linux-kernel - Re: [patch 2/5] mm: fault vs invalidate/truncate race fix

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Date:	Wed, 11 Oct 2006 10:38:31 -0700 (PDT)
From:	Linus Torvalds <torvalds@...l.org>
To:	Nick Piggin <npiggin@...e.de>
cc:	Andrew Morton <akpm@...l.org>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Linux Memory Management <linux-mm@...ck.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [patch 2/5] mm: fault vs invalidate/truncate race fix



On Wed, 11 Oct 2006, Nick Piggin wrote:
> 
> I mean filemap_nopage does *two* synchronous reads when finding a !uptodate
> page. This is despite the comment saying that it retries once on error.

Ahh. 

Yes, now that you point to the actual code, that does look ugly.

I think it's related to the

	ClearPageError(page);

thing, and probably related to that function being rather old and having 
gone through several re-organizations. I suspect we used to fall through 
to the error handling code regardless of whether we did the read ourselves 
etc.

Are you saying that something like this would be preferable?

		Linus

---
diff --git a/mm/filemap.c b/mm/filemap.c
index 3464b68..e5ecf42 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1496,6 +1496,8 @@ page_not_uptodate:
 		goto success;
 	}
 
+	/* Clear any potential old errors, and try to read.. */
+	ClearPageError(page);
 	error = mapping->a_ops->readpage(file, page);
 	if (!error) {
 		wait_on_page_locked(page);
@@ -1526,21 +1528,12 @@ page_not_uptodate:
 		unlock_page(page);
 		goto success;
 	}
-	ClearPageError(page);
-	error = mapping->a_ops->readpage(file, page);
-	if (!error) {
-		wait_on_page_locked(page);
-		if (PageUptodate(page))
-			goto success;
-	} else if (error == AOP_TRUNCATED_PAGE) {
-		page_cache_release(page);
-		goto retry_find;
-	}
 
 	/*
 	 * Things didn't work out. Return zero to tell the
 	 * mm layer so, possibly freeing the page cache page first.
 	 */
+	unlock_page(page);
 	shrink_readahead_size_eio(file, ra);
 	page_cache_release(page);
 	return NOPAGE_SIGBUS;
-
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