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

ruansy.fnst
2021-04-01T07:04:35Z

> -----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
> >
> >
> >