linux-kernel - Re: [PATCH 2/3] mm,thp,shm: limit gfp mask to no more than specified

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Date:   Thu, 26 Nov 2020 14:40:34 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Rik van Riel <riel@...riel.com>
Cc:     hughd@...gle.com, 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, vbabka@...e.cz
Subject: Re: [PATCH 2/3] mm,thp,shm: limit gfp mask to no more than specified

On Tue 24-11-20 14:49:24, Rik van Riel wrote:
> Matthew Wilcox pointed out that the i915 driver opportunistically
> allocates tmpfs memory, but will happily reclaim some of its
> pool if no memory is available.
> 
> Make sure the gfp mask used to opportunistically allocate a THP
> is always at least as restrictive as the original gfp mask.

I have brought this up in the previous version review and I feel my
feedback hasn't been addressed. Please describe the expected behavior by
those shmem users including GFP_KERNEL restriction which would make the
THP flags incompatible. Is this a problem? Is there any actual problem
if the THP uses its own set of flags?

I am also not happy how those two sets of flags are completely detached
and we can only expect surprises there. 

> Signed-off-by: Rik van Riel <riel@...riel.com>
> Suggested-by: Matthew Wilcox <willy@...radead.org>
> ---
>  mm/shmem.c | 21 +++++++++++++++++++++
>  1 file changed, 21 insertions(+)
> 
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 6c3cb192a88d..ee3cea10c2a4 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -1531,6 +1531,26 @@ static struct page *shmem_swapin(swp_entry_t swap, gfp_t gfp,
>  	return page;
>  }
>  
> +/*
> + * Make sure huge_gfp is always more limited than limit_gfp.
> + * Some of the flags set permissions, while others set limitations.
> + */
> +static gfp_t limit_gfp_mask(gfp_t huge_gfp, gfp_t limit_gfp)
> +{
> +	gfp_t allowflags = __GFP_IO | __GFP_FS | __GFP_RECLAIM;
> +	gfp_t denyflags = __GFP_NOWARN | __GFP_NORETRY;
> +	gfp_t result = huge_gfp & ~allowflags;
> +
> +	/*
> +	 * Minimize the result gfp by taking the union with the deny flags,
> +	 * and the intersection of the allow flags.
> +	 */
> +	result |= (limit_gfp & denyflags);
> +	result |= (huge_gfp & limit_gfp) & allowflags;
> +
> +	return result;
> +}
> +
>  static struct page *shmem_alloc_hugepage(gfp_t gfp,
>  		struct shmem_inode_info *info, pgoff_t index)
>  {
> @@ -1889,6 +1909,7 @@ static int shmem_getpage_gfp(struct inode *inode, pgoff_t index,
>  
>  alloc_huge:
>  	huge_gfp = vma_thp_gfp_mask(vma);
> +	huge_gfp = limit_gfp_mask(huge_gfp, gfp);
>  	page = shmem_alloc_and_acct_page(huge_gfp, inode, index, true);
>  	if (IS_ERR(page)) {
>  alloc_nohuge:
> -- 
> 2.25.4
> 

-- 
Michal Hocko
SUSE Labs