Message-Id: <20061010213843.4478ddfc.akpm@osdl.org>
Date:	Tue, 10 Oct 2006 21:38:43 -0700
From:	Andrew Morton <akpm@...l.org>
To:	Nick Piggin <npiggin@...e.de>
Cc:	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 Tue, 10 Oct 2006 16:21:49 +0200 (CEST)
Nick Piggin <npiggin@...e.de> wrote:

> Fix the race between invalidate_inode_pages and do_no_page.
> 
> Andrea Arcangeli identified a subtle race between invalidation of
> pages from pagecache with userspace mappings, and do_no_page.
> 
> The issue is that invalidation has to shoot down all mappings to the
> page, before it can be discarded from the pagecache. Between shooting
> down ptes to a particular page, and actually dropping the struct page
> from the pagecache, do_no_page from any process might fault on that
> page and establish a new mapping to the page just before it gets
> discarded from the pagecache.
> 
> The most common case where such invalidation is used is in file
> truncation. This case was catered for by doing a sort of open-coded
> seqlock between the file's i_size, and its truncate_count.
> 
> Truncation will decrease i_size, then increment truncate_count before
> unmapping userspace pages; do_no_page will read truncate_count, then
> find the page if it is within i_size, and then check truncate_count
> under the page table lock and back out and retry if it had
> subsequently been changed (ptl will serialise against unmapping, and
> ensure a potentially updated truncate_count is actually visible).
> 
> Complexity and documentation issues aside, the locking protocol fails
> in the case where we would like to invalidate pagecache inside i_size.
> do_no_page can come in anytime and filemap_nopage is not aware of the
> invalidation in progress (as it is when it is outside i_size). The
> end result is that dangling (->mapping == NULL) pages that appear to
> be from a particular file may be mapped into userspace with nonsense
> data. Valid mappings to the same place will see a different page.
> 
> Andrea implemented two working fixes, one using a real seqlock,
> another using a page->flags bit. He also proposed using the page lock
> in do_no_page, but that was initially considered too heavyweight.
> However, it is not a global or per-file lock, and the page cacheline
> is modified in do_no_page to increment _count and _mapcount anyway, so
> a further modification should not be a large performance hit.
> Scalability is not an issue.
> 
> This patch implements this latter approach. ->nopage implementations
> return with the page locked if it is possible for their underlying
> file to be invalidated (in that case, they must set a special vm_flags
> bit to indicate so). do_no_page only unlocks the page after setting
> up the mapping completely. invalidation is excluded because it holds
> the page lock during invalidation of each page (and ensures that the
> page is not mapped while holding the lock).
> 
> This allows significant simplifications in do_no_page.
> 

The (unchangelogged) changes to filemap_nopage() appear to have switched
the try-the-read-twice logic into try-it-forever logic.  I think it'll hang
if there's a bad sector?
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