The patch titled
Subject: dax: improve documentation for fsync/msync
has been added to the -mm tree. Its filename is
dax-add-support-for-fsync-msync-v8-fix-2.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/dax-add-support-for-fsync-msync-v8-fix-2.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/dax-add-support-for-fsync-msync-v8-fix-2.patch
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------------------------------------------------------
From: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
Subject: dax: improve documentation for fsync/msync
Several of the subtleties and assumptions of the DAX fsync/msync
implementation are not immediately obvious, so document them with comments.
Signed-off-by: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
Reported-by: Jan Kara <jack@xxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
fs/dax.c | 30 ++++++++++++++++++++++++++++++
1 file changed, 30 insertions(+)
diff -puN fs/dax.c~dax-add-support-for-fsync-msync-v8-fix-2 fs/dax.c
--- a/fs/dax.c~dax-add-support-for-fsync-msync-v8-fix-2
+++ a/fs/dax.c
@@ -349,6 +349,13 @@ static int dax_radix_entry(struct addres
if (!pmd_entry || type == RADIX_DAX_PMD)
goto dirty;
+
+ /*
+ * We only insert dirty PMD entries into the radix tree. This
+ * means we don't need to worry about removing a dirty PTE
+ * entry and inserting a clean PMD entry, thus reducing the
+ * range we would flush with a follow-up fsync/msync call.
+ */
radix_tree_delete(&mapping->page_tree, index);
mapping->nrexceptional--;
}
@@ -910,6 +917,21 @@ int __dax_pmd_fault(struct vm_area_struc
}
dax_unmap_atomic(bdev, &dax);
+ /*
+ * For PTE faults we insert a radix tree entry for reads, and
+ * leave it clean. Then on the first write we dirty the radix
+ * tree entry via the dax_pnf_mkwrite() path. This sequence
+ * allows the dax_pfn_mkwrite() call to be simpler and avoid a
+ * call into get_block() to translate the pgoff to a sector in
+ * order to be able to create a new radix tree entry.
+ *
+ * The PMD path doesn't have an equivalent to
+ * dax_pfn_mkwrite(), though, so for a read followed by a
+ * write we traverse all the way through __dax_pmd_fault()
+ * twice. This means we can just skip inserting a radix tree
+ * entry completely on the initial read and just wait until
+ * the write to insert a dirty entry.
+ */
if (write) {
error = dax_radix_entry(mapping, pgoff, dax.sector,
true, true);
@@ -983,6 +1005,14 @@ int dax_pfn_mkwrite(struct vm_area_struc
{
struct file *file = vma->vm_file;
+ /*
+ * We pass NO_SECTOR to dax_radix_entry() because we expect that a
+ * RADIX_DAX_PTE entry already exists in the radix tree from a
+ * previous call to __dax_fault(). We just want to look up that PTE
+ * entry using vmf->pgoff and make sure the dirty tag is set. This
+ * saves us from having to make a call to get_block() here to look
+ * up the sector.
+ */
dax_radix_entry(file->f_mapping, vmf->pgoff, NO_SECTOR, false, true);
return VM_FAULT_NOPAGE;
}
_
Patches currently in -mm which might be from ross.zwisler@xxxxxxxxxxxxxxx are
dax-fix-null-pointer-dereference-in-__dax_dbg.patch
dax-fix-conversion-of-holes-to-pmds.patch
pmem-add-wb_cache_pmem-to-the-pmem-api.patch
pmem-add-wb_cache_pmem-to-the-pmem-api-v6.patch
dax-support-dirty-dax-entries-in-radix-tree.patch
dax-support-dirty-dax-entries-in-radix-tree-v6.patch
mm-add-find_get_entries_tag.patch
dax-add-support-for-fsync-sync.patch
dax-add-support-for-fsync-sync-v6.patch
dax-add-support-for-fsync-msync-v7.patch
dax-add-support-for-fsync-msync-v8.patch
dax-add-support-for-fsync-msync-v8-fix.patch
dax-add-support-for-fsync-msync-v8-fix-2.patch
dax-add-support-for-fsync-msync-v8-fix-3.patch
dax-add-support-for-fsync-sync-v8-fix-4.patch
ext2-call-dax_pfn_mkwrite-for-dax-fsync-msync.patch
ext4-call-dax_pfn_mkwrite-for-dax-fsync-msync.patch
xfs-call-dax_pfn_mkwrite-for-dax-fsync-msync.patch
dax-never-rely-on-bhb_dev-being-set-by-get_block.patch
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