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SubjectRe: [RFC 3/7] mm: introduce MADV_COLD
On Mon, May 20, 2019 at 10:27:03AM +0200, Michal Hocko wrote:
> [Cc linux-api]
>
> On Mon 20-05-19 12:52:50, Minchan Kim wrote:
> > When a process expects no accesses to a certain memory range
> > for a long time, it could hint kernel that the pages can be
> > reclaimed instantly but data should be preserved for future use.
> > This could reduce workingset eviction so it ends up increasing
> > performance.
> >
> > This patch introduces the new MADV_COLD hint to madvise(2)
> > syscall. MADV_COLD can be used by a process to mark a memory range
> > as not expected to be used for a long time. The hint can help
> > kernel in deciding which pages to evict proactively.
>
> As mentioned in other email this looks like a non-destructive
> MADV_DONTNEED alternative.
>
> > Internally, it works via reclaiming memory in process context
> > the syscall is called. If the page is dirty but backing storage
> > is not synchronous device, the written page will be rotate back
> > into LRU's tail once the write is done so they will reclaim easily
> > when memory pressure happens. If backing storage is
> > synchrnous device(e.g., zram), hte page will be reclaimed instantly.
>
> Why do we special case async backing storage? Please always try to
> explain _why_ the decision is made.

I didn't make any decesion. ;-) That's how current reclaim works to
avoid latency of freeing page in interrupt context. I had a patchset
to resolve the concern a few years ago but got distracted.

>
> I haven't checked the implementation yet so I cannot comment on that.
>
> > Signed-off-by: Minchan Kim <minchan@kernel.org>
> > ---
> > include/linux/swap.h | 1 +
> > include/uapi/asm-generic/mman-common.h | 1 +
> > mm/madvise.c | 123 +++++++++++++++++++++++++
> > mm/vmscan.c | 74 +++++++++++++++
> > 4 files changed, 199 insertions(+)
> >
> > diff --git a/include/linux/swap.h b/include/linux/swap.h
> > index 64795abea003..7f32a948fc6a 100644
> > --- a/include/linux/swap.h
> > +++ b/include/linux/swap.h
> > @@ -365,6 +365,7 @@ extern int vm_swappiness;
> > extern int remove_mapping(struct address_space *mapping, struct page *page);
> > extern unsigned long vm_total_pages;
> >
> > +extern unsigned long reclaim_pages(struct list_head *page_list);
> > #ifdef CONFIG_NUMA
> > extern int node_reclaim_mode;
> > extern int sysctl_min_unmapped_ratio;
> > diff --git a/include/uapi/asm-generic/mman-common.h b/include/uapi/asm-generic/mman-common.h
> > index f7a4a5d4b642..b9b51eeb8e1a 100644
> > --- a/include/uapi/asm-generic/mman-common.h
> > +++ b/include/uapi/asm-generic/mman-common.h
> > @@ -43,6 +43,7 @@
> > #define MADV_WILLNEED 3 /* will need these pages */
> > #define MADV_DONTNEED 4 /* don't need these pages */
> > #define MADV_COOL 5 /* deactivatie these pages */
> > +#define MADV_COLD 6 /* reclaim these pages */
> >
> > /* common parameters: try to keep these consistent across architectures */
> > #define MADV_FREE 8 /* free pages only if memory pressure */
> > diff --git a/mm/madvise.c b/mm/madvise.c
> > index c05817fb570d..9a6698b56845 100644
> > --- a/mm/madvise.c
> > +++ b/mm/madvise.c
> > @@ -42,6 +42,7 @@ static int madvise_need_mmap_write(int behavior)
> > case MADV_WILLNEED:
> > case MADV_DONTNEED:
> > case MADV_COOL:
> > + case MADV_COLD:
> > case MADV_FREE:
> > return 0;
> > default:
> > @@ -416,6 +417,125 @@ static long madvise_cool(struct vm_area_struct *vma,
> > return 0;
> > }
> >
> > +static int madvise_cold_pte_range(pmd_t *pmd, unsigned long addr,
> > + unsigned long end, struct mm_walk *walk)
> > +{
> > + pte_t *orig_pte, *pte, ptent;
> > + spinlock_t *ptl;
> > + LIST_HEAD(page_list);
> > + struct page *page;
> > + int isolated = 0;
> > + struct vm_area_struct *vma = walk->vma;
> > + unsigned long next;
> > +
> > + next = pmd_addr_end(addr, end);
> > + if (pmd_trans_huge(*pmd)) {
> > + spinlock_t *ptl;
> > +
> > + ptl = pmd_trans_huge_lock(pmd, vma);
> > + if (!ptl)
> > + return 0;
> > +
> > + if (is_huge_zero_pmd(*pmd))
> > + goto huge_unlock;
> > +
> > + page = pmd_page(*pmd);
> > + if (page_mapcount(page) > 1)
> > + goto huge_unlock;
> > +
> > + if (next - addr != HPAGE_PMD_SIZE) {
> > + int err;
> > +
> > + get_page(page);
> > + spin_unlock(ptl);
> > + lock_page(page);
> > + err = split_huge_page(page);
> > + unlock_page(page);
> > + put_page(page);
> > + if (!err)
> > + goto regular_page;
> > + return 0;
> > + }
> > +
> > + if (isolate_lru_page(page))
> > + goto huge_unlock;
> > +
> > + list_add(&page->lru, &page_list);
> > +huge_unlock:
> > + spin_unlock(ptl);
> > + reclaim_pages(&page_list);
> > + return 0;
> > + }
> > +
> > + if (pmd_trans_unstable(pmd))
> > + return 0;
> > +regular_page:
> > + orig_pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
> > + for (pte = orig_pte; addr < end; pte++, addr += PAGE_SIZE) {
> > + ptent = *pte;
> > + if (!pte_present(ptent))
> > + continue;
> > +
> > + page = vm_normal_page(vma, addr, ptent);
> > + if (!page)
> > + continue;
> > +
> > + if (page_mapcount(page) > 1)
> > + continue;
> > +
> > + if (isolate_lru_page(page))
> > + continue;
> > +
> > + isolated++;
> > + list_add(&page->lru, &page_list);
> > + if (isolated >= SWAP_CLUSTER_MAX) {
> > + pte_unmap_unlock(orig_pte, ptl);
> > + reclaim_pages(&page_list);
> > + isolated = 0;
> > + pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
> > + orig_pte = pte;
> > + }
> > + }
> > +
> > + pte_unmap_unlock(orig_pte, ptl);
> > + reclaim_pages(&page_list);
> > + cond_resched();
> > +
> > + return 0;
> > +}
> > +
> > +static void madvise_cold_page_range(struct mmu_gather *tlb,
> > + struct vm_area_struct *vma,
> > + unsigned long addr, unsigned long end)
> > +{
> > + struct mm_walk warm_walk = {
> > + .pmd_entry = madvise_cold_pte_range,
> > + .mm = vma->vm_mm,
> > + };
> > +
> > + tlb_start_vma(tlb, vma);
> > + walk_page_range(addr, end, &warm_walk);
> > + tlb_end_vma(tlb, vma);
> > +}
> > +
> > +
> > +static long madvise_cold(struct vm_area_struct *vma,
> > + unsigned long start_addr, unsigned long end_addr)
> > +{
> > + struct mm_struct *mm = vma->vm_mm;
> > + struct mmu_gather tlb;
> > +
> > + if (vma->vm_flags & (VM_LOCKED|VM_HUGETLB|VM_PFNMAP))
> > + return -EINVAL;
> > +
> > + lru_add_drain();
> > + tlb_gather_mmu(&tlb, mm, start_addr, end_addr);
> > + madvise_cold_page_range(&tlb, vma, start_addr, end_addr);
> > + tlb_finish_mmu(&tlb, start_addr, end_addr);
> > +
> > + return 0;
> > +}
> > +
> > static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
> > unsigned long end, struct mm_walk *walk)
> >
> > @@ -806,6 +926,8 @@ madvise_vma(struct vm_area_struct *vma, struct vm_area_struct **prev,
> > return madvise_willneed(vma, prev, start, end);
> > case MADV_COOL:
> > return madvise_cool(vma, start, end);
> > + case MADV_COLD:
> > + return madvise_cold(vma, start, end);
> > case MADV_FREE:
> > case MADV_DONTNEED:
> > return madvise_dontneed_free(vma, prev, start, end, behavior);
> > @@ -828,6 +950,7 @@ madvise_behavior_valid(int behavior)
> > case MADV_DONTNEED:
> > case MADV_FREE:
> > case MADV_COOL:
> > + case MADV_COLD:
> > #ifdef CONFIG_KSM
> > case MADV_MERGEABLE:
> > case MADV_UNMERGEABLE:
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index a28e5d17b495..1701b31f70a8 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -2096,6 +2096,80 @@ static void shrink_active_list(unsigned long nr_to_scan,
> > nr_deactivate, nr_rotated, sc->priority, file);
> > }
> >
> > +unsigned long reclaim_pages(struct list_head *page_list)
> > +{
> > + int nid = -1;
> > + unsigned long nr_isolated[2] = {0, };
> > + 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);
> > + list_del(&page->lru);
> > +
> > + if (nid == -1) {
> > + nid = page_to_nid(page);
> > + INIT_LIST_HEAD(&node_page_list);
> > + nr_isolated[0] = nr_isolated[1] = 0;
> > + }
> > +
> > + if (nid == page_to_nid(page)) {
> > + list_add(&page->lru, &node_page_list);
> > + nr_isolated[!!page_is_file_cache(page)] +=
> > + hpage_nr_pages(page);
> > + continue;
> > + }
> > +
> > + nid = page_to_nid(page);
> > +
> > + mod_node_page_state(NODE_DATA(nid), NR_ISOLATED_ANON,
> > + nr_isolated[0]);
> > + mod_node_page_state(NODE_DATA(nid), NR_ISOLATED_FILE,
> > + nr_isolated[1]);
> > + nr_reclaimed += shrink_page_list(&node_page_list,
> > + NODE_DATA(nid), &sc, TTU_IGNORE_ACCESS,
> > + &dummy_stat, true);
> > + while (!list_empty(&node_page_list)) {
> > + struct page *page = lru_to_page(page_list);
> > +
> > + list_del(&page->lru);
> > + putback_lru_page(page);
> > + }
> > + mod_node_page_state(NODE_DATA(nid), NR_ISOLATED_ANON,
> > + -nr_isolated[0]);
> > + mod_node_page_state(NODE_DATA(nid), NR_ISOLATED_FILE,
> > + -nr_isolated[1]);
> > + nr_isolated[0] = nr_isolated[1] = 0;
> > + INIT_LIST_HEAD(&node_page_list);
> > + }
> > +
> > + if (!list_empty(&node_page_list)) {
> > + mod_node_page_state(NODE_DATA(nid), NR_ISOLATED_ANON,
> > + nr_isolated[0]);
> > + mod_node_page_state(NODE_DATA(nid), NR_ISOLATED_FILE,
> > + nr_isolated[1]);
> > + nr_reclaimed += shrink_page_list(&node_page_list,
> > + NODE_DATA(nid), &sc, TTU_IGNORE_ACCESS,
> > + &dummy_stat, true);
> > + mod_node_page_state(NODE_DATA(nid), NR_ISOLATED_ANON,
> > + -nr_isolated[0]);
> > + mod_node_page_state(NODE_DATA(nid), NR_ISOLATED_FILE,
> > + -nr_isolated[1]);
> > + }
> > +
> > + return nr_reclaimed;
> > +}
> > +
> > /*
> > * The inactive anon list should be small enough that the VM never has
> > * to do too much work.
> > --
> > 2.21.0.1020.gf2820cf01a-goog
> >
>
> --
> Michal Hocko
> SUSE Labs

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