On Thu 21-01-16 10:46:04, Ross Zwisler wrote:
> When the user reads from a DAX hole via a mmap we service page faults using
> zero-filled page cache pages. These zero pages are also placed into the
> address_space radix tree. When we get our first write for that space, we
> can allocate a PMD page worth of DAX storage to replace that hole.
>
> When this happens we need to unmap the zero pages and remove them from the
> radix tree. Prior to this patch we were unmapping *all* storage in our
> PMD's range, which is incorrect because it removed DAX entries as well on
> non-allocating page faults.
>
> Instead, keep track of when get_block() actually gives us storage so that
> we can be sure to only remove zero pages that were covering holes.
>
> Signed-off-by: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
> Reported-by: Jan Kara <jack@xxxxxxx>
So the get_block() calls below are still racy so your allocation detection
doesn't quite work reliably anyway. For that we need a change in the fault
locking we were talking about and at that point we'll need to somewhat
update __dax_pmd_fault() anyway. So I'm not sure if this intermediate
change really makes sense... I'd rather first change the locking in the
filesystems and then fixup remaining issues with __dax_pmd_fault().
Honza
> ---
> fs/dax.c | 32 ++++++++++++++++++++++----------
> 1 file changed, 22 insertions(+), 10 deletions(-)
>
> diff --git a/fs/dax.c b/fs/dax.c
> index afacc30..263aed1 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -790,9 +790,9 @@ 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 error, result = 0;
> + bool alloc = false;
>
> /* dax pmd mappings require pfn_t_devmap() */
> if (!IS_ENABLED(CONFIG_FS_DAX_PMD))
> @@ -830,10 +830,17 @@ int __dax_pmd_fault(struct vm_area_struct *vma, unsigned long address,
> block = (sector_t)pgoff << (PAGE_SHIFT - blkbits);
>
> bh.b_size = PMD_SIZE;
> - if (get_block(inode, block, &bh, write) != 0)
> +
> + if (get_block(inode, block, &bh, 0) != 0)
> return VM_FAULT_SIGBUS;
> +
> + if (!buffer_mapped(&bh) && write) {
> + if (get_block(inode, block, &bh, 1) != 0)
> + return VM_FAULT_SIGBUS;
> + alloc = true;
> + }
> +
> bdev = bh.b_bdev;
> - i_mmap_lock_read(mapping);
>
> /*
> * If the filesystem isn't willing to tell us the length of a hole,
> @@ -842,15 +849,20 @@ int __dax_pmd_fault(struct vm_area_struct *vma, unsigned long address,
> */
> if (!buffer_size_valid(&bh) || bh.b_size < PMD_SIZE) {
> dax_pmd_dbg(&bh, address, "allocated block too small");
> - goto fallback;
> + return VM_FAULT_FALLBACK;
> + }
> +
> + /*
> + * If we allocated new storage, make sure no process has any
> + * zero pages covering this hole
> + */
> + if (alloc) {
> + loff_t lstart = pgoff << PAGE_SHIFT;
> + loff_t lend = lstart + PMD_SIZE - 1; /* inclusive */
> +
> + truncate_pagecache_range(inode, lstart, lend);
> }
>
> - /* 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);
> - unmap_mapping_range(mapping, lstart, PMD_SIZE, 0);
> - truncate_inode_pages_range(mapping, lstart, lend);
> i_mmap_lock_read(mapping);
>
> /*
> --
> 2.5.0
>
>
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
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