linux-kernel - Re: [PATCH 3/3] mm,thp,shmem: make khugepaged obey tmpfs mount flags

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Date:   Thu, 26 Nov 2020 18:18:29 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Rik van Riel <riel@...riel.com>, hughd@...gle.com
Cc:     xuyu@...ux.alibaba.com, akpm@...ux-foundation.org, mgorman@...e.de,
        aarcange@...hat.com, willy@...radead.org,
        linux-kernel@...r.kernel.org, kernel-team@...com,
        linux-mm@...ck.org, mhocko@...e.com
Subject: Re: [PATCH 3/3] mm,thp,shmem: make khugepaged obey tmpfs mount flags

On 11/24/20 8:49 PM, Rik van Riel wrote:
> Currently if thp enabled=[madvise], mounting a tmpfs filesystem
> with huge=always and mmapping files from that tmpfs does not
> result in khugepaged collapsing those mappings, despite the
> mount flag indicating that it should.
> 
> Fix that by breaking up the blocks of tests in hugepage_vma_check
> a little bit, and testing things in the correct order.
> 
> Signed-off-by: Rik van Riel <riel@...riel.com>
> Fixes: c2231020ea7b ("mm: thp: register mm for khugepaged when merging vma for shmem")

Looks ok. But, it we have sysfs thp enabled=never, and shmem mount explicitly 
thp enabled, then shmem mount overrides the global sysfs setting and thp's will 
be allocated there, right? However, khugepaged_enabled() will be false and thus 
khugepaged won't run at all? So a similar situation than what you're fixing here.

> ---
>   include/linux/khugepaged.h |  2 ++
>   mm/khugepaged.c            | 22 ++++++++++++++++------
>   2 files changed, 18 insertions(+), 6 deletions(-)
> 
> diff --git a/include/linux/khugepaged.h b/include/linux/khugepaged.h
> index c941b7377321..2fcc01891b47 100644
> --- a/include/linux/khugepaged.h
> +++ b/include/linux/khugepaged.h
> @@ -3,6 +3,7 @@
>   #define _LINUX_KHUGEPAGED_H
>   
>   #include <linux/sched/coredump.h> /* MMF_VM_HUGEPAGE */
> +#include <linux/shmem_fs.h>
>   
>   
>   #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> @@ -57,6 +58,7 @@ static inline int khugepaged_enter(struct vm_area_struct *vma,
>   {
>   	if (!test_bit(MMF_VM_HUGEPAGE, &vma->vm_mm->flags))
>   		if ((khugepaged_always() ||
> +		     (shmem_file(vma->vm_file) && shmem_huge_enabled(vma)) ||
>   		     (khugepaged_req_madv() && (vm_flags & VM_HUGEPAGE))) &&
>   		    !(vm_flags & VM_NOHUGEPAGE) &&
>   		    !test_bit(MMF_DISABLE_THP, &vma->vm_mm->flags))
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 4e3dff13eb70..abab394c4206 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -440,18 +440,28 @@ static inline int khugepaged_test_exit(struct mm_struct *mm)
>   static bool hugepage_vma_check(struct vm_area_struct *vma,
>   			       unsigned long vm_flags)
>   {
> -	if ((!(vm_flags & VM_HUGEPAGE) && !khugepaged_always()) ||
> -	    (vm_flags & VM_NOHUGEPAGE) ||
> +	/* Explicitly disabled through madvise. */
> +	if ((vm_flags & VM_NOHUGEPAGE) ||
>   	    test_bit(MMF_DISABLE_THP, &vma->vm_mm->flags))
>   		return false;
>   
> -	if (shmem_file(vma->vm_file) ||
> -	    (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) &&
> -	     vma->vm_file &&
> -	     (vm_flags & VM_DENYWRITE))) {
> +	/* Enabled via shmem mount options or sysfs settings. */
> +	if (shmem_file(vma->vm_file) && shmem_huge_enabled(vma)) {
>   		return IS_ALIGNED((vma->vm_start >> PAGE_SHIFT) - vma->vm_pgoff,
>   				HPAGE_PMD_NR);
>   	}
> +
> +	/* THP settings require madvise. */
> +	if (!(vm_flags & VM_HUGEPAGE) && !khugepaged_always())
> +		return false;
> +
> +	/* Read-only file mappings need to be aligned for THP to work. */
> +	if (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) && vma->vm_file &&
> +	    (vm_flags & VM_DENYWRITE)) {
> +		return IS_ALIGNED((vma->vm_start >> PAGE_SHIFT) - vma->vm_pgoff,
> +				HPAGE_PMD_NR);
> +	}
> +
>   	if (!vma->anon_vma || vma->vm_ops)
>   		return false;
>   	if (vma_is_temporary_stack(vma))
>