Re: [PATCH] mm: shmem: fix too little space for tmpfs only fallback 4KB

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 2025/9/8 20:31, Vernon Yang wrote:
> From: Vernon Yang <yanglincheng@kylinos.cn>
> 
> When the system memory is sufficient, allocating memory is always
> successful, but when tmpfs size is low (e.g. 1MB), it falls back
> directly from 2MB to 4KB, and other small granularity (8KB ~ 1024KB)
> will not be tried.
> 
> Therefore add check whether the remaining space of tmpfs is sufficient
> for allocation. If there is too little space left, try smaller large
> folio.

I don't think so.

For a tmpfs mount with 'huge=within_size' and 'size=1M', if you try to 
write 1M data, it will allocate an order 8 large folio and will not 
fallback to order 0.

For a tmpfs mount with 'huge=always' and 'size=1M', if you try to write 
1M data, it will not completely fallback to order 0 either, instead, it 
will still allocate some order 1 to order 7 large folios.

I'm not sure if this is your actual user scenario. If your files are 
small and you are concerned about not getting large folio allocations, I 
recommend using the 'huge=within_size' mount option.

> Fixes: acd7ccb284b8 ("mm: shmem: add large folio support for tmpfs")

No, this doesn't fix anything.

> Signed-off-by: Vernon Yang <yanglincheng@kylinos.cn>
> ---
>   mm/shmem.c | 13 +++++++++++++
>   1 file changed, 13 insertions(+)
> 
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 8c592c6db2a0..b20affd57b23 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -1820,6 +1820,7 @@ static unsigned long shmem_suitable_orders(struct inode *inode, struct vm_fault
>   					   unsigned long orders)
>   {
>   	struct vm_area_struct *vma = vmf ? vmf->vma : NULL;
> +	struct shmem_sb_info *sbinfo = SHMEM_SB(inode->i_sb);
>   	pgoff_t aligned_index;
>   	unsigned long pages;
>   	int order;
> @@ -1835,6 +1836,18 @@ static unsigned long shmem_suitable_orders(struct inode *inode, struct vm_fault
>   	while (orders) {
>   		pages = 1UL << order;
>   		aligned_index = round_down(index, pages);
> +
> +		/*
> +		 * Check whether the remaining space of tmpfs is sufficient for
> +		 * allocation. If there is too little space left, try smaller
> +		 * large folio.
> +		 */
> +		if (sbinfo->max_blocks && percpu_counter_read(&sbinfo->used_blocks)
> +						+ pages > sbinfo->max_blocks) {
> +			order = next_order(&orders, order);
> +			continue;
> +		}
> +
>   		/*
>   		 * Check for conflict before waiting on a huge allocation.
>   		 * Conflict might be that a huge page has just been allocated