Re: [PATCH 1/3] mm: Handle MADV_WILLNEED through vfs_fadvise()

["Darrick J. Wong" <darrick.wong@xxxxxxxxxx>]

On Thu, Jul 11, 2019 at 04:00:10PM +0200, Jan Kara wrote:
> Currently handling of MADV_WILLNEED hint calls directly into readahead
> code. Handle it by calling vfs_fadvise() instead so that filesystem can
> use its ->fadvise() callback to acquire necessary locks or otherwise
> prepare for the request.
> 
> Suggested-by: Amir Goldstein <amir73il@xxxxxxxxx>
> CC: stable@xxxxxxxxxxxxxxx # Needed by "xfs: Fix stale data exposure
> 					when readahead races with hole punch"
> Signed-off-by: Jan Kara <jack@xxxxxxx>

Looks reasonable to me, though is this race between readahead and
truncate severe enough to try to push it as a fix for 5.3 or are you
targetting 5.4?

--D

> ---
>  mm/madvise.c | 22 ++++++++++++++++------
>  1 file changed, 16 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/madvise.c b/mm/madvise.c
> index 628022e674a7..ae56d0ef337d 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -14,6 +14,7 @@
>  #include <linux/userfaultfd_k.h>
>  #include <linux/hugetlb.h>
>  #include <linux/falloc.h>
> +#include <linux/fadvise.h>
>  #include <linux/sched.h>
>  #include <linux/ksm.h>
>  #include <linux/fs.h>
> @@ -275,6 +276,7 @@ static long madvise_willneed(struct vm_area_struct *vma,
>  			     unsigned long start, unsigned long end)
>  {
>  	struct file *file = vma->vm_file;
> +	loff_t offset;
>  
>  	*prev = vma;
>  #ifdef CONFIG_SWAP
> @@ -298,12 +300,20 @@ static long madvise_willneed(struct vm_area_struct *vma,
>  		return 0;
>  	}
>  
> -	start = ((start - vma->vm_start) >> PAGE_SHIFT) + vma->vm_pgoff;
> -	if (end > vma->vm_end)
> -		end = vma->vm_end;
> -	end = ((end - vma->vm_start) >> PAGE_SHIFT) + vma->vm_pgoff;
> -
> -	force_page_cache_readahead(file->f_mapping, file, start, end - start);
> +	/*
> +	 * Filesystem's fadvise may need to take various locks.  We need to
> +	 * explicitly grab a reference because the vma (and hence the
> +	 * vma's reference to the file) can go away as soon as we drop
> +	 * mmap_sem.
> +	 */
> +	*prev = NULL;	/* tell sys_madvise we drop mmap_sem */
> +	get_file(file);
> +	up_read(&current->mm->mmap_sem);
> +	offset = (loff_t)(start - vma->vm_start)
> +			+ ((loff_t)vma->vm_pgoff << PAGE_SHIFT);
> +	vfs_fadvise(file, offset, end - start, POSIX_FADV_WILLNEED);
> +	fput(file);
> +	down_read(&current->mm->mmap_sem);
>  	return 0;
>  }
>  
> -- 
> 2.16.4
>