Re: [PATCH v4 10/12] dax: add struct iomap based DAX PMD support

Ross Zwisler
2016-10-03T21:16:48Z

On Fri, Sep 30, 2016 at 02:56:27AM -0700, Christoph Hellwig wrote:
> > -/*
> > - * We use lowest available bit in exceptional entry for locking, other two
> > - * bits to determine entry type. In total 3 special bits.
> > - */
> > -#define RADIX_DAX_SHIFT (RADIX_TREE_EXCEPTIONAL_SHIFT + 3)
> > -#define RADIX_DAX_PTE (1 << (RADIX_TREE_EXCEPTIONAL_SHIFT + 1))
> > -#define RADIX_DAX_PMD (1 << (RADIX_TREE_EXCEPTIONAL_SHIFT + 2))
> > -#define RADIX_DAX_TYPE_MASK (RADIX_DAX_PTE | RADIX_DAX_PMD)
> > -#define RADIX_DAX_TYPE(entry) ((unsigned long)entry & RADIX_DAX_TYPE_MASK)
> > -#define RADIX_DAX_SECTOR(entry) (((unsigned long)entry >> RADIX_DAX_SHIFT))
> > -#define RADIX_DAX_ENTRY(sector, pmd) ((void *)((unsigned long)sector << \
> > - RADIX_DAX_SHIFT | (pmd ? RADIX_DAX_PMD : RADIX_DAX_PTE) | \
> > - RADIX_TREE_EXCEPTIONAL_ENTRY))
> > -
>
> Please split the move of these constants into a separate patch.

Will do for v5.

> > -static void *grab_mapping_entry(struct address_space *mapping, pgoff_t index)
> > +static void *grab_mapping_entry(struct address_space *mapping, pgoff_t index,
> > + unsigned long new_type)
> > {
> > + bool pmd_downgrade = false; /* splitting 2MiB entry into 4k entries? */
> > void *entry, **slot;
> >
> > restart:
> > spin_lock_irq(&mapping->tree_lock);
> > entry = get_unlocked_mapping_entry(mapping, index, &slot);
> > +
> > + if (entry && new_type == RADIX_DAX_PMD) {
> > + if (!radix_tree_exceptional_entry(entry) ||
> > + RADIX_DAX_TYPE(entry) == RADIX_DAX_PTE) {
> > + spin_unlock_irq(&mapping->tree_lock);
> > + return ERR_PTR(-EEXIST);
> > + }
> > + } else if (entry && new_type == RADIX_DAX_PTE) {
> > + if (radix_tree_exceptional_entry(entry) &&
> > + RADIX_DAX_TYPE(entry) == RADIX_DAX_PMD &&
> > + (unsigned long)entry & (RADIX_DAX_HZP|RADIX_DAX_EMPTY)) {
> > + pmd_downgrade = true;
> > + }
> > + }
>
> Would be nice to use switch on the type here:
>
> old_type = RADIX_DAX_TYPE(entry);
>
> if (entry) {
> switch (new_type) {
> case RADIX_DAX_PMD:
> if (!radix_tree_exceptional_entry(entry) ||
> oldentry == RADIX_DAX_PTE) {
> entry = ERR_PTR(-EEXIST);
> goto out_unlock;
> }
> break;
> case RADIX_DAX_PTE:
> if (radix_tree_exceptional_entry(entry) &&
> old_entry = RADIX_DAX_PMD &&
> (unsigned long)entry &
> (RADIX_DAX_HZP|RADIX_DAX_EMPTY))
> ..

Will do. This definitely makes things less dense and more readable.

> Btw, why are only RADIX_DAX_PTE and RADIX_DAX_PMD in the type mask,
> and not RADIX_DAX_HZP and RADIX_DAX_EMPTY? With that we could use the
> above old_entry local variable over this function and make it a lot les
> of a mess.

So essentially in this revision of the series 'type' just means PMD or PTE
(perhaps this should have been 'size' or something), and the Zero Page / Empty
Entry / Regular DAX choice was something separate. Jan also commented on
this, and I agree there's room for improvement. Let me try and rework it a
bit for v5.

> > static void *dax_insert_mapping_entry(struct address_space *mapping,
> > struct vm_fault *vmf,
> > - void *entry, sector_t sector)
> > + void *entry, sector_t sector,
> > + unsigned long new_type, bool hzp)
>
> And then we could also drop the hzp argument here..

Yep, that would be good.

> > #ifdef CONFIG_FS_IOMAP
> > +static inline sector_t dax_iomap_sector(struct iomap *iomap, loff_t pos)
> > +{
> > + return iomap->blkno + (((pos & PAGE_MASK) - iomap->offset) >> 9);
> > +}
>
> Please split adding this new helper into a separate patch.

Done for v5.

> > +#if defined(CONFIG_FS_DAX_PMD)
>
> Please use #ifdef here.

Will do.

> > +#define RADIX_DAX_TYPE(entry) ((unsigned long)entry & RADIX_DAX_TYPE_MASK)
> > +#define RADIX_DAX_SECTOR(entry) (((unsigned long)entry >> RADIX_DAX_SHIFT))
> > +
> > +/* entries begin locked */
> > +#define RADIX_DAX_ENTRY(sector, type) ((void *)(RADIX_TREE_EXCEPTIONAL_ENTRY |\
> > + type | (unsigned long)sector << RADIX_DAX_SHIFT | RADIX_DAX_ENTRY_LOCK))
> > +#define RADIX_DAX_HZP_ENTRY() ((void *)(RADIX_TREE_EXCEPTIONAL_ENTRY | \
> > + RADIX_DAX_PMD | RADIX_DAX_HZP | RADIX_DAX_EMPTY | RADIX_DAX_ENTRY_LOCK))
> > +#define RADIX_DAX_EMPTY_ENTRY(type) ((void *)(RADIX_TREE_EXCEPTIONAL_ENTRY | \
> > + type | RADIX_DAX_EMPTY | RADIX_DAX_ENTRY_LOCK))
> > +
> > +#define RADIX_DAX_ORDER(type) (type == RADIX_DAX_PMD ? PMD_SHIFT-PAGE_SHIFT : 0)
>
> All these macros don't properly brace their arguments. I think
> you'd make your life a lot easier by making them inline functions.

Oh, great point. Will do, although depending on how the 'type' thing works
out we may end up with just a dax_radix_entry(sector, type) inline function,
where 'type' can be (RADIX_DAX_PMD | RADIX_DAX_EMPTY), etc. That would
prevent the need for a bunch of different ways of making a new entry.

> > +#if defined(CONFIG_FS_DAX_PMD)
>
> #ifdef, please

Will do.

Thank you for the review, Christoph.