linux-kernel - Re: [PATCH v4 05/11] mm: do not split a folio if it has minimum folio order requirement

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Date: Sat, 27 Apr 2024 17:57:17 -0700
From: Luis Chamberlain <mcgrof@...nel.org>
To: Matthew Wilcox <willy@...radead.org>, ziy@...dia.com
Cc: "Pankaj Raghav (Samsung)" <kernel@...kajraghav.com>, djwong@...nel.org,
	brauner@...nel.org, david@...morbit.com, chandan.babu@...cle.com,
	akpm@...ux-foundation.org, linux-fsdevel@...r.kernel.org,
	hare@...e.de, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linux-xfs@...r.kernel.org, gost.dev@...sung.com,
	p.raghav@...sung.com
Subject: Re: [PATCH v4 05/11] mm: do not split a folio if it has minimum
 folio order requirement

On Fri, Apr 26, 2024 at 04:46:11PM -0700, Luis Chamberlain wrote:
> On Thu, Apr 25, 2024 at 05:47:28PM -0700, Luis Chamberlain wrote:
> > On Thu, Apr 25, 2024 at 09:10:16PM +0100, Matthew Wilcox wrote:
> > > On Thu, Apr 25, 2024 at 01:37:40PM +0200, Pankaj Raghav (Samsung) wrote:
> > > > From: Pankaj Raghav <p.raghav@...sung.com>
> > > > 
> > > > using that API for LBS is resulting in an NULL ptr dereference
> > > > error in the writeback path [1].
> > > >
> > > > [1] https://gist.github.com/mcgrof/d12f586ec6ebe32b2472b5d634c397df
> > > 
> > >  How would I go about reproducing this?

Well so the below fixes this but I am not sure if this is correct.
folio_mark_dirty() at least says that a folio should not be truncated
while its running. I am not sure if we should try to split folios then
even though we check for writeback once. truncate_inode_partial_folio()
will folio_wait_writeback() but it will split_folio() before checking
for claiming to fail to truncate with folio_test_dirty(). But since the
folio is locked its not clear why this should be possible.

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 83955362d41c..90195506211a 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3058,7 +3058,7 @@ int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
 	if (new_order >= folio_order(folio))
 		return -EINVAL;
 
-	if (folio_test_writeback(folio))
+	if (folio_test_dirty(folio) || folio_test_writeback(folio))
 		return -EBUSY;
 
 	if (!folio_test_anon(folio)) {