linux-kernel - Re: [PATCH v2 4/5] mm: introduce MADV

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Date:   Thu, 20 Jun 2019 13:16:20 +0900
From:   Minchan Kim <minchan@...nel.org>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>, linux-api@...r.kernel.org,
        Johannes Weiner <hannes@...xchg.org>,
        Tim Murray <timmurray@...gle.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Daniel Colascione <dancol@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Sonny Rao <sonnyrao@...gle.com>,
        Brian Geffon <bgeffon@...gle.com>, jannh@...gle.com,
        oleg@...hat.com, christian@...uner.io, oleksandr@...hat.com,
        hdanton@...a.com, lizeb@...gle.com
Subject: Re: [PATCH v2 4/5] mm: introduce MADV_PAGEOUT

On Wed, Jun 19, 2019 at 03:24:50PM +0200, Michal Hocko wrote:
> On Mon 10-06-19 20:12:51, Minchan Kim wrote:
> [...]
> > +static int madvise_pageout_pte_range(pmd_t *pmd, unsigned long addr,
> > +				unsigned long end, struct mm_walk *walk)
> 
> Again the same question about a potential code reuse...
> [...]
> > +regular_page:
> > +	tlb_change_page_size(tlb, PAGE_SIZE);
> > +	orig_pte = pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
> > +	flush_tlb_batched_pending(mm);
> > +	arch_enter_lazy_mmu_mode();
> > +	for (; addr < end; pte++, addr += PAGE_SIZE) {
> > +		ptent = *pte;
> > +		if (!pte_present(ptent))
> > +			continue;
> > +
> > +		page = vm_normal_page(vma, addr, ptent);
> > +		if (!page)
> > +			continue;
> > +
> > +		if (isolate_lru_page(page))
> > +			continue;
> > +
> > +		isolated++;
> > +		if (pte_young(ptent)) {
> > +			ptent = ptep_get_and_clear_full(mm, addr, pte,
> > +							tlb->fullmm);
> > +			ptent = pte_mkold(ptent);
> > +			set_pte_at(mm, addr, pte, ptent);
> > +			tlb_remove_tlb_entry(tlb, pte, addr);
> > +		}
> > +		ClearPageReferenced(page);
> > +		test_and_clear_page_young(page);
> > +		list_add(&page->lru, &page_list);
> > +		if (isolated >= SWAP_CLUSTER_MAX) {
> 
> Why do we need SWAP_CLUSTER_MAX batching? Especially when we need ...
> [...]

It aims for preventing early OOM kill since we isolate too many LRU
pages concurrently.

> 
> > +unsigned long reclaim_pages(struct list_head *page_list)
> > +{
> > +	int nid = -1;
> > +	unsigned long nr_reclaimed = 0;
> > +	LIST_HEAD(node_page_list);
> > +	struct reclaim_stat dummy_stat;
> > +	struct scan_control sc = {
> > +		.gfp_mask = GFP_KERNEL,
> > +		.priority = DEF_PRIORITY,
> > +		.may_writepage = 1,
> > +		.may_unmap = 1,
> > +		.may_swap = 1,
> > +	};
> > +
> > +	while (!list_empty(page_list)) {
> > +		struct page *page;
> > +
> > +		page = lru_to_page(page_list);
> > +		if (nid == -1) {
> > +			nid = page_to_nid(page);
> > +			INIT_LIST_HEAD(&node_page_list);
> > +		}
> > +
> > +		if (nid == page_to_nid(page)) {
> > +			list_move(&page->lru, &node_page_list);
> > +			continue;
> > +		}
> > +
> > +		nr_reclaimed += shrink_page_list(&node_page_list,
> > +						NODE_DATA(nid),
> > +						&sc, 0,
> > +						&dummy_stat, false);
> 
> per-node batching in fact. Other than that nothing really jumped at me.
> Except for the shared page cache side channel timing aspect not being
> considered AFAICS. To be more specific. Pushing out a shared page cache
> is possible even now but this interface gives a much easier tool to
> evict shared state and perform all sorts of timing attacks. Unless I am
> missing something we should be doing something similar to mincore and
> ignore shared pages without a writeable access or at least document why
> we do not care.

I'm not sure IIUC side channel attach. As you mentioned, without this syscall,
1. they already can do that simply by memory hogging
2. If we need fix MADV_PAGEOUT, that means we need to fix MADV_DONTNEED, too?