Subject: Re: [PATCH v6 2/2] mm: use mapping_max_folio_order() for force_thp_readahead order

From: Usama Arif <[email-masked]>
Date: Fri, 29 May 2026 13:36:10 +0100

On 28/05/2026 17:55, Usama Arif wrote:
> The force_thp_readahead path in do_sync_mmap_readahead() is gated on
> HPAGE_PMD_ORDER <= MAX_PAGECACHE_ORDER and always requests
> HPAGE_PMD_ORDER / HPAGE_PMD_NR. On configurations where HPAGE_PMD_ORDER
> exceeds MAX_PAGECACHE_ORDER, notably arm64 with a 64K base page size,
> VM_HUGEPAGE mappings cannot use this path and fall back to the non-forced
> mmap readahead path even when the mapping supports useful large folios.
> 
> Keep the existing PMD-sized behavior when HPAGE_PMD_ORDER fits in the
> page cache. When it does not, enable forced readahead for mappings that
> support large folios and request an order capped by both
> mapping_max_folio_order(mapping) and 2MB.
> 
> 2MB is chosen as the cap because it matches the PMD size on x86_64
> and on arm64 with 4K or 16K base pages, so the size/memory-pressure
> tradeoff for folios of that size is already well understood. On arm64
> with a 64K base page size, 2MB is also the contiguous-PTE (contpte)
> block size, so the resulting folios coalesce into a single TLB entry
> and reduce TLB pressure on the readahead path.
> 
> The final allocation order may still be clamped by page_cache_ra_order()
> to the mapping and request geometry, but this gives VM_HUGEPAGE mappings
> on such configurations a large-folio readahead request instead of
> dropping back to base-page readahead.
> 
> Signed-off-by: Usama Arif <[email-masked]>
> ---
>  mm/filemap.c | 27 +++++++++++++++++++--------
>  1 file changed, 19 insertions(+), 8 deletions(-)
> 
> diff --git a/mm/filemap.c b/mm/filemap.c
> index a16b33e0fc71..bfb891d9da1f 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -3312,14 +3312,23 @@ static struct file *do_sync_mmap_readahead(struct vm_fault *vmf)
>  	struct file *fpin = NULL;
>  	vm_flags_t vm_flags = vmf->vma->vm_flags;
>  	bool force_thp_readahead = false;
> +	unsigned int thp_order = 0;
>  	unsigned short mmap_miss;
>  
>  	ractl._max_index = vmf->vma->vm_pgoff + vma_pages(vmf->vma) - 1;
>  
>  	/* Use the readahead code, even if readahead is disabled */
> -	if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) &&
> -	    (vm_flags & VM_HUGEPAGE) && HPAGE_PMD_ORDER <= MAX_PAGECACHE_ORDER)
> -		force_thp_readahead = true;
> +	if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) && (vm_flags & VM_HUGEPAGE)) {
> +		if (HPAGE_PMD_ORDER <= MAX_PAGECACHE_ORDER) {
> +			force_thp_readahead = true;
> +			thp_order = HPAGE_PMD_ORDER;
> +		} else if (mapping_large_folio_support(mapping)) {
> +			force_thp_readahead = true;
> +			thp_order = min_t(unsigned int,
> +					  mapping_max_folio_order(mapping),
> +					  get_order(SZ_2M));
> +		}
> +	}
>  

I think might be good to include the below comment to explain the decision being made
here:

>From 6673c04a434df01d449c1bdb9ac8de74e19d6b7e Mon Sep 17 00:00:00 2001
From: Usama Arif <[email-masked]>
Date: Fri, 29 May 2026 05:31:05 -0700
Subject: [PATCH] [fixlet] mm/filemap: add comment explaining design decision

Signed-off-by: Usama Arif <[email-masked]>
---
 mm/filemap.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/mm/filemap.c b/mm/filemap.c
index bfb891d9da1f..0a3facf452b3 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -3319,6 +3319,14 @@ static struct file *do_sync_mmap_readahead(struct vm_fault *vmf)
 
 	/* Use the readahead code, even if readahead is disabled */
 	if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) && (vm_flags & VM_HUGEPAGE)) {
+		/*
+		 * Preserve PMD-sized readahead where it already fits in
+		 * the page cache. Otherwise cap the new fallback path at
+		 * 2MB: this is the common PMD-sized hugepage size, and it
+		 * avoids memory pressure from very large forced readahead
+		 * when mapping_max_folio_order() is high (for example,
+		 * 128MB with 64K base pages on arm64).
+		 */
 		if (HPAGE_PMD_ORDER <= MAX_PAGECACHE_ORDER) {
 			force_thp_readahead = true;
 			thp_order = HPAGE_PMD_ORDER;
-- 
2.53.0-Meta