Re: [PATCH v7 07/14] fs, dax: use page->mapping to warn if truncate collides with a busy page

Jan Kara
2018-03-29T16:02:19Z

On Wed 21-03-18 15:57:48, Dan Williams wrote:
> Catch cases where extent unmap operations encounter pages that are
> pinned / busy. Typically this is pinned pages that are under active dma.
> This warning is a canary for potential data corruption as truncated
> blocks could be allocated to a new file while the device is still
> performing i/o.
>
> Here is an example of a collision that this implementation catches:
>
> WARNING: CPU: 2 PID: 1286 at fs/dax.c:343 dax_disassociate_entry+0x55/0x80
> [..]
> Call Trace:
> __dax_invalidate_mapping_entry+0x6c/0xf0
> dax_delete_mapping_entry+0xf/0x20
> truncate_exceptional_pvec_entries.part.12+0x1af/0x200
> truncate_inode_pages_range+0x268/0x970
> ? tlb_gather_mmu+0x10/0x20
> ? up_write+0x1c/0x40
> ? unmap_mapping_range+0x73/0x140
> xfs_free_file_space+0x1b6/0x5b0 [xfs]
> ? xfs_file_fallocate+0x7f/0x320 [xfs]
> ? down_write_nested+0x40/0x70
> ? xfs_ilock+0x21d/0x2f0 [xfs]
> xfs_file_fallocate+0x162/0x320 [xfs]
> ? rcu_read_lock_sched_held+0x3f/0x70
> ? rcu_sync_lockdep_assert+0x2a/0x50
> ? __sb_start_write+0xd0/0x1b0
> ? vfs_fallocate+0x20c/0x270
> vfs_fallocate+0x154/0x270
> SyS_fallocate+0x43/0x80
> entry_SYSCALL_64_fastpath+0x1f/0x96
>
> Cc: Jeff Moyer <jmoyer@redhat.com>
> Cc: Matthew Wilcox <mawilcox@microsoft.com>
> Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
> Reviewed-by: Jan Kara <jack@suse.cz>
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> Signed-off-by: Dan Williams <dan.j.williams@intel.com>

Two comments when looking at this now:

> +#define for_each_entry_pfn(entry, pfn, end_pfn) \
> + for (pfn = dax_radix_pfn(entry), \
> + end_pfn = pfn + dax_entry_size(entry) / PAGE_SIZE; \
> + pfn < end_pfn; \
> + pfn++)

Why don't you declare 'end_pfn' inside the for() block? That way you don't
have to pass the variable as an argument to for_each_entry_pfn(). It's not
like you need end_pfn anywhere in the loop body, you just use it to cache
loop termination index.

> @@ -547,6 +599,10 @@ static void *dax_insert_mapping_entry(struct address_space *mapping,
>
> spin_lock_irq(&mapping->tree_lock);
> new_entry = dax_radix_locked_entry(pfn, flags);
> + if (dax_entry_size(entry) != dax_entry_size(new_entry)) {
> + dax_disassociate_entry(entry, mapping, false);
> + dax_associate_entry(new_entry, mapping);
> + }

I find it quite tricky that in case we pass zero page / empty entry into
dax_[dis]associate_entry(), it will not do anything because
dax_entry_size() will return 0. Can we add an explicit check into
dax_[dis]associate_entry() or at least a comment there?

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