re: [PATCH v3 05/10] fsdax: Replace mmap entry in case of CoW
> 
> On 21/03/19 09:52AM, 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.
> >
> 
> Please correct me here. So the flow is like this.
> 1. In case of CoW or a reflinked file, on an mmaped file if write is attempted,
>    Then in DAX fault handler code, ->iomap_begin() on a given filesystem will
>    populate iomap and srcmap. srcmap being from where the read needs to be
>    attempted from and iomap on where the new write should go to.
> 2. So the dax_insert_entry() code as part of the fault handling will take care
>    of removing the old entry and inserting the new pfn entry to xas and mark
>    it with PAGECACHE_TAG_TOWRITE so that dax writeback can mark the
> entry as
>    write protected.
> Is my above understanding correct?

Yes, you are right.

> 
> > Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
> > Signed-off-by: Shiyang Ruan <ruansy.fnst@fujitsu.com>
> > Reviewed-by: Christoph Hellwig <hch@lst.de>
> > ---
> >  fs/dax.c | 37 ++++++++++++++++++++++++++-----------
> >  1 file changed, 26 insertions(+), 11 deletions(-)
> >
> > diff --git a/fs/dax.c b/fs/dax.c
> > index 181aad97136a..cfe513eb111e 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		(1 << 0)
> > +#define DAX_IF_COW		(1 << 1)
> > +
> >
> small comment expalining this means DAX insert flags used in dax_insert_entry()

OK.  I'll add it.
> 
> >
> >  /*
> >   * 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 @@ -729,16 +732,19 @@ static int
> copy_cow_page_dax(struct block_device *bdev, struct dax_device *dax_d
> >   * already in the tree, we will skip the insertion and just dirty the PMD as
> >   * appropriate.
> >   */
> > -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)
> > +static void *dax_insert_entry(struct xa_state *xas, struct vm_fault *vmf,
> > +		void *entry, pfn_t pfn, unsigned long flags,
> > +		unsigned int insert_flags)
> >  {
> > +	struct address_space *mapping = vmf->vma->vm_file->f_mapping;
> >  	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))
> > @@ -750,7 +756,7 @@ 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 +780,9
> @@
> > 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;
> >  }
> > @@ -1098,8 +1107,7 @@ static vm_fault_t dax_load_hole(struct xa_state
> *xas,
> >  	pfn_t pfn = pfn_to_pfn_t(my_zero_pfn(vaddr));
> >  	vm_fault_t ret;
> >
> > -	*entry = dax_insert_entry(xas, mapping, vmf, *entry, pfn,
> > -			DAX_ZERO_PAGE, false);
> > +	*entry = dax_insert_entry(xas, vmf, *entry, pfn, DAX_ZERO_PAGE, 0);
> >
> >  	ret = vmf_insert_mixed(vmf->vma, vaddr, pfn);
> >  	trace_dax_load_hole(inode, vmf, ret); @@ -1126,8 +1134,8 @@ static
> > vm_fault_t dax_pmd_load_hole(struct xa_state *xas, struct vm_fault *vmf,
> >  		goto fallback;
> >
> >  	pfn = page_to_pfn_t(zero_page);
> > -	*entry = dax_insert_entry(xas, mapping, vmf, *entry, pfn,
> > -			DAX_PMD | DAX_ZERO_PAGE, false);
> > +	*entry = dax_insert_entry(xas, vmf, *entry, pfn,
> > +				  DAX_PMD | DAX_ZERO_PAGE, 0);
> >
> >  	if (arch_needs_pgtable_deposit()) {
> >  		pgtable = pte_alloc_one(vma->vm_mm); @@ -1431,6 +1439,7 @@
> static
> > vm_fault_t dax_fault_actor(struct vm_fault *vmf, pfn_t *pfnp,
> >  	loff_t pos = (loff_t)xas->xa_offset << PAGE_SHIFT;
> >  	bool write = vmf->flags & FAULT_FLAG_WRITE;
> >  	bool sync = dax_fault_is_synchronous(flags, vmf->vma, iomap);
> > +	unsigned int insert_flags = 0;
> >  	int err = 0;
> >  	pfn_t pfn;
> >  	void *kaddr;
> > @@ -1453,8 +1462,14 @@ static vm_fault_t dax_fault_actor(struct vm_fault
> *vmf, pfn_t *pfnp,
> >  	if (err)
> >  		return dax_fault_return(err);
> >
> > -	entry = dax_insert_entry(xas, mapping, vmf, entry, pfn, 0,
> > -				 write && !sync);
> > +	if (write) {
> > +		if (!sync)
> > +			insert_flags |= DAX_IF_DIRTY;
> > +		if (iomap->flags & IOMAP_F_SHARED)
> > +			insert_flags |= DAX_IF_COW;
> > +	}
> > +
> > +	entry = dax_insert_entry(xas, vmf, entry, pfn, 0, insert_flags);
> >
> >  	if (write && srcmap->addr != iomap->addr) {
> >  		err = dax_iomap_cow_copy(pos, size, size, srcmap, kaddr, false);
> >
> 
> Rest looks good to me. Please feel free to add
> Reviewed-by: Ritesh Harjani <riteshh@gmail.com>
> 
> sorry about changing my email in between of this code review.
> I am planning to use above gmail id as primary account for all upstream work
> from now.
> 

--
Thanks,
Ruan Shiyang.

> > --
> > 2.30.1
> >
> >
> >
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