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SubjectRe: [PATCH v12 12/14] mm: multi-gen LRU: debugfs interface
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On 2022/6/14 15:16, Yu Zhao wrote:
> Add /sys/kernel/debug/lru_gen for working set estimation and proactive
> reclaim. These techniques are commonly used to optimize job scheduling
> (bin packing) in data centers [1][2].
>
> Compared with the page table-based approach and the PFN-based
> approach, this lruvec-based approach has the following advantages:
> 1. It offers better choices because it is aware of memcgs, NUMA nodes,
> shared mappings and unmapped page cache.
> 2. It is more scalable because it is O(nr_hot_pages), whereas the
> PFN-based approach is O(nr_total_pages).
>
> Add /sys/kernel/debug/lru_gen_full for debugging.
>
> [1] https://dl.acm.org/doi/10.1145/3297858.3304053
> [2] https://dl.acm.org/doi/10.1145/3503222.3507731
>
> Signed-off-by: Yu Zhao <yuzhao@google.com>
> Acked-by: Brian Geffon <bgeffon@google.com>
> Acked-by: Jan Alexander Steffens (heftig) <heftig@archlinux.org>
> Acked-by: Oleksandr Natalenko <oleksandr@natalenko.name>
> Acked-by: Steven Barrett <steven@liquorix.net>
> Acked-by: Suleiman Souhlal <suleiman@google.com>
> Tested-by: Daniel Byrne <djbyrne@mtu.edu>
> Tested-by: Donald Carr <d@chaos-reins.com>
> Tested-by: Holger Hoffstätte <holger@applied-asynchrony.com>
> Tested-by: Konstantin Kharlamov <Hi-Angel@yandex.ru>
> Tested-by: Shuang Zhai <szhai2@cs.rochester.edu>
> Tested-by: Sofia Trinh <sofia.trinh@edi.works>
> Tested-by: Vaibhav Jain <vaibhav@linux.ibm.com>
> ---
> include/linux/nodemask.h | 1 +
> mm/vmscan.c | 412 ++++++++++++++++++++++++++++++++++++++-
> 2 files changed, 403 insertions(+), 10 deletions(-)
>

Hi Yu,

> +static ssize_t lru_gen_seq_write(struct file *file, const char __user *src,
> + size_t len, loff_t *pos)
> +{
> + void *buf;
> + char *cur, *next;
> + unsigned int flags;
> + struct blk_plug plug;
> + int err = -EINVAL;
> + struct scan_control sc = {
> + .may_writepage = true,
> + .may_unmap = true,
> + .may_swap = true,
> + .reclaim_idx = MAX_NR_ZONES - 1,
> + .gfp_mask = GFP_KERNEL,
> + };
> +
> + buf = kvmalloc(len + 1, GFP_KERNEL);
> + if (!buf)
> + return -ENOMEM;
> +
> + if (copy_from_user(buf, src, len)) {
> + kvfree(buf);
> + return -EFAULT;
> + }
> +
> + if (!set_mm_walk(NULL)) {

The current->reclaim_state will be dereferenced in set_mm_walk(), so
calling set_mm_walk() before set_task_reclaim_state(current,
&sc.reclaim_state) will cause panic:

[ 1861.154916] BUG: kernel NULL pointer dereference, address:
0000000000000008
[ 1861.155720] #PF: supervisor read access in kernel mode
[ 1861.156263] #PF: error_code(0x0000) - not-present page
[ 1861.156805] PGD 0 P4D 0
[ 1861.157107] Oops: 0000 [#1] PREEMPT SMP PTI
[ 1861.157560] CPU: 5 PID: 1017 Comm: bash Not tainted 5.19.0-rc2+ #244
[ 1861.158227] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996),
BIOS rel-1.14.0-0-g14
[ 1861.159419] RIP: 0010:set_mm_walk+0x15/0x60
[ 1861.159878] Code: e8 30 5f 01 00 48 c7 43 70 00 00 00 00 5b c3 31 f6
eb e2 66 90 0f 1f f
[ 1861.161806] RSP: 0018:ffffc90006dd3d58 EFLAGS: 00010246
[ 1861.162356] RAX: 0000000000000000 RBX: 00005582747a70b0 RCX:
0000000000000000
[ 1861.163109] RDX: ffff88810a198000 RSI: 00005582747a70c1 RDI:
0000000000000000
[ 1861.163855] RBP: ffff888104f4e400 R08: 0000000000000000 R09:
ffff888100042400
[ 1861.164597] R10: 0000000000000000 R11: 0000000000000000 R12:
ffff888685896fc0
[ 1861.165334] R13: 00005582747a70b0 R14: ffff888103ef2e40 R15:
0000000000000011
[ 1861.166083] FS: 00007f843df57740(0000) GS:ffff888666b40000(0000)
knlGS:0000000000000000
[ 1861.166921] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 1861.167527] CR2: 0000000000000008 CR3: 0000000684e7e000 CR4:
00000000000006e0
[ 1861.168272] DR0: 0000000000000000 DR1: 0000000000000000 DR2:
0000000000000000
[ 1861.169020] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7:
0000000000000400
[ 1861.169867] Call Trace:
[ 1861.170159] <TASK>
[ 1861.170396] lru_gen_seq_write+0xbf/0x600
[ 1861.170837] ? _raw_spin_unlock+0x15/0x30
[ 1861.171272] ? wp_page_reuse+0x5f/0x70
[ 1861.171678] ? do_wp_page+0xda/0x3e0
[ 1861.172063] ? __handle_mm_fault+0x92f/0xeb0
[ 1861.172529] full_proxy_write+0x4d/0x70
[ 1861.172941] vfs_write+0xb8/0x2a0
[ 1861.173302] ksys_write+0x59/0xd0
[ 1861.173667] do_syscall_64+0x34/0x80
[ 1861.174055] entry_SYSCALL_64_after_hwframe+0x46/0xb0

> + kvfree(buf);
> + return -ENOMEM;
> + }
> +
> + set_task_reclaim_state(current, &sc.reclaim_state);
> + flags = memalloc_noreclaim_save();
> + blk_start_plug(&plug);
> +
> + next = buf;
> + next[len] = '\0';
> +
> + while ((cur = strsep(&next, ",;\n"))) {
> + int n;
> + int end;
> + char cmd;
> + unsigned int memcg_id;
> + unsigned int nid;
> + unsigned long seq;
> + unsigned int swappiness = -1;
> + unsigned long opt = -1;
> +
> + cur = skip_spaces(cur);
> + if (!*cur)
> + continue;
> +
> + n = sscanf(cur, "%c %u %u %lu %n %u %n %lu %n", &cmd, &memcg_id, &nid,
> + &seq, &end, &swappiness, &end, &opt, &end);
> + if (n < 4 || cur[end]) {
> + err = -EINVAL;
> + break;
> + }
> +
> + err = run_cmd(cmd, memcg_id, nid, seq, &sc, swappiness, opt);
> + if (err)
> + break;
> + }
> +
> + blk_finish_plug(&plug);
> + memalloc_noreclaim_restore(flags);
> + set_task_reclaim_state(current, NULL);
> +
> + clear_mm_walk();

Ditto, we can't call clear_mm_walk() after
set_task_reclaim_state(current, NULL).

Maybe it can be modified as follows:

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 2422edc786eb..552e6ae5243e 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -5569,12 +5569,12 @@ static ssize_t lru_gen_seq_write(struct file
*file, const char __user *src,
return -EFAULT;
}

+ set_task_reclaim_state(current, &sc.reclaim_state);
if (!set_mm_walk(NULL)) {
kvfree(buf);
return -ENOMEM;
}

- set_task_reclaim_state(current, &sc.reclaim_state);
flags = memalloc_noreclaim_save();
blk_start_plug(&plug);

@@ -5609,9 +5609,9 @@ static ssize_t lru_gen_seq_write(struct file
*file, const char __user *src,

blk_finish_plug(&plug);
memalloc_noreclaim_restore(flags);
+ clear_mm_walk();
set_task_reclaim_state(current, NULL);

- clear_mm_walk();
kvfree(buf);

return err ? : len;
Thanks,
Qi

> + kvfree(buf);
> +
> + return err ? : len;
> +}
> +


--
Thanks,
Qi

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