linux-kernel - Re: [PATCH v4 00/11] synchronously scan and reclaim empty user PTE pages

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Message-ID: <CAG48ez19udcgNARKsizFbbnO6ATFGTSX2KEd2D5aC2Wnxy+Jwg@mail.gmail.com>
Date: Wed, 4 Dec 2024 23:56:49 +0100
From: Jann Horn <jannh@...gle.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Qi Zheng <zhengqi.arch@...edance.com>, david@...hat.com, hughd@...gle.com, 
	willy@...radead.org, muchun.song@...ux.dev, vbabka@...nel.org, 
	peterx@...hat.com, mgorman@...e.de, catalin.marinas@....com, will@...nel.org, 
	dave.hansen@...ux.intel.com, luto@...nel.org, peterz@...radead.org, 
	x86@...nel.org, lorenzo.stoakes@...cle.com, zokeefe@...gle.com, 
	rientjes@...gle.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 00/11] synchronously scan and reclaim empty user PTE pages

On Wed, Dec 4, 2024 at 11:49 PM Andrew Morton <akpm@...ux-foundation.org> wrote:
> On Wed,  4 Dec 2024 19:09:40 +0800 Qi Zheng <zhengqi.arch@...edance.com> wrote:
> > But this is not enough to free the empty PTE page table pages in paths other
> > that munmap and exit_mmap path, because IPI cannot be synchronized with
> > rcu_read_lock() in pte_offset_map{_lock}(). So we should let single table also
> > be freed by RCU like batch table freeing.
> >
> > As a first step, we supported this feature on x86_64 and selectd the newly
> > introduced CONFIG_ARCH_SUPPORTS_PT_RECLAIM.
> >
> > For other cases such as madvise(MADV_FREE), consider scanning and freeing empty
> > PTE pages asynchronously in the future.
>
> Handling MADV_FREE sounds fairly straightforward?

AFAIU MADV_FREE usually doesn't immediately clear PTEs (except if they
are swap/hwpoison/... PTEs). So the easy thing to do would be to check
whether the page table has become empty within madvise(), but I think
the most likely case would be that PTEs still remain (and will be
asynchronously zapped later when memory pressure causes reclaim, or
something like that).

So I don't see an easy path to doing it for MADV_FREE.