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

[Jann Horn <jannh@google.com>]

On Wed, Dec 4, 2024 at 11:49 PM Andrew Morton <akpm@linux-foundation.org> wrote:
> On Wed,  4 Dec 2024 19:09:40 +0800 Qi Zheng <zhengqi.arch@bytedance.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.