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

Dan Williams
2018-03-21T23:06:58Z

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>
---
fs/dax.c | 56 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 56 insertions(+)

diff --git a/fs/dax.c b/fs/dax.c
index b646a46e4d12..f21a8e7e47f6 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -298,6 +298,56 @@ static void put_unlocked_mapping_entry(struct address_space *mapping,
dax_wake_mapping_entry_waiter(mapping, index, entry, false);
}

+static unsigned long dax_entry_size(void *entry)
+{
+ if (dax_is_zero_entry(entry))
+ return 0;
+ else if (dax_is_empty_entry(entry))
+ return 0;
+ else if (dax_is_pmd_entry(entry))
+ return PMD_SIZE;
+ else
+ return PAGE_SIZE;
+}
+
+#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++)
+
+static void dax_associate_entry(void *entry, struct address_space *mapping)
+{
+ unsigned long pfn, end_pfn;
+
+ if (IS_ENABLED(CONFIG_FS_DAX_LIMITED))
+ return;
+
+ for_each_entry_pfn(entry, pfn, end_pfn) {
+ struct page *page = pfn_to_page(pfn);
+
+ WARN_ON_ONCE(page->mapping);
+ page->mapping = mapping;
+ }
+}
+
+static void dax_disassociate_entry(void *entry, struct address_space *mapping,
+ bool trunc)
+{
+ unsigned long pfn, end_pfn;
+
+ if (IS_ENABLED(CONFIG_FS_DAX_LIMITED))
+ return;
+
+ for_each_entry_pfn(entry, pfn, end_pfn) {
+ struct page *page = pfn_to_page(pfn);
+
+ WARN_ON_ONCE(trunc && page_ref_count(page) > 1);
+ WARN_ON_ONCE(page->mapping && page->mapping != mapping);
+ page->mapping = NULL;
+ }
+}
+
/*
* Find radix tree entry at given index. If it points to an exceptional entry,
* return it with the radix tree entry locked. If the radix tree doesn't
@@ -404,6 +454,7 @@ static void *grab_mapping_entry(struct address_space *mapping, pgoff_t index,
}

if (pmd_downgrade) {
+ dax_disassociate_entry(entry, mapping, false);
radix_tree_delete(&mapping->page_tree, index);
mapping->nrexceptional--;
dax_wake_mapping_entry_waiter(mapping, index, entry,
@@ -453,6 +504,7 @@ static int __dax_invalidate_mapping_entry(struct address_space *mapping,
(radix_tree_tag_get(page_tree, index, PAGECACHE_TAG_DIRTY) ||
radix_tree_tag_get(page_tree, index, PAGECACHE_TAG_TOWRITE)))
goto out;
+ dax_disassociate_entry(entry, mapping, trunc);
radix_tree_delete(page_tree, index);
mapping->nrexceptional--;
ret = 1;
@@ -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);
+ }

if (dax_is_zero_entry(entry) || dax_is_empty_entry(entry)) {
/*