Re: [PATCH] mm: avoid blocking lock_page() in kcompactd

[Matthew Wilcox <willy@infradead.org>]

On Thu, Feb 13, 2020 at 08:48:47AM +0100, Michal Hocko wrote:
> Can we pursue on this please? An explicit NOFS scope annotation with a
> reference to compaction potentially locking up on pages in the readahead
> would be a great start.

How about this (on top of the current readahead series):

diff --git a/mm/readahead.c b/mm/readahead.c
index 29ca25c8f01e..32fd32b913da 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -160,6 +160,16 @@ unsigned long page_cache_readahead_limit(struct address_space *mapping,
 		.nr_pages = 0,
 	};
 
+	/*
+	 * Partway through the readahead operation, we will have added
+	 * locked pages to the page cache, but will not yet have submitted
+	 * them for I/O.  Adding another page may need to allocate
+	 * memory, which can trigger memory migration.	Telling the VM
+	 * we're in the middle of a filesystem operation will cause it
+	 * to not touch file-backed pages, preventing a deadlock.
+	 */
+	unsigned int nofs = memalloc_nofs_save();
+
 	/*
 	 * Preallocate as many pages as we will need.
 	 */
@@ -217,6 +227,7 @@ unsigned long page_cache_readahead_limit(struct address_space *mapping,
 	 */
 	read_pages(&rac, &page_pool);
 	BUG_ON(!list_empty(&page_pool));
+	memalloc_nofs_restore(nofs);
 	return rac.nr_pages;
 }
 EXPORT_SYMBOL_GPL(page_cache_readahead_limit);