linux-kernel - Re: [PATCH] mm: split thp synchronously on MADV

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Date:   Sat, 20 Nov 2021 21:25:27 -0800
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     David Hildenbrand <david@...hat.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Yang Shi <shy828301@...il.com>, Zi Yan <ziy@...dia.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: split thp synchronously on MADV_DONTNEED

On Sat, Nov 20, 2021 at 8:35 PM Matthew Wilcox <willy@...radead.org> wrote:
>
> On Sat, Nov 20, 2021 at 12:12:30PM -0800, Shakeel Butt wrote:
> > This patch let such applications not worry about the low level handling
> > of THPs in the kernel and splits the THPs synchronously on
> > MADV_DONTNEED.
>
> I don't mind the synchronous split, but I have concerns about the
> implementation.  I don't think it's worth another pointer in task_struct.

Are you concerned about the size of task_struct? At least on my config
this additional pointer was just filling the holes and not increasing
the size. I can check a couple other configs as well.

> It's also the case that splitting is likely to succeed, so I think a
> better implementation would try to split and then put the page on the
> global deferred list if splitting fails.

Actually this is what this patch is doing. See the second loop in
split_local_deferred_list().