From 441c228f817f7597e090d84aca74bdb7c2bd5040 Mon Sep 17 00:00:00 2001 From: Mel Gorman Date: Fri, 12 Dec 2014 16:56:33 -0800 Subject: [PATCH] mm: fadvise: document the fadvise(FADV_DONTNEED) behaviour for partial pages 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 Cc: Michael Kerrisk Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/fadvise.c | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-) diff --git a/mm/fadvise.c b/mm/fadvise.c index 3bcfd81db45e..2ad7adf4f0a4 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); -- 2.20.1