From dd7e52e3d20299714f1beed68ec71ceecda5e521 Mon Sep 17 00:00:00 2001 From: Hugh Dickins Date: Wed, 23 Jul 2014 14:00:15 -0700 Subject: [PATCH] mm/fs: fix pessimization in hole-punching pagecache I wanted to revert my v3.1 commit b24af47b9a20 ("mm: pincer in truncate_inode_pages_range"), to keep truncate_inode_pages_range() in synch with shmem_undo_range(); but have stepped back - a change to hole-punching in truncate_inode_pages_range() is a change to hole-punching in every filesystem (except tmpfs) that supports it. If there's a logical proof why no filesystem can depend for its own correctness on the pincer guarantee in truncate_inode_pages_range() - an instant when the entire hole is removed from pagecache - then let's revisit later. But the evidence is that only tmpfs suffered from the livelock, and we have no intention of extending hole-punch to ramfs. So for now just add a few comments (to match or differ from those in shmem_undo_range()), and fix one silliness noticed in b24af47b9a20... Its "index == start" addition to the hole-punch termination test was incomplete: it opened a way for the end condition to be missed, and the loop go on looking through the radix_tree, all the way to end of file. Fix that pessimization by resetting index when detected in inner loop. Note that it's actually hard to hit this case, without the obsessive concurrent faulting that trinity does: normally all pages are removed in the initial trylock_page() pass, and this loop finds nothing to do. I had to "#if 0" out the initial pass to reproduce bug and test fix. Signed-off-by: Hugh Dickins Cc: Sasha Levin Cc: Konstantin Khlebnikov Cc: Lukas Czerner Cc: Dave Jones Acked-by: Vlastimil Babka Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/truncate.c | 11 ++++++++--- 1 file changed, 8 insertions(+), 3 deletions(-) diff --git a/mm/truncate.c b/mm/truncate.c index 6a78c814bebfb..eda2473071648 100644 --- a/mm/truncate.c +++ b/mm/truncate.c @@ -355,14 +355,16 @@ void truncate_inode_pages_range(struct address_space *mapping, for ( ; ; ) { cond_resched(); if (!pagevec_lookup_entries(&pvec, mapping, index, - min(end - index, (pgoff_t)PAGEVEC_SIZE), - indices)) { + min(end - index, (pgoff_t)PAGEVEC_SIZE), indices)) { + /* If all gone from start onwards, we're done */ if (index == start) break; + /* Otherwise restart to make sure all gone */ index = start; continue; } if (index == start && indices[0] >= end) { + /* All gone out of hole to be punched, we're done */ pagevec_remove_exceptionals(&pvec); pagevec_release(&pvec); break; @@ -373,8 +375,11 @@ void truncate_inode_pages_range(struct address_space *mapping, /* We rely upon deletion not changing page->index */ index = indices[i]; - if (index >= end) + if (index >= end) { + /* Restart punch to make sure all gone */ + index = start - 1; break; + } if (radix_tree_exceptional_entry(page)) { clear_exceptional_entry(mapping, index, page); -- 2.39.5