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

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Message-ID: <87play1fzy.fsf@linux.dev>
Date: Tue, 07 Oct 2025 15:52:49 -0700
From: Roman Gushchin <roman.gushchin@...ux.dev>
To: Andrew Morton <akpm@...ux-foundation.org>
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

Andrew Morton <akpm@...ux-foundation.org> writes:

> 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.

Yes, this was my thinking. Also you need a very specific setup to reveal
this regression.

>> 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.

I'd do exactly what you suggested: Fixes + no stable backport.

But I guess it still might end up in the LTS tree thanks to
the automation picking up all fixes. Should be ok too.

Thanks!