[PATCH 1/2] mm: fadvise: Document the fadvise(FADV_DONTNEED) behaviour for partial pages

[Mel Gorman <mgorman@suse.de>]

A random seek IO benchmark appeared to regress because of a change to
readahead but the real problem was the benchmark. To ensure the IO request
accesssed disk, it used fadvise(FADV_DONTNEED) on a block boundary (512K)
but the hint is ignored by the kernel. This is correct but not necessarily
obvious behaviour.  As much as I dislike comment patches, the explanation
for this behaviour predates current git history. Clarify why it behaves
like this in case someone "fixes" fadvise or readahead for the wrong reasons.

Signed-off-by: Mel Gorman <mgorman@suse.de>
---
 mm/fadvise.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/mm/fadvise.c b/mm/fadvise.c
index 3bcfd81d..c908c72 100644
--- a/mm/fadvise.c
+++ b/mm/fadvise.c
@@ -117,7 +117,11 @@ SYSCALL_DEFINE4(fadvise64_64, int, fd, loff_t, offset, loff_t, len, int, advice)
 			__filemap_fdatawrite_range(mapping, offset, endbyte,
 						   WB_SYNC_NONE);
 
-		/* First and last FULL page! */
+		/*
+		 * First and last FULL page! Partial pages are deliberately
+		 * preserved on the expectation that it is better to preserve
+		 * needed memory than to discard unneeded memory.
+		 */
 		start_index = (offset+(PAGE_CACHE_SIZE-1)) >> PAGE_CACHE_SHIFT;
 		end_index = (endbyte >> PAGE_CACHE_SHIFT);
 
-- 
2.1.2