Re: [PATCH] hugetlbfs: dirty pages as they are added to pagecache

[Andrew Morton]

On Wed, 17 Oct 2018 21:10:22 -0700 Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:

> Some test systems were experiencing negative huge page reserve
> counts and incorrect file block counts.  This was traced to
> /proc/sys/vm/drop_caches removing clean pages from hugetlbfs
> file pagecaches.  When non-hugetlbfs explicit code removes the
> pages, the appropriate accounting is not performed.
> 
> This can be recreated as follows:
>  fallocate -l 2M /dev/hugepages/foo
>  echo 1 > /proc/sys/vm/drop_caches
>  fallocate -l 2M /dev/hugepages/foo
>  grep -i huge /proc/meminfo
>    AnonHugePages:         0 kB
>    ShmemHugePages:        0 kB
>    HugePages_Total:    2048
>    HugePages_Free:     2047
>    HugePages_Rsvd:    18446744073709551615
>    HugePages_Surp:        0
>    Hugepagesize:       2048 kB
>    Hugetlb:         4194304 kB
>  ls -lsh /dev/hugepages/foo
>    4.0M -rw-r--r--. 1 root root 2.0M Oct 17 20:05 /dev/hugepages/foo
> 
> To address this issue, dirty pages as they are added to pagecache.
> This can easily be reproduced with fallocate as shown above. Read
> faulted pages will eventually end up being marked dirty.  But there
> is a window where they are clean and could be impacted by code such
> as drop_caches.  So, just dirty them all as they are added to the
> pagecache.
> 
> In addition, it makes little sense to even try to drop hugetlbfs
> pagecache pages, so disable calls to these filesystems in drop_caches
> code.
> 
> ...
>
> --- a/fs/drop_caches.c
> +++ b/fs/drop_caches.c
> @@ -9,6 +9,7 @@
>  #include <linux/writeback.h>
>  #include <linux/sysctl.h>
>  #include <linux/gfp.h>
> +#include <linux/magic.h>
>  #include "internal.h"
>  
>  /* A global variable is a bit ugly, but it keeps the code simple */
> @@ -18,6 +19,12 @@ static void drop_pagecache_sb(struct super_block *sb, void *unused)
>  {
>  	struct inode *inode, *toput_inode = NULL;
>  
> +	/*
> +	 * It makes no sense to try and drop hugetlbfs page cache pages.
> +	 */
> +	if (sb->s_magic == HUGETLBFS_MAGIC)
> +		return;

Hardcoding hugetlbfs seems wrong here.  There are other filesystems
where it makes no sense to try to drop pagecache.  ramfs and, errrr...

I'm struggling to remember which is the correct thing to test here. 
BDI_CAP_NO_WRITEBACK should get us there, but doesn't seem quite
appropriate.