Re: [PATCH] AFS: Implement shared-writable mmap [try #2]

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David Howells wrote:
Implement shared-writable mmap for AFS.

The key with which to access the file is obtained from the VMA at the point
where the PTE is made writable by the page_mkwrite() VMA op and cached in the
affected page.

If there's an outstanding write on the page made with a different key, then
page_mkwrite() will flush it before attaching a record of the new key.

[try #2] Only flush the page if the page is still part of the mapping (truncate
may have discarded it).

Couple more issues...

Signed-off-by: David Howells <[email protected]>

+/*
+ * notification that a previously read-only page is about to become writable
+ * - if it returns an error, the caller will deliver a bus error signal
+ *
+ * we use this to make a record of the key with which the writeback should be
+ * performed and to flush any outstanding writes made with a different key
+ *
+ * the key to be used is attached to the file pinned by the VMA
+ */
+int afs_page_mkwrite(struct vm_area_struct *vma, struct page *page)
+{
+	struct afs_vnode *vnode = AFS_FS_I(vma->vm_file->f_mapping->host);
+	struct key *key = vma->vm_file->private_data;
+	int ret;
+
+	_enter("{{%x:%u},%x},{%lx}",
+	       vnode->fid.vid, vnode->fid.vnode, key_serial(key), page->index);
+
+	lock_page(page);
+	if (page->mapping == vma->vm_file->f_mapping)
+		ret = afs_prepare_write(vma->vm_file, page, 0, 0);

I would strongly suggest you used (0, PAGE_CACHE_SIZE) for the range, and
have your nopage function DTRT.

Minor issue: you can just check for `if (!page->mapping)` for truncation,
which is the usual signal to tell the reader you're checking for truncate.
Then you can remove the comment...


+	else
+		ret = 0; /* seems truncate interfered - let the caller deal
+			  * with it (presumably the PTE changed too) */

Rather than add this (not always correct) comment about the VM workings, I'd
just add a directive in the page_mkwrite API documentation that the filesystem
is to return 0 if the page has been truncated.


+	unlock_page(page);
+
+	_leave(" = %d", ret);
+	return ret;
+}

--
SUSE Labs, Novell Inc.
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