On Tue, Jan 12, 2016 at 10:44:51AM +0100, Jan Kara wrote:
> On Thu 07-01-16 22:27:52, Ross Zwisler wrote:
> > When we get a DAX PMD fault for a write it is possible that there could be
> > some number of 4k zero pages already present for the same range that were
> > inserted to service reads from a hole. These 4k zero pages need to be
> > unmapped from the VMAs and removed from the struct address_space radix tree
> > before the real DAX PMD entry can be inserted.
> >
> > For PTE faults this same use case also exists and is handled by a
> > combination of unmap_mapping_range() to unmap the VMAs and
> > delete_from_page_cache() to remove the page from the address_space radix
> > tree.
> >
> > For PMD faults we do have a call to unmap_mapping_range() (protected by a
> > buffer_new() check), but nothing clears out the radix tree entry. The
> > buffer_new() check is also incorrect as the current ext4 and XFS filesystem
> > code will never return a buffer_head with BH_New set, even when allocating
> > new blocks over a hole. Instead the filesystem will zero the blocks
> > manually and return a buffer_head with only BH_Mapped set.
> >
> > Fix this situation by removing the buffer_new() check and adding a call to
> > truncate_inode_pages_range() to clear out the radix tree entries before we
> > insert the DAX PMD.
> >
> > Signed-off-by: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
> > Reported-by: Dan Williams <dan.j.williams@xxxxxxxxx>
> > Tested-by: Dan Williams <dan.j.williams@xxxxxxxxx>
>
> Just two nits below. Nothing serious so you can add:
>
> Reviewed-by: Jan Kara <jack@xxxxxxx>
Cool, thank you for the review!
> > ---
> > fs/dax.c | 20 ++++++++++----------
> > 1 file changed, 10 insertions(+), 10 deletions(-)
> >
> > diff --git a/fs/dax.c b/fs/dax.c
> > index 513bba5..5b84a46 100644
> > --- a/fs/dax.c
> > +++ b/fs/dax.c
> > @@ -589,6 +589,7 @@ int __dax_pmd_fault(struct vm_area_struct *vma, unsigned long address,
> > bool write = flags & FAULT_FLAG_WRITE;
> > struct block_device *bdev;
> > pgoff_t size, pgoff;
> > + loff_t lstart, lend;
> > sector_t block;
> > int result = 0;
> >
> > @@ -643,15 +644,13 @@ int __dax_pmd_fault(struct vm_area_struct *vma, unsigned long address,
> > goto fallback;
> > }
> >
> > - /*
> > - * If we allocated new storage, make sure no process has any
> > - * zero pages covering this hole
> > - */
> > - if (buffer_new(&bh)) {
> > - i_mmap_unlock_read(mapping);
> > - unmap_mapping_range(mapping, pgoff << PAGE_SHIFT, PMD_SIZE, 0);
> > - i_mmap_lock_read(mapping);
> > - }
> > + /* make sure no process has any zero pages covering this hole */
> > + lstart = pgoff << PAGE_SHIFT;
> > + lend = lstart + PMD_SIZE - 1; /* inclusive */
> > + i_mmap_unlock_read(mapping);
>
> Just a nit but is there reason why we grab i_mmap_lock_read(mapping) only
> to release it a few lines below? The bh checks inside the locked region
> don't seem to rely on i_mmap_lock...
I think we can probably just take it when we're done with the truncate() -
I'll fix for v9.
> > + unmap_mapping_range(mapping, lstart, PMD_SIZE, 0);
> > + truncate_inode_pages_range(mapping, lstart, lend);
>
> These two calls can be shortened as:
>
> truncate_pagecache_range(inode, lstart, lend);
Nice. I'll change it for v9.
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