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SubjectRe: [PATCH v5 15/17] dax: add struct iomap based DAX PMD support
On Fri 07-10-16 15:09:02, Ross Zwisler wrote:
> diff --git a/fs/dax.c b/fs/dax.c
> index ac3cd05..e51d51f 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -281,7 +281,7 @@ static wait_queue_head_t *dax_entry_waitqueue(struct address_space *mapping,
> * queue to the start of that PMD. This ensures that all offsets in
> * the range covered by the PMD map to the same bit lock.
> */
> - if (RADIX_DAX_TYPE(entry) == RADIX_DAX_PMD)
> + if ((unsigned long)entry & RADIX_DAX_PMD)
> index &= ~((1UL << (PMD_SHIFT - PAGE_SHIFT)) - 1);

I agree with Christoph - helper for masking type bits would make this
nicer.

...
> -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 size_flag)
> {
> + 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) {
> + if (size_flag & RADIX_DAX_PMD) {
> + if (!radix_tree_exceptional_entry(entry) ||
> + !((unsigned long)entry & RADIX_DAX_PMD)) {
> + entry = ERR_PTR(-EEXIST);
> + goto out_unlock;

You need to call put_unlocked_mapping_entry() if you use
get_unlocked_mapping_entry() and then decide not to lock it. The reason is
that the waitqueues we use are exclusive (we wake up only a single waiter
waiting for the lock) and so there can be some waiters for the entry lock
although we have not locked the entry ourselves.

> + }
> + } else { /* trying to grab a PTE entry */
> + if (radix_tree_exceptional_entry(entry) &&
> + ((unsigned long)entry & RADIX_DAX_PMD) &&
> + ((unsigned long)entry &
> + (RADIX_DAX_HZP|RADIX_DAX_EMPTY))) {
> + pmd_downgrade = true;
> + }
> + }
> + }
> +
> /* No entry for given index? Make sure radix tree is big enough. */
> - if (!entry) {
> + if (!entry || pmd_downgrade) {
> int err;
>
> + if (pmd_downgrade) {
> + /*
> + * Make sure 'entry' remains valid while we drop
> + * mapping->tree_lock.
> + */
> + entry = lock_slot(mapping, slot);
> + }
> +
> spin_unlock_irq(&mapping->tree_lock);
> err = radix_tree_preload(
> mapping_gfp_mask(mapping) & ~__GFP_HIGHMEM);
> if (err)
> return ERR_PTR(err);

You need to unlock the locked entry before you return here...

> - entry = (void *)(RADIX_TREE_EXCEPTIONAL_ENTRY |
> - RADIX_DAX_ENTRY_LOCK);
> +
> + /*
> + * Besides huge zero pages the only other thing that gets
> + * downgraded are empty entries which don't need to be
> + * unmapped.
> + */
> + if (pmd_downgrade && ((unsigned long)entry & RADIX_DAX_HZP))
> + unmap_mapping_range(mapping,
> + (index << PAGE_SHIFT) & PMD_MASK, PMD_SIZE, 0);
> +
> spin_lock_irq(&mapping->tree_lock);
> - err = radix_tree_insert(&mapping->page_tree, index, entry);
> +
> + if (pmd_downgrade) {
> + radix_tree_delete(&mapping->page_tree, index);
> + mapping->nrexceptional--;
> + dax_wake_mapping_entry_waiter(mapping, index, entry,
> + false);

You need to set 'wake_all' argument here to true. Otherwise there could be
waiters waiting for non-existent entry forever...

> + }
> +
> + entry = dax_radix_entry(0, size_flag | RADIX_DAX_EMPTY);
> +
> + err = __radix_tree_insert(&mapping->page_tree, index,
> + dax_radix_order(entry), entry);
> radix_tree_preload_end();
> if (err) {
> spin_unlock_irq(&mapping->tree_lock);
> - /* Someone already created the entry? */
> - if (err == -EEXIST)
> + /*
> + * Someone already created the entry? This is a
> + * normal failure when inserting PMDs in a range
> + * that already contains PTEs. In that case we want
> + * to return -EEXIST immediately.
> + */
> + if (err == -EEXIST && !(size_flag & RADIX_DAX_PMD))
> goto restart;

Add a comment here that we can get here only if there was no radix tree
entry at 'index' and thus there can be no waiters to wake.

> return ERR_PTR(err);
> }
> @@ -441,6 +509,7 @@ restart:
> return page;
> }
> entry = lock_slot(mapping, slot);
> + out_unlock:
> spin_unlock_irq(&mapping->tree_lock);
> return entry;
> }
> @@ -581,11 +650,17 @@ static int copy_user_dax(struct block_device *bdev, sector_t sector, size_t size
> return 0;
> }
>
> -#define DAX_PMD_INDEX(page_index) (page_index & (PMD_MASK >> PAGE_SHIFT))
> -
> +/*
> + * 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
> + * PTEs. If we happen to be trying to insert a PTE and there is a PMD
> + * already in the tree, we will skip the insertion and just dirty the PMD as
> + * appropriate.
> + */
> 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 flags)
> {
> struct radix_tree_root *page_tree = &mapping->page_tree;
> int error = 0;
> @@ -608,22 +683,28 @@ static void *dax_insert_mapping_entry(struct address_space *mapping,
> error = radix_tree_preload(vmf->gfp_mask & ~__GFP_HIGHMEM);
> if (error)
> return ERR_PTR(error);
> + } else if (((unsigned long)entry & RADIX_DAX_HZP) &&
> + !(flags & RADIX_DAX_HZP)) {
> + /* replacing huge zero page with PMD block mapping */
> + unmap_mapping_range(mapping,
> + (vmf->pgoff << PAGE_SHIFT) & PMD_MASK, PMD_SIZE, 0);
> }
>
> spin_lock_irq(&mapping->tree_lock);
> - new_entry = (void *)((unsigned long)RADIX_DAX_ENTRY(sector, false) |
> - RADIX_DAX_ENTRY_LOCK);
> + new_entry = dax_radix_entry(sector, flags);
> +

You've lost the RADIX_DAX_ENTRY_LOCK flag here?

> if (hole_fill) {
> __delete_from_page_cache(entry, NULL);
> /* Drop pagecache reference */
> put_page(entry);
> - error = radix_tree_insert(page_tree, index, new_entry);
> + error = __radix_tree_insert(page_tree, index,
> + dax_radix_order(new_entry), new_entry);
> if (error) {
> new_entry = ERR_PTR(error);
> goto unlock;
> }
> mapping->nrexceptional++;
> - } else {
> + } else if ((unsigned long)entry & (RADIX_DAX_HZP|RADIX_DAX_EMPTY)) {
> void **slot;
> void *ret;

Uh, why this condition need to change? Is it some protection so that we
don't replace a mapped PMD entry with PTE one?

<snip>

> @@ -1261,4 +1338,186 @@ int dax_iomap_fault(struct vm_area_struct *vma, struct vm_fault *vmf,
> return VM_FAULT_NOPAGE | major;
> }
> EXPORT_SYMBOL_GPL(dax_iomap_fault);
> +
> +#ifdef CONFIG_FS_DAX_PMD
> +/*
> + * The 'colour' (ie low bits) within a PMD of a page offset. This comes up
> + * more often than one might expect in the below functions.
> + */
> +#define PG_PMD_COLOUR ((PMD_SIZE >> PAGE_SHIFT) - 1)

Just out of curiosity: Why the british spelling of 'colour'?

> +
> +static int dax_pmd_insert_mapping(struct vm_area_struct *vma, pmd_t *pmd,
> + struct vm_fault *vmf, unsigned long address,
> + struct iomap *iomap, loff_t pos, bool write, void **entryp)
> +{
> + struct address_space *mapping = vma->vm_file->f_mapping;
> + struct block_device *bdev = iomap->bdev;
> + struct blk_dax_ctl dax = {
> + .sector = dax_iomap_sector(iomap, pos),
> + .size = PMD_SIZE,
> + };
> + long length = dax_map_atomic(bdev, &dax);
> + void *ret;
> +
> + if (length < 0) /* dax_map_atomic() failed */
> + return VM_FAULT_FALLBACK;
> + if (length < PMD_SIZE)
> + goto unmap_fallback;
> + if (pfn_t_to_pfn(dax.pfn) & PG_PMD_COLOUR)
> + goto unmap_fallback;
> + if (!pfn_t_devmap(dax.pfn))
> + goto unmap_fallback;
> +
> + dax_unmap_atomic(bdev, &dax);
> +
> + ret = dax_insert_mapping_entry(mapping, vmf, *entryp, dax.sector,
> + RADIX_DAX_PMD);
> + if (IS_ERR(ret))
> + return VM_FAULT_FALLBACK;
> + *entryp = ret;
> +
> + return vmf_insert_pfn_pmd(vma, address, pmd, dax.pfn, write);
> +
> + unmap_fallback:
> + dax_unmap_atomic(bdev, &dax);
> + return VM_FAULT_FALLBACK;
> +}
> +
> +static int dax_pmd_load_hole(struct vm_area_struct *vma, pmd_t *pmd,
> + struct vm_fault *vmf, unsigned long address,
> + struct iomap *iomap, void **entryp)
> +{
> + struct address_space *mapping = vma->vm_file->f_mapping;
> + unsigned long pmd_addr = address & PMD_MASK;
> + struct page *zero_page;
> + spinlock_t *ptl;
> + pmd_t pmd_entry;
> + void *ret;
> +
> + zero_page = get_huge_zero_page();
> +
> + if (unlikely(!zero_page))
> + return VM_FAULT_FALLBACK;
> +
> + ret = dax_insert_mapping_entry(mapping, vmf, *entryp, 0,
> + RADIX_DAX_PMD | RADIX_DAX_HZP);
> + if (IS_ERR(ret))
> + return VM_FAULT_FALLBACK;
> + *entryp = ret;
> +
> + ptl = pmd_lock(vma->vm_mm, pmd);
> + if (!pmd_none(*pmd)) {
> + spin_unlock(ptl);
> + return VM_FAULT_FALLBACK;
> + }
> +
> + pmd_entry = mk_pmd(zero_page, vma->vm_page_prot);
> + pmd_entry = pmd_mkhuge(pmd_entry);
> + set_pmd_at(vma->vm_mm, pmd_addr, pmd, pmd_entry);
> + spin_unlock(ptl);
> + return VM_FAULT_NOPAGE;
> +}
> +
> +int dax_iomap_pmd_fault(struct vm_area_struct *vma, unsigned long address,
> + pmd_t *pmd, unsigned int flags, struct iomap_ops *ops)
> +{
> + struct address_space *mapping = vma->vm_file->f_mapping;
> + unsigned long pmd_addr = address & PMD_MASK;
> + bool write = flags & FAULT_FLAG_WRITE;
> + unsigned int iomap_flags = write ? IOMAP_WRITE : 0;
> + struct inode *inode = mapping->host;
> + int result = VM_FAULT_FALLBACK;
> + struct iomap iomap = { 0 };

Why the 0 here? Just empty braces are enough to initialize the structure to
zeros.

> + pgoff_t size, pgoff;
> + struct vm_fault vmf;
> + void *entry;
> + loff_t pos;
> + int error;
> +
> + /* Fall back to PTEs if we're going to COW */
> + if (write && !(vma->vm_flags & VM_SHARED)) {
> + split_huge_pmd(vma, pmd, address);
> + goto fallback;
> + }
> +
> + /* If the PMD would extend outside the VMA */
> + if (pmd_addr < vma->vm_start)
> + goto fallback;
> + if ((pmd_addr + PMD_SIZE) > vma->vm_end)
> + goto fallback;
> +
> + /*
> + * Check whether offset isn't beyond end of file now. Caller is
> + * supposed to hold locks serializing us with truncate / punch hole so
> + * this is a reliable test.
> + */
> + pgoff = linear_page_index(vma, pmd_addr);
> + size = (i_size_read(inode) + PAGE_SIZE - 1) >> PAGE_SHIFT;

Nitpick - 'size' does not express that this is in pages and rounded up.
Maybe we could have:

max_pgoff = (i_size_read(inode) - 1) >> PAGE_SHIFT;

and then use strict inequalities below?


Honza
--
Jan Kara <jack@suse.com>
SUSE Labs, CR

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