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

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Message-ID: <4d3f1078-0938-4920-9a09-805a54643419@bytedance.com>
Date: Thu, 5 Dec 2024 11:59:21 +0800
From: Qi Zheng <zhengqi.arch@...edance.com>
To: Jann Horn <jannh@...gle.com>, Andrew Morton <akpm@...ux-foundation.org>
Cc: 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 2024/12/5 06:56, Jann Horn wrote:
> 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.

+1. Thanks for helping explain!