Re: [syzbot] [mm?] possible deadlock in shmem_uncharge (2)

[Hugh Dickins]

On Tue, 18 Jul 2023, Carlos Maiolino wrote:
> On Tue, Jul 18, 2023 at 11:22:59AM -0700, Hugh Dickins wrote:
> > On Tue, 18 Jul 2023, syzbot wrote:
> > 
> > > Hello,
> > > 
> > > syzbot found the following issue on:
> > 
> > Yes, this doesn't require any syzbot trickery, it showed up as soon as
> > I tried a shmem quota linux-next with lockdep and shmem huge last week.
> > 
> > There's some other things wrong with the accounting there (in the non-
> > quota case anyway): I have been working up a patch to fix them, but need
> > to consider what must go in quickly, and what should wait until later.
> > 
> > Carlos, in brief: don't worry about this syzbot report, I'm on it (but
> > there's a risk that any patch I send may turn out to break your quotas).
> 
> Ok, thanks Hugh, I have the next version ready to go when I saw those syzkaller
> reports.
> 
> I will send the new series tomorrow morning then.
> Could you please keep me in the loop, I'm interested to see what's going on
> there.

I never saw a new series on Wednesday, just a new series on Monday which
I had ignored, knowing that another was coming.  I was waiting for your
new series before sending my patch to that, but perhaps you were waiting
for my patch before sending your new series?

Then when I went to check my patch on next-20230721, found that your
series has been dropped from mm-unstable meanwhile.  Maybe Andrew
dropped it because of the lockdep reports (I see now there was another),
or maybe he dropped it because of new series coming too thick and fast,
or maybe he dropped it because of the awkward merge commit which Stephen
Rothwell had to write, to reconcile the vfs and mm trees.  (But I was
surprised not to have seen any mm-commits mail on the dropping.)

So the patch below is against next-20230714, and will hopefully still
apply in your next posting: please include it there (since it modifies
THP source, I think it's best kept as a separate patch in your series).

But when you repost, if you are still going for the next release, the
more you can do to minimize that merge commit the better.  In particular,
the reindentations of shmem_get_inode() and other functions in
"shmem: make shmem_get_inode() return ERR_PTR instead of NULL"
don't help review, and could well be left until some later cleanup -
but I don't know how much that would actually eliminate the clashes.

(And "Add default quota limit mount options" needs to say "shmem: ...";
I'd have preferred "tmpfs: ...", but the others say shmem, so okay.)

[PATCH] shmem: fix quota lock nesting in huge hole handling

i_pages lock nests inside i_lock, but shmem_charge() and shmem_uncharge()
were being called from THP splitting or collapsing while i_pages lock was
held, and now go on to call dquot_alloc_block_nodirty() which takes
i_lock to update i_blocks.

We may well want to take i_lock out of this path later, in the non-quota
case even if it's left in the quota case (or perhaps use i_lock instead
of shmem's info->lock throughout); but don't get into that at this time.

Move the shmem_charge() and shmem_uncharge() calls out from under i_pages
lock, accounting the full batch of holes in a single call.

Still pass the pages argument to shmem_uncharge(), but it happens now to
be unused: shmem_recalc_inode() is designed to account for clean pages
freed behind shmem's back, so it gets the accounting right by itself;
then the later call to shmem_inode_unacct_blocks() led to imbalance
(that WARN_ON(inode->i_blocks) in shmem_evict_inode()).

Reported-by: syzbot+38ca19393fb3344f57e6@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://lore.kernel.org/lkml/0000000000008e62f40600bfe080@xxxxxxxxxx/
Reported-by: syzbot+440ff8cca06ee7a1d4db@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://lore.kernel.org/lkml/00000000000076a7840600bfb6e8@xxxxxxxxxx/
Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
---
 mm/huge_memory.c |  6 ++++--
 mm/khugepaged.c  | 13 +++++++------
 mm/shmem.c       | 19 +++++++++----------
 3 files changed, 20 insertions(+), 18 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 762be2f4244c..01f6838329a1 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2521,7 +2521,7 @@ static void __split_huge_page(struct page *page, struct list_head *list,
 	struct address_space *swap_cache = NULL;
 	unsigned long offset = 0;
 	unsigned int nr = thp_nr_pages(head);
-	int i;
+	int i, nr_dropped = 0;
 
 	/* complete memcg works before add pages to LRU */
 	split_page_memcg(head, nr);
@@ -2546,7 +2546,7 @@ static void __split_huge_page(struct page *page, struct list_head *list,
 			struct folio *tail = page_folio(head + i);
 
 			if (shmem_mapping(head->mapping))
-				shmem_uncharge(head->mapping->host, 1);
+				nr_dropped++;
 			else if (folio_test_clear_dirty(tail))
 				folio_account_cleaned(tail,
 					inode_to_wb(folio->mapping->host));
@@ -2583,6 +2583,8 @@ static void __split_huge_page(struct page *page, struct list_head *list,
 	}
 	local_irq_enable();
 
+	if (nr_dropped)
+		shmem_uncharge(head->mapping->host, nr_dropped);
 	remap_page(folio, nr);
 
 	if (PageSwapCache(head)) {
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 6bad69c0e4bd..366ee467fb83 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1828,10 +1828,6 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
 						goto xa_locked;
 					}
 				}
-				if (!shmem_charge(mapping->host, 1)) {
-					result = SCAN_FAIL;
-					goto xa_locked;
-				}
 				nr_none++;
 				continue;
 			}
@@ -2017,8 +2013,13 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
 	 */
 	try_to_unmap_flush();
 
-	if (result != SCAN_SUCCEED)
+	if (result == SCAN_SUCCEED && nr_none &&
+	    !shmem_charge(mapping->host, nr_none))
+		result = SCAN_FAIL;
+	if (result != SCAN_SUCCEED) {
+		nr_none = 0;
 		goto rollback;
+	}
 
 	/*
 	 * The old pages are locked, so they won't change anymore.
@@ -2157,8 +2158,8 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
 	if (nr_none) {
 		xas_lock_irq(&xas);
 		mapping->nrpages -= nr_none;
-		shmem_uncharge(mapping->host, nr_none);
 		xas_unlock_irq(&xas);
+		shmem_uncharge(mapping->host, nr_none);
 	}
 
 	list_for_each_entry_safe(page, tmp, &pagelist, lru) {
diff --git a/mm/shmem.c b/mm/shmem.c
index 161592a8d3fe..521a6302dc37 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -424,18 +424,20 @@ static void shmem_recalc_inode(struct inode *inode)
 bool shmem_charge(struct inode *inode, long pages)
 {
 	struct shmem_inode_info *info = SHMEM_I(inode);
-	unsigned long flags;
+	struct address_space *mapping = inode->i_mapping;
 
 	if (shmem_inode_acct_block(inode, pages))
 		return false;
 
 	/* nrpages adjustment first, then shmem_recalc_inode() when balanced */
-	inode->i_mapping->nrpages += pages;
+	xa_lock_irq(&mapping->i_pages);
+	mapping->nrpages += pages;
+	xa_unlock_irq(&mapping->i_pages);
 
-	spin_lock_irqsave(&info->lock, flags);
+	spin_lock_irq(&info->lock);
 	info->alloced += pages;
 	shmem_recalc_inode(inode);
-	spin_unlock_irqrestore(&info->lock, flags);
+	spin_unlock_irq(&info->lock);
 
 	return true;
 }
@@ -443,16 +445,13 @@ bool shmem_charge(struct inode *inode, long pages)
 void shmem_uncharge(struct inode *inode, long pages)
 {
 	struct shmem_inode_info *info = SHMEM_I(inode);
-	unsigned long flags;
 
 	/* nrpages adjustment done by __filemap_remove_folio() or caller */
 
-	spin_lock_irqsave(&info->lock, flags);
-	info->alloced -= pages;
+	spin_lock_irq(&info->lock);
 	shmem_recalc_inode(inode);
-	spin_unlock_irqrestore(&info->lock, flags);
-
-	shmem_inode_unacct_blocks(inode, pages);
+	/* which has called shmem_inode_unacct_blocks() if necessary */
+	spin_unlock_irq(&info->lock);
 }
 
 /*
-- 
2.35.3