linux-kernel - Re: [PATCH 7/8] mm/vmalloc: Support non-blocking GFP flags in __vmalloc_area

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Message-ID: <aKKthIZbD4oNywY4@MiWiFi-R3L-srv>
Date: Mon, 18 Aug 2025 12:35:16 +0800
From: Baoquan He <bhe@...hat.com>
To: "Uladzislau Rezki (Sony)" <urezki@...il.com>
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
	Vlastimil Babka <vbabka@...e.cz>, Michal Hocko <mhocko@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 7/8] mm/vmalloc: Support non-blocking GFP flags in
 __vmalloc_area_node()

On 08/07/25 at 09:58am, Uladzislau Rezki (Sony) wrote:
> This patch makes __vmalloc_area_node() to correctly handle non-blocking
> allocation requests, such as GFP_ATOMIC and GFP_NOWAIT. Main changes:
> 
> - Add a __GFP_HIGHMEM to gfp_mask only for blocking requests
>   if there are no DMA constraints.
> 
> - vmap_page_range() is wrapped by memalloc_noreclaim_save/restore()
>   to avoid memory reclaim related operations that could sleep during
>   page table setup or mapping pages.
> 
> This is particularly important for page table allocations that
> internally use GFP_PGTABLE_KERNEL, which may sleep unless such
> scope restrictions are applied. For example:
> 
> <snip>
> __pte_alloc_kernel()
>     pte_alloc_one_kernel(&init_mm);
>         pagetable_alloc_noprof(GFP_PGTABLE_KERNEL & ~__GFP_HIGHMEM, 0);
> <snip>
> 
> Note: in most cases, PTE entries are established only up to the level
> required by current vmap space usage, meaning the page tables are typically
> fully populated during the mapping process.
> 
> Signed-off-by: Uladzislau Rezki (Sony) <urezki@...il.com>
> ---
>  mm/vmalloc.c | 20 ++++++++++++++++----
>  1 file changed, 16 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 2424f80d524a..8a7eab810561 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -3721,12 +3721,20 @@ static void *__vmalloc_area_node(struct vm_struct *area, gfp_t gfp_mask,
>  	unsigned int nr_small_pages = size >> PAGE_SHIFT;
>  	unsigned int page_order;
>  	unsigned int flags;
> +	bool noblock;
>  	int ret;
>  
>  	array_size = (unsigned long)nr_small_pages * sizeof(struct page *);
> +	noblock = !gfpflags_allow_blocking(gfp_mask);
>  
> -	if (!(gfp_mask & (GFP_DMA | GFP_DMA32)))
> -		gfp_mask |= __GFP_HIGHMEM;
> +	if (noblock) {
> +		/* __GFP_NOFAIL and "noblock" flags are mutually exclusive. */
> +		nofail = false;
> +	} else {
> +		/* Allow highmem allocations if there are no DMA constraints. */
> +		if (!(gfp_mask & (GFP_DMA | GFP_DMA32)))
> +			gfp_mask |= __GFP_HIGHMEM;
> +	}
>  
>  	/* Please note that the recursion is strictly bounded. */
>  	if (array_size > PAGE_SIZE) {
> @@ -3790,7 +3798,9 @@ static void *__vmalloc_area_node(struct vm_struct *area, gfp_t gfp_mask,
>  	 * page tables allocations ignore external gfp mask, enforce it
>  	 * by the scope API
>  	 */
> -	if ((gfp_mask & (__GFP_FS | __GFP_IO)) == __GFP_IO)
> +	if (noblock)
> +		flags = memalloc_noreclaim_save();
> +	else if ((gfp_mask & (__GFP_FS | __GFP_IO)) == __GFP_IO)
>  		flags = memalloc_nofs_save();
>  	else if ((gfp_mask & (__GFP_FS | __GFP_IO)) == 0)
>  		flags = memalloc_noio_save();
> @@ -3802,7 +3812,9 @@ static void *__vmalloc_area_node(struct vm_struct *area, gfp_t gfp_mask,
>  			schedule_timeout_uninterruptible(1);
>  	} while (nofail && (ret < 0));
>  
> -	if ((gfp_mask & (__GFP_FS | __GFP_IO)) == __GFP_IO)
> +	if (noblock)
> +		memalloc_noreclaim_restore(flags);
> +	else if ((gfp_mask & (__GFP_FS | __GFP_IO)) == __GFP_IO)
>  		memalloc_nofs_restore(flags);
>  	else if ((gfp_mask & (__GFP_FS | __GFP_IO)) == 0)
>  		memalloc_noio_restore(flags);

Can we use memalloc_flags_restore(flags) directly to replace above if
else checking? It can reduce LOC, might be not as readable as the change
in patch surely. Not strong opinion.

	memalloc_flags_restore(flags);