linux-ext4 - Re: [v2 PATCH 3/8] mm: khugepaged: skip DAX vma

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Date:   Mon, 21 Mar 2022 12:04:20 +0000
From:   Hyeonggon Yoo <42.hyeyoo@...il.com>
To:     Yang Shi <shy828301@...il.com>
Cc:     vbabka@...e.cz, kirill.shutemov@...ux.intel.com,
        linmiaohe@...wei.com, songliubraving@...com, riel@...riel.com,
        willy@...radead.org, ziy@...dia.com, akpm@...ux-foundation.org,
        tytso@....edu, adilger.kernel@...ger.ca, darrick.wong@...cle.com,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [v2 PATCH 3/8] mm: khugepaged: skip DAX vma

On Thu, Mar 17, 2022 at 04:48:22PM -0700, Yang Shi wrote:
> The DAX vma may be seen by khugepaged when the mm has other khugepaged
> suitable vmas.  So khugepaged may try to collapse THP for DAX vma, but
> it will fail due to page sanity check, for example, page is not
> on LRU.
> 
> So it is not harmful, but it is definitely pointless to run khugepaged
> against DAX vma, so skip it in early check.

in fs/xfs/xfs_file.c:
1391 STATIC int
1392 xfs_file_mmap(
1393         struct file             *file,
1394         struct vm_area_struct   *vma)
1395 {
1396         struct inode            *inode = file_inode(file);
1397         struct xfs_buftarg      *target = xfs_inode_buftarg(XFS_I(inode));
1398 
1399         /*
1400          * We don't support synchronous mappings for non-DAX files and
1401          * for DAX files if underneath dax_device is not synchronous.
1402          */
1403         if (!daxdev_mapping_supported(vma, target->bt_daxdev))
1404                 return -EOPNOTSUPP;
1405 
1406         file_accessed(file);
1407         vma->vm_ops = &xfs_file_vm_ops;
1408         if (IS_DAX(inode))
1409                 vma->vm_flags |= VM_HUGEPAGE;

Are xfs and other filesystems setting VM_HUGEPAGE flag even if it can
never be collapsed?

1410         return 0;
1411 }


> Reviewed-by: Miaohe Lin <linmiaohe@...wei.com>
> Signed-off-by: Yang Shi <shy828301@...il.com>
> ---
>  mm/khugepaged.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 82c71c6da9ce..a0e4fa33660e 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -448,6 +448,10 @@ static bool hugepage_vma_check(struct vm_area_struct *vma,
>  	if (vm_flags & VM_NO_KHUGEPAGED)
>  		return false;
>  
> +	/* Don't run khugepaged against DAX vma */
> +	if (vma_is_dax(vma))
> +		return false;
> +
>  	if (vma->vm_file && !IS_ALIGNED((vma->vm_start >> PAGE_SHIFT) -
>  				vma->vm_pgoff, HPAGE_PMD_NR))
>  		return false;
> -- 
> 2.26.3
> 
> 

-- 
Thank you, You are awesome!
Hyeonggon :-)