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    SubjectRe: [RFC PATCH v1 12/13] mm: split up release_pages into non-sentinel and sentinel passes
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    On 01/02/2018 00:04, daniel.m.jordan@oracle.com wrote:
    > A common case in release_pages is for the 'pages' list to be in roughly
    > the same order as they are in their LRU. With LRU batch locking, when a
    > sentinel page is removed, an adjacent non-sentinel page must be promoted
    > to a sentinel page to follow the locking scheme. So we can get behavior
    > where nearly every page in the 'pages' array is treated as a sentinel
    > page, hurting the scalability of this approach.
    >
    > To address this, split up release_pages into non-sentinel and sentinel
    > passes so that the non-sentinel pages can be locked with an LRU batch
    > lock before the sentinel pages are removed.
    >
    > For the prototype, just use a bitmap and a temporary outer loop to
    > implement this.
    >
    > Performance numbers from a single microbenchmark at this point in the
    > series are included in the next patch.
    >
    > Signed-off-by: Daniel Jordan <daniel.m.jordan@oracle.com>
    > ---
    > mm/swap.c | 20 +++++++++++++++++++-
    > 1 file changed, 19 insertions(+), 1 deletion(-)
    >
    > diff --git a/mm/swap.c b/mm/swap.c
    > index fae766e035a4..a302224293ad 100644
    > --- a/mm/swap.c
    > +++ b/mm/swap.c
    > @@ -731,6 +731,7 @@ void lru_add_drain_all(void)
    > put_online_cpus();
    > }
    >
    > +#define LRU_BITMAP_SIZE 512
    > /**
    > * release_pages - batched put_page()
    > * @pages: array of pages to release
    > @@ -742,16 +743,32 @@ void lru_add_drain_all(void)
    > */
    > void release_pages(struct page **pages, int nr)
    > {
    > - int i;
    > + int h, i;
    > LIST_HEAD(pages_to_free);
    > struct pglist_data *locked_pgdat = NULL;
    > spinlock_t *locked_lru_batch = NULL;
    > struct lruvec *lruvec;
    > unsigned long uninitialized_var(flags);
    > + DECLARE_BITMAP(lru_bitmap, LRU_BITMAP_SIZE);
    > +
    > + VM_BUG_ON(nr > LRU_BITMAP_SIZE);

    While running your series rebased on v4.15-mmotm-2018-01-31-16-51, I'm
    hitting this VM_BUG sometimes on a ppc64 system where page size is set to 64K.
    In my case, nr=537 while LRU_BITMAP_SIZE is 512. Here is the stack trace
    displayed :

    kernel BUG at /local/laurent/work/glinux/mm/swap.c:728!
    Oops: Exception in kernel mode, sig: 5 [#1]
    LE SMP NR_CPUS=2048 NUMA pSeries
    Modules linked in: pseries_rng rng_core vmx_crypto virtio_balloon ip_tables
    x_tables autofs4 virtio_net virtio_blk virtio_pci virtio_ring virtio
    CPU: 41 PID: 3485 Comm: cc1 Not tainted 4.15.0-mm1-lru+ #2
    NIP: c0000000002b0784 LR: c0000000002b0780 CTR: c0000000007bab20
    REGS: c0000005e126b740 TRAP: 0700 Not tainted (4.15.0-mm1-lru+)
    MSR: 8000000000029033 <SF,EE,ME,IR,DR,RI,LE> CR: 28002422 XER: 20000000
    CFAR: c000000000192ae4 SOFTE: 0
    GPR00: c0000000002b0780 c0000005e126b9c0 c00000000103c100 000000000000001c
    GPR04: c0000005ffc4ce38 c0000005ffc63d00 0000000000000000 0000000000000001
    GPR08: 0000000000000007 c000000000ec3a4c 00000005fed90000 0000000000000000
    GPR12: 0000000000002200 c00000000fd8cd00 0000000000000000 0000000000000000
    GPR16: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
    GPR20: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
    GPR24: c0000005e11ab980 0000000000000000 0000000000000000 c0000005e126ba60
    GPR28: 0000000000000219 c0000005e126bc40 0000000000000000 c0000005ec5f0000
    NIP [c0000000002b0784] release_pages+0x864/0x880
    LR [c0000000002b0780] release_pages+0x860/0x880
    Call Trace:
    [c0000005e126b9c0] [c0000000002b0780] release_pages+0x860/0x880 (unreliable)
    [c0000005e126bb30] [c00000000031da3c] free_pages_and_swap_cache+0x11c/0x150
    [c0000005e126bb80] [c0000000002ef5f8] tlb_flush_mmu_free+0x68/0xa0
    [c0000005e126bbc0] [c0000000002f1568] arch_tlb_finish_mmu+0x58/0xf0
    [c0000005e126bbf0] [c0000000002f19d4] tlb_finish_mmu+0x34/0x60
    [c0000005e126bc20] [c0000000003031e8] exit_mmap+0xd8/0x1d0
    [c0000005e126bce0] [c0000000000f3188] mmput+0x78/0x160
    [c0000005e126bd10] [c0000000000ff568] do_exit+0x348/0xd00
    [c0000005e126bdd0] [c0000000000fffd8] do_group_exit+0x58/0xd0
    [c0000005e126be10] [c00000000010006c] SyS_exit_group+0x1c/0x20
    [c0000005e126be30] [c00000000000ba60] system_call+0x58/0x6c
    Instruction dump:
    3949ffff 4bfffdc8 3c62ffce 38a00200 f9c100e0 f9e100e8 386345e8 fa0100f0
    fa2100f8 fa410100 4bee2329 60000000 <0fe00000> 3b400001 4bfff868 7d5d5378
    ---[ end trace 55b1651f9d92f14f ]---

    >
    > + bitmap_zero(lru_bitmap, nr);
    > +
    > + for (h = 0; h < 2; h++) {
    > for (i = 0; i < nr; i++) {
    > struct page *page = pages[i];
    >
    > + if (h == 0) {
    > + if (PageLRU(page) && page->lru_sentinel) {
    > + bitmap_set(lru_bitmap, i, 1);
    > + continue;
    > + }
    > + } else {
    > + if (!test_bit(i, lru_bitmap))
    > + continue;
    > + }
    > +
    > if (is_huge_zero_page(page))
    > continue;
    >
    > @@ -798,6 +815,7 @@ void release_pages(struct page **pages, int nr)
    >
    > list_add(&page->lru, &pages_to_free);
    > }
    > + }
    > if (locked_lru_batch) {
    > lru_batch_unlock(NULL, &locked_lru_batch, &locked_pgdat,
    > &flags);
    >

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