On Mon, 2007-03-05 at 13:43 -0800, Linus Torvalds wrote:
>
> On Mon, 5 Mar 2007, Dave Kleikamp wrote:
> >
> > This fixes a regression caused by 22c8ca78f20724676b6006232bf06cc3e9299539.
> >
> > nobh_prepare_write() no longer marks the page uptodate, so
> > nobh_truncate_page() needs to do it.
>
> I'm not convinced...
I should have described the problem better. Assuming Nick's patch
(22c8ca78f20724676b6006232bf06cc3e9299539) makes sense by moving the
SetPageUptodate() from nobh_prepare_write() to nobh_commit_write(), it
follows that nobh_truncate_page(), which doesn't call
nobh_commit_write() needs to call SetPageUpdate().
> If the page wasn't up-to-date from before, it's *not* necessarily
> up-to-date after the truncate either! So why do we have that at all?
> The same comment is true of "nobh_commit_write()" (which _does_ have the
> SetPageUptodate() there).
> So I have three questions:
>
> - why is that valid in the first place (the page is *not* guaranteed to
> be up-to-date as far as I can see!)
->prepare_write() guarantees that the parts of the page before offset
(and beyond to) are up-to-date, and nobh_truncate_page() zeros the rest
of the page.
>
> - why is it valid to do in "nobh_commit_write()"
->commit_write() is only called after bringing the page up-to-date.
> - why doesn't "nobh_truncate_page()"
> (a) call nobh_prepare_write() through an indirect pointer?
nobh_prepare_write() needs the filesystem's get_block() function, so
a_ops->prepare_write() will call nobh_prepare_write(...,
fs_specific_get_block)
> (b) call nobh_commit_write() at all? (Yeah, I realize it's because
> of brokenness with i_size, so this is more of a "those
> functions should be factored out properly" statement rather
> than a question.
It would be more consistent to call ->commit_write(). I assume it does
what it does as an optimization. The fact that it doesn't call
nobh_commit_write() can be blamed for this bug, since it wasn't clear
that a change to nobh_commit_write() had to be mirrored in
nobh_truncate_page().
> IOW, I'm sure your patch _fixes_ something, but no, it's certainly not
> obvious to me. A few added comments would be good.
I've added some comments to the patch below
> Why is it ok to do
> this on a page that wasn't up-to-date before (since obviously, if it *was*
> up-to-date, it's pointless).
Before Nick's patch, nobh_prepare_write() incorrectly marked the page
up-to-date. (It was just a little bit too early.) His patch moved
SetPageUptodate() to a better place, nobh_commit_write(), but it missed
that nobh_truncate_page() cheats by not calling nobh_commit_write().
fs: nobh_truncate_page() must set page up to date
This fixes a regression caused by 22c8ca78f20724676b6006232bf06cc3e9299539.
nobh_prepare_write() no longer marks the page uptodate, so
nobh_truncate_page() needs to do it.
Signed-off-by: Dave Kleikamp <[EMAIL PROTECTED]>
diff -Nurp linux-orig/fs/buffer.c linux/fs/buffer.c
--- linux-orig/fs/buffer.c 2007-02-22 07:59:01.000000000 -0600
+++ linux/fs/buffer.c 2007-03-05 16:56:24.000000000 -0600
@@ -2365,6 +2365,10 @@ failed:
}
EXPORT_SYMBOL(nobh_prepare_write);
+/*
+ * Make sure any changes to nobh_commit_write() are reflected in
+ * nobh_truncate_page(), since it doesn't call commit_write().
+ */
int nobh_commit_write(struct file *file, struct page *page,
unsigned from, unsigned to)
{
@@ -2466,6 +2470,11 @@ int nobh_truncate_page(struct address_sp
memset(kaddr + offset, 0, PAGE_CACHE_SIZE - offset);
flush_dcache_page(page);
kunmap_atomic(kaddr, KM_USER0);
+ /*
+ * It would be more correct to call aops->commit_write()
+ * here, but this is more efficient.
+ */
+ SetPageUptodate(page);
set_page_dirty(page);
}
unlock_page(page);
--
David Kleikamp
IBM Linux Technology Center
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