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

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Message-ID: <aN0i107IF0oQ_PQb@google.com>
Date: Wed, 1 Oct 2025 12:47:19 +0000
From: Roman Gushchin <roman.gushchin@...ux.dev>
To: Jan Kara <jack@...e.cz>
Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-kernel@...r.kernel.org,
	"Matthew Wilcox (Oracle)" <willy@...radead.org>, linux-mm@...ck.org
Subject: Re: [PATCH] mm: readahead: make thp readahead conditional to
 mmap_miss logic

On Wed, Oct 01, 2025 at 01:35:39PM +0200, Jan Kara wrote:
> On Tue 30-09-25 07:48:15, 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
> 
> It would be good to get confirmation from Matthew that indeed this
> preserves what he had in mind with commit 4687fdbb805a92 but the change
> looks good to me.

Hi Jan!

Matthew and myself had a chat about this issue last week at Kernel Recipes
conference and in general agreed on this approach. But of course,
an explicit Ack from him will be appreciated.

Long-term it would be great to use a better metric for memory pressure
here, e.g. PSI. But it's far from trivial.

> Feel free to add:
> 
> Reviewed-by: Jan Kara <jack@...e.cz>

Thank you!