From 3a77d214807c32f900618cee0432feb4f7f99c65 Mon Sep 17 00:00:00 2001 From: Shakeel Butt Date: Fri, 8 Sep 2017 16:13:05 -0700 Subject: [PATCH] mm: fadvise: avoid fadvise for fs without backing device The fadvise() manpage is silent on fadvise()'s effect on memory-based filesystems (shmem, hugetlbfs & ramfs) and pseudo file systems (procfs, sysfs, kernfs). The current implementaion of fadvise is mostly a noop for such filesystems except for FADV_DONTNEED which will trigger expensive remote LRU cache draining. This patch makes the noop of fadvise() on such file systems very explicit. However this change has two side effects for ramfs and one for tmpfs. First fadvise(FADV_DONTNEED) could remove the unmapped clean zero'ed pages of ramfs (allocated through read, readahead & read fault) and tmpfs (allocated through read fault). Also fadvise(FADV_WILLNEED) could create such clean zero'ed pages for ramfs. This change removes those possibilities. One of our generic libraries does fadvise(FADV_DONTNEED). Recently we observed high latency in fadvise() and noticed that the users have started using tmpfs files and the latency was due to expensive remote LRU cache draining. For normal tmpfs files (have data written on them), fadvise(FADV_DONTNEED) will always trigger the unneeded remote cache draining. Link: http://lkml.kernel.org/r/20170818011023.181465-1-shakeelb@google.com Signed-off-by: Shakeel Butt Cc: Mel Gorman Cc: Johannes Weiner Cc: Hillf Danton Cc: Vlastimil Babka Cc: Hugh Dickins Cc: Greg Thelen Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/fadvise.c | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-) diff --git a/mm/fadvise.c b/mm/fadvise.c index a43013112581..702f239cd6db 100644 --- a/mm/fadvise.c +++ b/mm/fadvise.c @@ -52,7 +52,9 @@ SYSCALL_DEFINE4(fadvise64_64, int, fd, loff_t, offset, loff_t, len, int, advice) goto out; } - if (IS_DAX(inode)) { + bdi = inode_to_bdi(mapping->host); + + if (IS_DAX(inode) || (bdi == &noop_backing_dev_info)) { switch (advice) { case POSIX_FADV_NORMAL: case POSIX_FADV_RANDOM: @@ -75,8 +77,6 @@ SYSCALL_DEFINE4(fadvise64_64, int, fd, loff_t, offset, loff_t, len, int, advice) else endbyte--; /* inclusive */ - bdi = inode_to_bdi(mapping->host); - switch (advice) { case POSIX_FADV_NORMAL: f.file->f_ra.ra_pages = bdi->ra_pages;