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

[Qi Zheng]

On 2024/12/5 06:56, Jann Horn wrote:
> On Wed, Dec 4, 2024 at 11:49 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> > On Wed,  4 Dec 2024 19:09:40 +0800 Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx> 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!