On Tue, 2006-12-19 at 21:58 +1100, Nick Piggin wrote:
> Peter Zijlstra wrote:
> > On Tue, 2006-12-19 at 02:32 -0800, Andrew Morton wrote:
>
> >>Well it used to be. After 2.6.19 it can do the wrong thing for mapped
> >>pages. But it turns out that we don't feed it mapped pages, apart from
> >>pagevec_strip() and possibly races against pagefaults.
> >
> >
> > So how about this:
>
> Well that's still racy. Anyway several earlier patches (including
> the one I posted) closed this race. Some were still reported to
> trigger corruption IIRC.
I can't remember a patch that removes mapped pages from this code path,
however I could have missed it. All out removing the mapping branch in
ttfb() did also fix the problem - which is a superset of page_mapped().
I'm now building a kernel with this patch, and will submit that to
rtorrent with mem=256M on a 1k ext3 filesystem on x86_64 smp preempt.
---
fs/buffer.c | 32 +++++++++++++++++++++++++++++++-
1 file changed, 31 insertions(+), 1 deletion(-)
Index: linux-2.6/fs/buffer.c
===================================================================
--- linux-2.6.orig/fs/buffer.c
+++ linux-2.6/fs/buffer.c
@@ -2798,11 +2798,38 @@ static inline int buffer_busy(struct buf
(bh->b_state & ((1 << BH_Dirty) | (1 << BH_Lock)));
}
+/*
+ * AKPM sayeth:
+ *
+ * - a process does a one-byte-write to a file on a 64k pagesize, 4k
+ * blocksize ext3 filesystem. The page is now PageDirty, !PgeUptodate and
+ * has one dirty buffer and 15 not uptodate buffers.
+ *
+ * - kjournald writes the dirty buffer. The page is now PageDirty,
+ * !PageUptodate and has a mix of clean and not uptodate buffers.
+ *
+ * - try_to_free_buffers() removes the page's buffers. It MUST now clear
+ * PageDirty. If we were to leave the page dirty then we'd have a dirty, not
+ * uptodate page with no buffer_heads.
+ *
+ * We're screwed: we cannot write the page because we don't know which
+ * sections of it contain garbage. We cannot read the page because we don't
+ * know which sections of it contain modified data. We cannot free the page
+ * because it is dirty.
+ *
+ * However for mapped pages this is not true; mapped pages will be fully
+ * loaded and thus cannot have not uptodate buffers.
+ *
+ * Hence allow the PG_dirty bit to stay for pages that had no not uptodate
+ * buffers (and assert that mapped pages never have those).
+ */
+
static int
drop_buffers(struct page *page, struct buffer_head **buffers_to_free)
{
struct buffer_head *head = page_buffers(page);
struct buffer_head *bh;
+ int uptodate = 1;
bh = head;
do {
@@ -2818,11 +2845,14 @@ drop_buffers(struct page *page, struct b
if (!list_empty(&bh->b_assoc_buffers))
__remove_assoc_queue(bh);
+ if (!buffer_uptodate(bh))
+ uptodate = 0;
bh = next;
} while (bh != head);
*buffers_to_free = head;
__clear_page_buffers(page);
- return 1;
+ VM_BUG_ON(page_mapped(page) && !uptodate);
+ return !uptodate;
failed:
return 0;
}
-
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