linux-kernel - Re: [PATCH] mm: shmem: use proper gfp flags for shmem

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Date:   Wed, 6 Nov 2019 16:18:20 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Yang Shi <yang.shi@...ux.alibaba.com>
Cc:     hughd@...gle.com, akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: shmem: use proper gfp flags for shmem_writepage()

On Wed 06-11-19 06:02:31, Yang Shi wrote:
> The shmem_writepage() uses GFP_ATOMIC to allocate swap cache.
> GFP_ATOMIC used to mean __GFP_HIGH, but now it means __GFP_HIGH |
> __GFP_ATOMIC | __GFP_KSWAPD_RECLAIM.  However, shmem_writepage() should
> write out to swap only in response to memory pressure, so
> __GFP_KSWAPD_RECLAIM looks useless since the caller may be kswapd itself
> or in direct reclaim already.

What kind of problem are you trying to fix here?

> In addition, XArray node allocations from PF_MEMALLOC contexts could
> completely exhaust the page allocator, __GFP_NOMEMALLOC stops emergency
> reserves from being allocated.

I am not really familiar with XArray much, could you be more specific
please?

> Here just copy the gfp flags used by add_to_swap().
> 
> Cc: Hugh Dickins <hughd@...gle.com>
> Signed-off-by: Yang Shi <yang.shi@...ux.alibaba.com>
> ---
>  mm/shmem.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 220be9f..9691dec 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -1369,7 +1369,8 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc)
>  	if (list_empty(&info->swaplist))
>  		list_add(&info->swaplist, &shmem_swaplist);
>  
> -	if (add_to_swap_cache(page, swap, GFP_ATOMIC) == 0) {
> +	if (add_to_swap_cache(page, swap,
> +			__GFP_HIGH | __GFP_NOMEMALLOC | __GFP_NOWARN) == 0) {
>  		spin_lock_irq(&info->lock);
>  		shmem_recalc_inode(inode);
>  		info->swapped++;
> -- 
> 1.8.3.1

-- 
Michal Hocko
SUSE Labs