Re: [PATCH v2 08/11] dax: add support for fsync/sync

Ross Zwisler
2015-11-17T18:30:21Z

On Tue, Nov 17, 2015 at 09:58:07AM +1100, Dave Chinner wrote:
> On Fri, Nov 13, 2015 at 05:06:47PM -0700, Ross Zwisler wrote:
> > To properly handle fsync/msync in an efficient way DAX needs to track dirty
> > pages so it is able to flush them durably to media on demand.
> >
> > The tracking of dirty pages is done via the radix tree in struct
> > address_space. This radix tree is already used by the page writeback
> > infrastructure for tracking dirty pages associated with an open file, and
> > it already has support for exceptional (non struct page*) entries. We
> > build upon these features to add exceptional entries to the radix tree for
> > DAX dirty PMD or PTE pages at fault time.
> >
> > When called as part of the msync/fsync flush path DAX queries the radix
> > tree for dirty entries, flushing them and then marking the PTE or PMD page
> > table entries as clean. The step of cleaning the PTE or PMD entries is
> > necessary so that on subsequent writes to the same page we get a new write
> > fault allowing us to once again dirty the DAX tag in the radix tree.
> >
> > Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
> > ---
> > fs/dax.c | 140 +++++++++++++++++++++++++++++++++++++++++++++++++---
> > include/linux/dax.h | 1 +
> > mm/huge_memory.c | 14 +++---
> > 3 files changed, 141 insertions(+), 14 deletions(-)
> >
> > diff --git a/fs/dax.c b/fs/dax.c
> > index 131fd35a..9ce6d1b 100644
> > --- a/fs/dax.c
> > +++ b/fs/dax.c
> > @@ -24,7 +24,9 @@
> > #include <linux/memcontrol.h>
> > #include <linux/mm.h>
> > #include <linux/mutex.h>
> > +#include <linux/pagevec.h>
> > #include <linux/pmem.h>
> > +#include <linux/rmap.h>
> > #include <linux/sched.h>
> > #include <linux/uio.h>
> > #include <linux/vmstat.h>
> > @@ -287,6 +289,53 @@ static int copy_user_bh(struct page *to, struct buffer_head *bh,
> > return 0;
> > }
> >
> > +static int dax_dirty_pgoff(struct address_space *mapping, unsigned long pgoff,
> > + void __pmem *addr, bool pmd_entry)
> > +{
> > + struct radix_tree_root *page_tree = &mapping->page_tree;
> > + int error = 0;
> > + void *entry;
> > +
> > + __mark_inode_dirty(mapping->host, I_DIRTY_PAGES);
> > +
> > + spin_lock_irq(&mapping->tree_lock);
> > + entry = radix_tree_lookup(page_tree, pgoff);
> > + if (addr == NULL) {
> > + if (entry)
> > + goto dirty;
> > + else {
> > + WARN(1, "DAX pfn_mkwrite failed to find an entry");
> > + goto out;
> > + }
> > + }
> > +
> > + if (entry) {
> > + if (pmd_entry && RADIX_DAX_TYPE(entry) == RADIX_DAX_PTE) {
> > + radix_tree_delete(&mapping->page_tree, pgoff);
> > + mapping->nrdax--;
> > + } else
> > + goto dirty;
> > + }
>
> Logic is pretty spagettied here. Perhaps:
>
> entry = radix_tree_lookup(page_tree, pgoff);
> if (entry) {
> if (!pmd_entry || RADIX_DAX_TYPE(entry) == RADIX_DAX_PMD))
> goto dirty;
> radix_tree_delete(&mapping->page_tree, pgoff);
> mapping->nrdax--;
> } else {
> WARN_ON(!addr);
> goto out_unlock;
> }
> ....

I don't think that this works because now if !entry we unconditionally goto
out_unlock without inserting a new entry. I'll try and simplify the logic and
add some comments.

> > +
> > + BUG_ON(RADIX_DAX_TYPE(addr));
> > + if (pmd_entry)
> > + error = radix_tree_insert(page_tree, pgoff,
> > + RADIX_DAX_PMD_ENTRY(addr));
> > + else
> > + error = radix_tree_insert(page_tree, pgoff,
> > + RADIX_DAX_PTE_ENTRY(addr));
> > +
> > + if (error)
> > + goto out;
> > +
> > + mapping->nrdax++;
> > + dirty:
> > + radix_tree_tag_set(page_tree, pgoff, PAGECACHE_TAG_DIRTY);
> > + out:
> > + spin_unlock_irq(&mapping->tree_lock);
>
> label should be "out_unlock" rather "out" to indicate in the code
> that we are jumping to the correct spot in the error stack...

Sure, will do.

> > + goto fallback;
> > }
> >
> > out:
> > @@ -689,15 +746,12 @@ EXPORT_SYMBOL_GPL(dax_pmd_fault);
> > * dax_pfn_mkwrite - handle first write to DAX page
> > * @vma: The virtual memory area where the fault occurred
> > * @vmf: The description of the fault
> > - *
> > */
> > int dax_pfn_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
> > {
> > - struct super_block *sb = file_inode(vma->vm_file)->i_sb;
> > + struct file *file = vma->vm_file;
> >
> > - sb_start_pagefault(sb);
> > - file_update_time(vma->vm_file);
> > - sb_end_pagefault(sb);
> > + dax_dirty_pgoff(file->f_mapping, vmf->pgoff, NULL, false);
> > return VM_FAULT_NOPAGE;
>
> This seems wrong - it's dropping the freeze protection on fault, and
> now the inode timestamp won't get updated, either.

Oh, that all still happens in the filesystem pfn_mkwrite code
(xfs_filemap_pfn_mkwrite() for XFS). It needs to happen there, I think,
because we wanted to order it so that the filesystem freeze happens outside of
the XFS_MMAPLOCK_SHARED locking, as it does with the regular PMD and PTE fault
paths.

Prior to this patch set dax_pfn_mkwrite() was completely unused an was ready
to be removed as dead code - it's now being used by all filesystems just to
make sure we re-add the newly dirtied page to the radix tree dirty list.

> > }
> > EXPORT_SYMBOL_GPL(dax_pfn_mkwrite);
> > @@ -772,3 +826,77 @@ int dax_truncate_page(struct inode *inode, loff_t from, get_block_t get_block)
> > return dax_zero_page_range(inode, from, length, get_block);
> > }
> > EXPORT_SYMBOL_GPL(dax_truncate_page);
> > +
> > +static void dax_sync_entry(struct address_space *mapping, pgoff_t pgoff,
> > + void *entry)
> > +{
>
> dax_writeback_pgoff() seems like a more consistent name (consider
> dax_dirty_pgoff), and that we are actually doing a writeback
> operation, not a "sync" operation.

Sure, I'm fine with that change.

> > + struct radix_tree_root *page_tree = &mapping->page_tree;
> > + int type = RADIX_DAX_TYPE(entry);
> > + size_t size;
> > +
> > + BUG_ON(type != RADIX_DAX_PTE && type != RADIX_DAX_PMD);
> > +
> > + spin_lock_irq(&mapping->tree_lock);
> > + if (!radix_tree_tag_get(page_tree, pgoff, PAGECACHE_TAG_TOWRITE)) {
> > + /* another fsync thread already wrote back this entry */
> > + spin_unlock_irq(&mapping->tree_lock);
> > + return;
> > + }
> > + radix_tree_tag_clear(page_tree, pgoff, PAGECACHE_TAG_TOWRITE);
> > + radix_tree_tag_clear(page_tree, pgoff, PAGECACHE_TAG_DIRTY);
> > + spin_unlock_irq(&mapping->tree_lock);
> > +
> > + if (type == RADIX_DAX_PMD)
> > + size = PMD_SIZE;
> > + else
> > + size = PAGE_SIZE;
> > +
> > + wb_cache_pmem(RADIX_DAX_ADDR(entry), size);
> > + pgoff_mkclean(pgoff, mapping);
>
> This looks racy w.r.t. another operation setting the radix tree
> dirty tags. i.e. there is no locking to serialise marking the
> vma/pte clean and another operation marking the radix tree dirty.

I think you're right - I'll look into how to protect us from this race. Thank
you for catching this.

> > +}
> > +
> > +/*
> > + * Flush the mapping to the persistent domain within the byte range of (start,
> > + * end). This is required by data integrity operations to ensure file data is on
> > + * persistent storage prior to completion of the operation. It also requires us
> > + * to clean the mappings (i.e. write -> RO) so that we'll get a new fault when
> > + * the file is written to again so we have an indication that we need to flush
> > + * the mapping if a data integrity operation takes place.
> > + *
> > + * We don't need commits to storage here - the filesystems will issue flushes
> > + * appropriately at the conclusion of the data integrity operation via REQ_FUA
> > + * writes or blkdev_issue_flush() commands. This requires the DAX block device
> > + * to implement persistent storage domain fencing/commits on receiving a
> > + * REQ_FLUSH or REQ_FUA request so that this works as expected by the higher
> > + * layers.
> > + */
> > +void dax_fsync(struct address_space *mapping, loff_t start, loff_t end)
> > +{
>
> dax_writeback_mapping_range()

Sure, I'm fine with that change.