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From: Shakeel Butt <shakeelb@google.com>
Newsgroups: linux.kernel
Subject: Re: [RFC PATCH] mm: fadvise: avoid fadvise for fs without backing device
Date: Fri, 18 Aug 2017 03:30:02 +0200
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+linux-mm, linux-kernel

On Thu, Aug 17, 2017 at 6:10 PM, Shakeel Butt <shakeelb@google.com> wrote:
> 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) can 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) on create such clean zero'ed pages for
> ramfs. This change removes these two interfaces.
>
> Signed-off-by: Shakeel Butt <shakeelb@google.com>
> ---
>  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;
> --
> 2.14.1.480.gb18f417b89-goog
>