Re: [PATCH] mm/shmem: Do not wait for lock_page() in shmem_unused_huge_shrink()

[Tetsuo Handa]

Kirill A. Shutemov wrote:

> On Fri, Mar 16, 2018 at 10:14:24PM +0900, Tetsuo Handa wrote:
> > f2fs is doing
> > 
> >   page = f2fs_pagecache_get_page(inode->i_mapping, 0, FGP_LOCK|FGP_NOWAIT, 0);
> > 
> > which calls
> > 
> >   struct page *pagecache_get_page(inode->i_mapping, 0, FGP_LOCK|FGP_NOWAIT, 0);
> > 
> > . Then, can't we define
> > 
> >   static inline struct page *find_trylock_page(struct address_space *mapping,
> >   					     pgoff_t offset)
> >   {
> >   	return pagecache_get_page(mapping, offset, FGP_LOCK|FGP_NOWAIT, 0);
> >   }
> > 
> > and replace find_lock_page() with find_trylock_page() ?
> 
> This won't work in this case. We need to destinct no-page-in-page-cache
> from failed-to-lock-page. We take different routes depending on this.
> 

OK. Then, I think we should avoid reordering trylock_page() and PageTransHuge()
without patch description why it is safe. Below patch preserves the ordering
and sounds safer for stable. But either patch, please add why it is safe to omit
"/* Has the page been truncated? */" check which would have been done for FGP_LOCK
in patch description.

---
 mm/shmem.c | 30 ++++++++++++++++++++----------
 1 file changed, 20 insertions(+), 10 deletions(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index 8ead6cb..5e94ca4 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -493,16 +493,27 @@ static unsigned long shmem_unused_huge_shrink(struct shmem_sb_info *sbinfo,
 		info = list_entry(pos, struct shmem_inode_info, shrinklist);
 		inode = &info->vfs_inode;
 
-		if (nr_to_split && split >= nr_to_split) {
-			iput(inode);
-			continue;
-		}
+		if (nr_to_split && split >= nr_to_split)
+			goto leave;
 
-		page = find_lock_page(inode->i_mapping,
+		page = find_get_page(inode->i_mapping,
 				(inode->i_size & HPAGE_PMD_MASK) >> PAGE_SHIFT);
 		if (!page)
 			goto drop;
 
+		/*
+		 * Leave the inode on the list if we failed to lock
+		 * the page at this time.
+		 *
+		 * Waiting for the lock may lead to deadlock in the
+		 * reclaim path.
+		 */
+		if (!trylock_page(page)) {
+			put_page(page);
+			goto leave;
+		}
+
+		/* No huge page at the end of the file: nothing to split */
 		if (!PageTransHuge(page)) {
 			unlock_page(page);
 			put_page(page);
@@ -513,16 +524,15 @@ static unsigned long shmem_unused_huge_shrink(struct shmem_sb_info *sbinfo,
 		unlock_page(page);
 		put_page(page);
 
-		if (ret) {
-			/* split failed: leave it on the list */
-			iput(inode);
-			continue;
-		}
+		/* If split failed leave the inode on the list */
+		if (ret)
+			goto leave;
 
 		split++;
 drop:
 		list_del_init(&info->shrinklist);
 		removed++;
+leave:
 		iput(inode);
 	}
 
--