linux-kernel - Re: [PATCH v2] mm: readahead: make thp readahead conditional to mmap

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Message-ID: <e2zblaknfnzjtlo3df4aozoxuir6zgnycdjj4ywbu7rsnpw6hr@ocemkpucjd2d>
Date: Mon, 6 Oct 2025 14:31:06 +0200
From: Jan Kara <jack@...e.cz>
To: Roman Gushchin <roman.gushchin@...ux.dev>
Cc: Andrew Morton <akpm@...ux-foundation.org>, 
	linux-kernel@...r.kernel.org, Jan Kara <jack@...e.cz>, 
	"Matthew Wilcox (Oracle)" <willy@...radead.org>, Dev Jain <dev.jain@....com>, linux-mm@...ck.org
Subject: Re: [PATCH v2] mm: readahead: make thp readahead conditional to
 mmap_miss logic

On Sun 05-10-25 18:54:09, Roman Gushchin wrote:
> Commit 4687fdbb805a ("mm/filemap: Support VM_HUGEPAGE for file mappings")
> introduced a special handling for VM_HUGEPAGE mappings: even if the
> readahead is disabled, 1 or 2 HPAGE_PMD_ORDER pages are
> allocated.
> 
> This change causes a significant regression for containers with a
> tight memory.max limit, if VM_HUGEPAGE is widely used. Prior to this
> commit, mmap_miss logic would eventually lead to the readahead
> disablement, effectively reducing the memory pressure in the
> cgroup. With this change the kernel is trying to allocate 1-2 huge
> pages for each fault, no matter if these pages are used or not
> before being evicted, increasing the memory pressure multi-fold.
> 
> To fix the regression, let's make the new VM_HUGEPAGE conditional
> to the mmap_miss check, but keep independent from the ra->ra_pages.
> This way the main intention of commit 4687fdbb805a ("mm/filemap:
> Support VM_HUGEPAGE for file mappings") stays intact, but the
> regression is resolved.
> 
> The logic behind this changes is simple: even if a user explicitly
> requests using huge pages to back the file mapping (using VM_HUGEPAGE
> flag), under a very strong memory pressure it's better to fall back
> to ordinary pages.
> 
> Signed-off-by: Roman Gushchin <roman.gushchin@...ux.dev>
> Reviewed-by: Jan Kara <jack@...e.cz>
> Cc: Matthew Wilcox (Oracle) <willy@...radead.org>
> Cc: Dev Jain <dev.jain@....com>
> Cc: linux-mm@...ck.org
> 
> --
> 
> v2: fixed VM_SEQ_READ handling (by Dev Jain)

OK, but now we'll do mmap_miss detection and bail-out even for VM_SEQ_READ
| VM_HUGEPAGE vmas. And without VM_HUGEPAGE we won't do it which is really
odd. So I think you want to make the whole mmap_miss logic conditional on
!VM_SEQ_READ...

								Honza

> ---
>  mm/filemap.c | 42 ++++++++++++++++++++++--------------------
>  1 file changed, 22 insertions(+), 20 deletions(-)
> 
> diff --git a/mm/filemap.c b/mm/filemap.c
> index a52dd38d2b4a..446e591d57e5 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -3235,37 +3235,23 @@ static struct file *do_sync_mmap_readahead(struct vm_fault *vmf)
>  	DEFINE_READAHEAD(ractl, file, ra, mapping, vmf->pgoff);
>  	struct file *fpin = NULL;
>  	vm_flags_t vm_flags = vmf->vma->vm_flags;
> +	bool force_thp_readahead = false;
>  	unsigned short mmap_miss;
>  
> -#ifdef CONFIG_TRANSPARENT_HUGEPAGE
>  	/* Use the readahead code, even if readahead is disabled */
> -	if ((vm_flags & VM_HUGEPAGE) && HPAGE_PMD_ORDER <= MAX_PAGECACHE_ORDER) {
> -		fpin = maybe_unlock_mmap_for_io(vmf, fpin);
> -		ractl._index &= ~((unsigned long)HPAGE_PMD_NR - 1);
> -		ra->size = HPAGE_PMD_NR;
> -		/*
> -		 * Fetch two PMD folios, so we get the chance to actually
> -		 * readahead, unless we've been told not to.
> -		 */
> -		if (!(vm_flags & VM_RAND_READ))
> -			ra->size *= 2;
> -		ra->async_size = HPAGE_PMD_NR;
> -		ra->order = HPAGE_PMD_ORDER;
> -		page_cache_ra_order(&ractl, ra);
> -		return fpin;
> -	}
> -#endif
> -
> +	if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) &&
> +	    (vm_flags & VM_HUGEPAGE) && HPAGE_PMD_ORDER <= MAX_PAGECACHE_ORDER)
> +		force_thp_readahead = true;
>  	/*
>  	 * If we don't want any read-ahead, don't bother. VM_EXEC case below is
>  	 * already intended for random access.
>  	 */
>  	if ((vm_flags & (VM_RAND_READ | VM_EXEC)) == VM_RAND_READ)
>  		return fpin;
> -	if (!ra->ra_pages)
> +	if (!ra->ra_pages && !force_thp_readahead)
>  		return fpin;
>  
> -	if (vm_flags & VM_SEQ_READ) {
> +	if ((vm_flags & VM_SEQ_READ) && !force_thp_readahead) {
>  		fpin = maybe_unlock_mmap_for_io(vmf, fpin);
>  		page_cache_sync_ra(&ractl, ra->ra_pages);
>  		return fpin;
> @@ -3283,6 +3269,22 @@ static struct file *do_sync_mmap_readahead(struct vm_fault *vmf)
>  	if (mmap_miss > MMAP_LOTSAMISS)
>  		return fpin;
>  
> +	if (force_thp_readahead) {
> +		fpin = maybe_unlock_mmap_for_io(vmf, fpin);
> +		ractl._index &= ~((unsigned long)HPAGE_PMD_NR - 1);
> +		ra->size = HPAGE_PMD_NR;
> +		/*
> +		 * Fetch two PMD folios, so we get the chance to actually
> +		 * readahead, unless we've been told not to.
> +		 */
> +		if (!(vm_flags & VM_RAND_READ))
> +			ra->size *= 2;
> +		ra->async_size = HPAGE_PMD_NR;
> +		ra->order = HPAGE_PMD_ORDER;
> +		page_cache_ra_order(&ractl, ra);
> +		return fpin;
> +	}
> +
>  	if (vm_flags & VM_EXEC) {
>  		/*
>  		 * Allow arch to request a preferred minimum folio order for
> -- 
> 2.51.0
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR