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

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Message-ID: <mxhut4yy3xqqv2gchlakiqu4bhjzbmu4bwl7xtdrwqqygcgwxk@cggnjvpola7g>
Date: Mon, 6 Oct 2025 10:20:03 +0200
From: Jan Kara <jack@...e.cz>
To: Dev Jain <dev.jain@....com>
Cc: Roman Gushchin <roman.gushchin@...ux.dev>, 
	Andrew Morton <akpm@...ux-foundation.org>, linux-kernel@...r.kernel.org, 
	"Matthew Wilcox (Oracle)" <willy@...radead.org>, Jan Kara <jack@...e.cz>, linux-mm@...ck.org
Subject: Re: [PATCH] mm: readahead: make thp readahead conditional to
 mmap_miss logic

On Sat 04-10-25 18:38:25, Dev Jain wrote:
> 
> On 30/09/25 11:18 am, 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>
> > Cc: Matthew Wilcox (Oracle) <willy@...radead.org>
> > Cc: Jan Kara <jack@...e.cz>
> > Cc: linux-mm@...ck.org
> > ---
> >   mm/filemap.c | 40 +++++++++++++++++++++-------------------
> >   1 file changed, 21 insertions(+), 19 deletions(-)
> > 
> > diff --git a/mm/filemap.c b/mm/filemap.c
> > index a52dd38d2b4a..b67d7981fafb 100644
> > --- a/mm/filemap.c
> > +++ b/mm/filemap.c
> > @@ -3235,34 +3235,20 @@ 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) {
> > @@ -3283,6 +3269,22 @@ static struct file *do_sync_mmap_readahead(struct vm_fault *vmf)
> >   	if (mmap_miss > MMAP_LOTSAMISS)
> >   		return fpin;
> 
> You have moved the PMD-THP logic below the VM_SEQ_READ check, is that intentional?
> So VMAs on which sequential read is expected will now use the common readahead algorithm,
> instead of always benefitting from reduced TLB pressure through PMD mapping, if my understanding
> is correct?

Hum, that's a good point. We should preserve the logic for VM_SEQ_READ
vmas. I've missed this during my review. Thanks for catching this.

								Honza

> 
> > +	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
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR