linux-kernel - Re: [PATCH v3] mm: readahead: make thp readahead conditional to mmap

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Message-Id: <20251007153424.1b46a4018e17d5dafd7fb126@linux-foundation.org>
Date: Tue, 7 Oct 2025 15:34:24 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Roman Gushchin <roman.gushchin@...ux.dev>
Cc: linux-kernel@...r.kernel.org, "Matthew Wilcox (Oracle)"
 <willy@...radead.org>, Jan Kara <jack@...e.cz>, Dev Jain
 <dev.jain@....com>, linux-mm@...ck.org
Subject: Re: [PATCH v3] mm: readahead: make thp readahead conditional to
 mmap_miss logic

On Mon,  6 Oct 2025 10:51:06 -0700 Roman Gushchin <roman.gushchin@...ux.dev> 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.

Three years ago.

> This change causes a significant regression

So no backport suggested?  I guess reasonable given how long 4687fdbb805a has
been in tree.

>
> ...
>

> 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.
>
> ...
>
> Signed-off-by: Roman Gushchin <roman.gushchin@...ux.dev>
> Cc: Matthew Wilcox (Oracle) <willy@...radead.org>
> Cc: Jan Kara <jack@...e.cz>
> Cc: Dev Jain <dev.jain@....com>

But I'll slap the Fixes: in there, it might help someone.