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

[Andrew Morton]

On Wed,  4 Dec 2024 19:09:40 +0800 Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx> wrote:

> 
> ...
>
> Previously, we tried to use a completely asynchronous method to reclaim empty
> user PTE pages [1]. After discussing with David Hildenbrand, we decided to
> implement synchronous reclaimation in the case of madvise(MADV_DONTNEED) as the
> first step.

Please help us understand what the other steps are.  Because we dont
want to commit to a particular partial implementation only to later
discover that completing that implementation causes us problems.

> So this series aims to synchronously free the empty PTE pages in
> madvise(MADV_DONTNEED) case. We will detect and free empty PTE pages in
> zap_pte_range(), and will add zap_details.reclaim_pt to exclude cases other than
> madvise(MADV_DONTNEED).
> 
> In zap_pte_range(), mmu_gather is used to perform batch tlb flushing and page
> freeing operations. Therefore, if we want to free the empty PTE page in this
> path, the most natural way is to add it to mmu_gather as well. Now, if
> CONFIG_MMU_GATHER_RCU_TABLE_FREE is selected, mmu_gather will free page table
> pages by semi RCU:
> 
>  - batch table freeing: asynchronous free by RCU
>  - single table freeing: IPI + synchronous free
> 
> 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?