Re: [PATCH] mm/huge_memory: Avoid PMD-size page cache if needed

[Matthew Wilcox]

On Thu, Jul 11, 2024 at 08:48:40PM +1000, Gavin Shan wrote:
> +++ b/mm/huge_memory.c
> @@ -136,7 +136,8 @@ unsigned long __thp_vma_allowable_orders(struct vm_area_struct *vma,
>  
>  		while (orders) {
>  			addr = vma->vm_end - (PAGE_SIZE << order);
> -			if (thp_vma_suitable_order(vma, addr, order))
> +			if (!(vma->vm_file && order > MAX_PAGECACHE_ORDER) &&
> +			    thp_vma_suitable_order(vma, addr, order))
>  				break;

Why does 'orders' even contain potential orders that are larger than
MAX_PAGECACHE_ORDER?

We do this at the top:

        orders &= vma_is_anonymous(vma) ?
                        THP_ORDERS_ALL_ANON : THP_ORDERS_ALL_FILE;

include/linux/huge_mm.h:#define THP_ORDERS_ALL_FILE     (BIT(PMD_ORDER) | BIT(PUD_ORDER))

... and that seems very wrong.  We support all kinds of orders for
files, not just PMD order.  We don't support PUD order at all.

What the hell is going on here?