I don't know why this was addressed to me in particular (easy to imagine
I've made a mod at some time that bears on this, but I haven't found it);
but have spent longer considering the patch than I should have done -
apologies to everyone else I should be replying to.
On Wed, 4 Nov 2020, Shakeel Butt wrote:
> Since the commit 369ea8242c0f ("mm/rmap: update to new mmu_notifier
> semantic v2"), the code to check the secondary MMU's page table access
> bit is broken for !(TTU_IGNORE_ACCESS) because the page is unmapped from
> the secondary MMU's page table before the check. More specifically for
> those secondary MMUs which unmap the memory in
> mmu_notifier_invalidate_range_start() like kvm.
Well, "broken" seems a bit unfair to 369ea8242c0f. It put a warning
mmu_notifier_invalidate_range_start() at the beginning, and matching
mmu_notifier_invalidate_range_end() at the end of try_to_unmap_one();
with its mmu_notifier_invalidate_range() exactly where the
mmu_notifier_invalidate_page() was before (I think the story gets
more complicated later). Yes, if notifiee takes invalidate_range_start()
as signal to invalidate all their own range, then that will sometimes
cause them unnecessary invalidations.
Not just for !TTU_IGNORE_ACCESS: there's also the !TTU_IGNORE_MLOCK
case meeting a VM_LOCKED vma and setting PageMlocked where that had
been missed earlier (and page_check_references() has intentionally but
confusingly marked this case as PAGEREF_RECLAIM, not to reclaim the page,
but to reach the try_to_unmap_one() which will recognize and fix it up -
historically easier to do there than in page_referenced_one()).
But I think mmu_notifier is a diversion from what needs thinking about.
>
> However memory reclaim is the only user of !(TTU_IGNORE_ACCESS) or the
> absence of TTU_IGNORE_ACCESS and it explicitly performs the page table
> access check before trying to unmap the page. So, at worst the reclaim
> will miss accesses in a very short window if we remove page table access
> check in unmapping code.
I agree with you and Johannes that the short race window when the page
might be re-referenced is no issue at all: the functional issue is the
one in your next paragraph. If that's agreed by memcg guys, great,
then this patch is a nice observation and a welcome cleanup.
>
> There is an unintented consequence of !(TTU_IGNORE_ACCESS) for the memcg
> reclaim. From memcg reclaim the page_referenced() only account the
> accesses from the processes which are in the same memcg of the target
> page but the unmapping code is considering accesses from all the
> processes, so, decreasing the effectiveness of memcg reclaim.
Are you sure it was unintended?
Since the dawn of memcg reclaim, it has been the case that a recent
reference in a "foreign" vma has rescued that page from being reclaimed:
now you propose to change that. I expect some workflows will benefit
and others be disadvantaged. I have no objection myself to the change,
but I do think it needs to be better highlighted here, and explicitly
agreed by those more familiar with memcg reclaim.
Hugh
>
> The simplest solution is to always assume TTU_IGNORE_ACCESS in unmapping
> code.
>
> Signed-off-by: Shakeel Butt <[email protected]>
> ---
> include/linux/rmap.h | 1 -
> mm/huge_memory.c | 2 +-
> mm/memory-failure.c | 2 +-
> mm/memory_hotplug.c | 2 +-
> mm/migrate.c | 8 +++-----
> mm/rmap.c | 9 ---------
> mm/vmscan.c | 14 +++++---------
> 7 files changed, 11 insertions(+), 27 deletions(-)
>
> diff --git a/include/linux/rmap.h b/include/linux/rmap.h
> index 3a6adfa70fb0..70085ca1a3fc 100644
> --- a/include/linux/rmap.h
> +++ b/include/linux/rmap.h
> @@ -91,7 +91,6 @@ enum ttu_flags {
>
> TTU_SPLIT_HUGE_PMD = 0x4, /* split huge PMD if any */
> TTU_IGNORE_MLOCK = 0x8, /* ignore mlock */
> - TTU_IGNORE_ACCESS = 0x10, /* don't age */
> TTU_IGNORE_HWPOISON = 0x20, /* corrupted page is recoverable */
> TTU_BATCH_FLUSH = 0x40, /* Batch TLB flushes where possible
> * and caller guarantees they will
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 08a183f6c3ab..8b235b4abf73 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2324,7 +2324,7 @@ void vma_adjust_trans_huge(struct vm_area_struct *vma,
>
> static void unmap_page(struct page *page)
> {
> - enum ttu_flags ttu_flags = TTU_IGNORE_MLOCK | TTU_IGNORE_ACCESS |
> + enum ttu_flags ttu_flags = TTU_IGNORE_MLOCK |
> TTU_RMAP_LOCKED | TTU_SPLIT_HUGE_PMD;
> bool unmap_success;
>
> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> index c0bb186bba62..b6d6d5cdb435 100644
> --- a/mm/memory-failure.c
> +++ b/mm/memory-failure.c
> @@ -989,7 +989,7 @@ static int get_hwpoison_page(struct page *page)
> static bool hwpoison_user_mappings(struct page *p, unsigned long pfn,
> int flags, struct page **hpagep)
> {
> - enum ttu_flags ttu = TTU_IGNORE_MLOCK | TTU_IGNORE_ACCESS;
> + enum ttu_flags ttu = TTU_IGNORE_MLOCK;
> struct address_space *mapping;
> LIST_HEAD(tokill);
> bool unmap_success = true;
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 8c1b7182bb08..968e6916d297 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1303,7 +1303,7 @@ do_migrate_range(unsigned long start_pfn, unsigned long
> end_pfn)
> if (WARN_ON(PageLRU(page)))
> isolate_lru_page(page);
> if (page_mapped(page))
> - try_to_unmap(page, TTU_IGNORE_MLOCK |
> TTU_IGNORE_ACCESS);
> + try_to_unmap(page, TTU_IGNORE_MLOCK);
> continue;
> }
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index c1585ec29827..e434d49fd428 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1122,8 +1122,7 @@ static int __unmap_and_move(struct page *page, struct
> page *newpage,
> /* Establish migration ptes */
> VM_BUG_ON_PAGE(PageAnon(page) && !PageKsm(page) && !anon_vma,
> page);
> - try_to_unmap(page,
> - TTU_MIGRATION|TTU_IGNORE_MLOCK|TTU_IGNORE_ACCESS);
> + try_to_unmap(page, TTU_MIGRATION|TTU_IGNORE_MLOCK);
> page_was_mapped = 1;
> }
>
> @@ -1339,8 +1338,7 @@ static int unmap_and_move_huge_page(new_page_t
> get_new_page,
> goto unlock_put_anon;
>
> try_to_unmap(hpage,
> - TTU_MIGRATION|TTU_IGNORE_MLOCK|TTU_IGNORE_ACCESS|
> - TTU_RMAP_LOCKED);
> + TTU_MIGRATION|TTU_IGNORE_MLOCK|TTU_RMAP_LOCKED);
> page_was_mapped = 1;
> /*
> * Leave mapping locked until after subsequent call to
> @@ -2684,7 +2682,7 @@ static void migrate_vma_prepare(struct migrate_vma
> *migrate)
> */
> static void migrate_vma_unmap(struct migrate_vma *migrate)
> {
> - int flags = TTU_MIGRATION | TTU_IGNORE_MLOCK | TTU_IGNORE_ACCESS;
> + int flags = TTU_MIGRATION | TTU_IGNORE_MLOCK;
> const unsigned long npages = migrate->npages;
> const unsigned long start = migrate->start;
> unsigned long addr, i, restore = 0;
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 1b84945d655c..6cd9d4512117 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1536,15 +1536,6 @@ static bool try_to_unmap_one(struct page *page, struct
> vm_area_struct *vma,
> goto discard;
> }
>
> - if (!(flags & TTU_IGNORE_ACCESS)) {
> - if (ptep_clear_flush_young_notify(vma, address,
> - pvmw.pte)) {
> - ret = false;
> - page_vma_mapped_walk_done(&pvmw);
> - break;
> - }
> - }
> -
> /* Nuke the page table entry. */
> flush_cache_page(vma, address, pte_pfn(*pvmw.pte));
> if (should_defer_flush(mm, flags)) {
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index a8611dce7a95..e71b563cda7b 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1072,7 +1072,6 @@ static void page_check_dirty_writeback(struct page
> *page,
> static unsigned int shrink_page_list(struct list_head *page_list,
> struct pglist_data *pgdat,
> struct scan_control *sc,
> - enum ttu_flags ttu_flags,
> struct reclaim_stat *stat,
> bool ignore_references)
> {
> @@ -1297,7 +1296,7 @@ static unsigned int shrink_page_list(struct list_head
> *page_list,
> * processes. Try to unmap it here.
> */
> if (page_mapped(page)) {
> - enum ttu_flags flags = ttu_flags | TTU_BATCH_FLUSH;
> + enum ttu_flags flags = TTU_BATCH_FLUSH;
> bool was_swapbacked = PageSwapBacked(page);
>
> if (unlikely(PageTransHuge(page)))
> @@ -1514,7 +1513,7 @@ unsigned int reclaim_clean_pages_from_list(struct zone
> *zone,
> }
>
> nr_reclaimed = shrink_page_list(&clean_pages, zone->zone_pgdat, &sc,
> - TTU_IGNORE_ACCESS, &stat, true);
> + &stat, true);
> list_splice(&clean_pages, page_list);
> mod_node_page_state(zone->zone_pgdat, NR_ISOLATED_FILE,
> -(long)nr_reclaimed);
> @@ -1958,8 +1957,7 @@ shrink_inactive_list(unsigned long nr_to_scan, struct
> lruvec *lruvec,
> if (nr_taken == 0)
> return 0;
>
> - nr_reclaimed = shrink_page_list(&page_list, pgdat, sc, 0,
> - &stat, false);
> + nr_reclaimed = shrink_page_list(&page_list, pgdat, sc, &stat, false);
>
> spin_lock_irq(&pgdat->lru_lock);
>
> @@ -2131,8 +2129,7 @@ unsigned long reclaim_pages(struct list_head *page_list)
>
> nr_reclaimed += shrink_page_list(&node_page_list,
> NODE_DATA(nid),
> - &sc, 0,
> - &dummy_stat, false);
> + &sc, &dummy_stat, false);
> while (!list_empty(&node_page_list)) {
> page = lru_to_page(&node_page_list);
> list_del(&page->lru);
> @@ -2145,8 +2142,7 @@ unsigned long reclaim_pages(struct list_head *page_list)
> if (!list_empty(&node_page_list)) {
> nr_reclaimed += shrink_page_list(&node_page_list,
> NODE_DATA(nid),
> - &sc, 0,
> - &dummy_stat, false);
> + &sc, &dummy_stat, false);
> while (!list_empty(&node_page_list)) {
> page = lru_to_page(&node_page_list);
> list_del(&page->lru);
> --
> 2.29.1.341.ge80a0c044ae-goog
