Re: [PATCH] mm: Require LRU reclaim progress before retrying direct reclaim

[Matt Fleming <matt@readmodwrite.com>]

On Thu, Apr 16, 2026 at 02:58:30PM -0700, Shakeel Butt wrote:
> On Thu, Apr 16, 2026 at 09:44:55AM +0800, Barry Song wrote:
> > 
> > I am still struggling to understand when zram-backed
> > reclamation cannot make progress. Is it because zram is
> > full, or because folio_alloc_swap() fails?
> > 
> > Or does zs_malloc() fail, causing pageout() to fail?
> > Even incompressible pages are still written as
> > ZRAM_HUGE pages and reclaimed successfully.
> 
> We should have counters for these, right?
 
Let me try and provide some more data for this. It's hard to replicate
on our production systems so I've resorted to creating a minimal Qemu
repro that has 1GiB RAM and zram disk = 1GiB. The workload is a simple
anon memory mapper that allocs 900MiB of memory and touches all pages
for 60s.

zs_malloc
---------
None of the zs_malloc() calls failed and we made ~1.2M of them during
the test. Here's a breakdown of allocation sizes:

@hist_zs_malloc_size: 
[32, 64)         4831015 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
[64, 128)            409 |                                                    |
[128, 256)          1090 |                                                    |
[256, 512)          2334 |                                                    |
[512, 1K)           5069 |                                                    |
[1K, 2K)           11174 |                                                    |
[2K, 4K)            2395 |                                                    |
[4K, 8K)             237 |                                                    |

During direct reclaim only:
@hist_zs_malloc_size_in_dr: 
[32, 64)         1268042 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
[64, 128)             52 |                                                    |
[128, 256)           149 |                                                    |
[256, 512)           292 |                                                    |
[512, 1K)           1234 |                                                    |
[1K, 2K)            3539 |                                                    |
[2K, 4K)            1156 |                                                    |
[4K, 8K)             135 |                                                    |


/sys/block/zram0/mm_stat
--------------------------
before: 4096       74    12288        0    12288        0        0        0        0
after:  42622976  9412667 10985472        0 34131968        0     1962        0      237


trace_mm_vmscan_lru_shrink_inactive
-----------------------------------
Anon LRU shrink events:                 397,949
  sum(args->nr_scanned):                11,837,216
  sum(args->nr_reclaimed):              4,871,775
  sum(args->nr_dirty):                  0
  sum(args->nr_writeback):              0
  sum(args->nr_congested):              0
  sum(args->nr_immediate):              0
  sum(args->nr_ref_keep):               5,200,896
  sum(args->nr_unmap_fail):             0

File LRU shrink events:                 2,632
  sum(args->nr_scanned):                26,048
  sum(args->nr_reclaimed):              12,681
  sum(args->nr_dirty):                  0
  sum(args->nr_writeback):              0
  sum(args->nr_congested):              0
  sum(args->nr_immediate):              0
  sum(args->nr_ref_keep):               476
  sum(args->nr_unmap_fail):             0


> > I would rather detect what causes the lack of progress
> > and implement a better fallback.
> 
> This is a good question. I think we have appropriate counters in /proc/vmstat
> for cases where pages keep getting recycled in the LRUs instead of reclaim.

Here's the output of /proc/vmstat before and after the test runs.

nr_free_pages                             210,825 ->       206,742  (delta=-4,083)
nr_free_pages_blocks                      209,920 ->        65,536  (delta=-144,384)
nr_zone_inactive_anon                       1,685 ->           136  (delta=-1,549)
nr_zone_active_anon                            15 ->         3,774  (delta=3,759)
nr_zone_inactive_file                         329 ->           591  (delta=262)
nr_zone_active_file                           673 ->           504  (delta=-169)
nr_zspages                                      3 ->         2,716  (delta=2,713)
nr_inactive_anon                            1,685 ->           136  (delta=-1,549)
nr_active_anon                                 15 ->         3,774  (delta=3,759)
nr_inactive_file                              329 ->           591  (delta=262)
nr_active_file                                673 ->           504  (delta=-169)
nr_slab_reclaimable                         1,352 ->         2,037  (delta=685)
nr_slab_unreclaimable                       9,581 ->        11,689  (delta=2,108)
nr_anon_pages                               1,526 ->           262  (delta=-1,264)
nr_mapped                                     912 ->           442  (delta=-470)
nr_file_pages                               1,132 ->         4,760  (delta=3,628)
nr_shmem                                      162 ->         3,608  (delta=3,446)
nr_swapcached                                   0 ->            19  (delta=19)
nr_vmscan_write                                 0 ->     4,872,846  (delta=4,872,846)
nr_written                                      1 ->     4,853,727  (delta=4,853,726)

pgpgin                                      1,200 ->    19,035,312  (delta=19,034,112)
pgpgout                                         4 ->    19,414,908  (delta=19,414,904)
pswpin                                          0 ->     4,758,528  (delta=4,758,528)
pswpout                                         0 ->     4,853,726  (delta=4,853,726)

pgalloc_dma                                    32 ->        84,262  (delta=84,230)
pgalloc_dma32                              45,989 ->     5,095,307  (delta=5,049,318)
pgfree                                    269,896 ->     5,415,629  (delta=5,145,733)
pgactivate                                  2,820 ->        14,490  (delta=11,670)
pgdeactivate                                   10 ->        10,924  (delta=10,914)
pgfault                                    29,321 ->     5,088,427  (delta=5,059,106)
pgmajfault                                  3,750 ->     4,794,781  (delta=4,791,031)
pgrefill                                        0 ->        13,733  (delta=13,733)
pgreuse                                     3,333 ->         5,852  (delta=2,519)

pgsteal_kswapd                                  0 ->     3,605,552  (delta=3,605,552)
pgsteal_direct                                  0 ->     1,280,091  (delta=1,280,091)
pgscan_kswapd                                   0 ->     6,579,240  (delta=6,579,240)
pgscan_direct                                   0 ->     5,290,778  (delta=5,290,778)
pgscan_anon                                     0 ->    11,843,970  (delta=11,843,970)
pgscan_file                                     0 ->        26,048  (delta=26,048)
pgsteal_anon                                    0 ->     4,872,962  (delta=4,872,962)
pgsteal_file                                    0 ->        12,681  (delta=12,681)

allocstall_normal                               0 ->           110  (delta=110)
allocstall_movable                              0 ->        32,088  (delta=32,088)
oom_kill                                        0 ->             0  (delta=0)

workingset_nodes                                0 ->           302  (delta=302)
workingset_refault_anon                         0 ->     4,777,591  (delta=4,777,591)
workingset_refault_file                         0 ->           870  (delta=870)
workingset_activate_anon                        0 ->           487  (delta=487)

kswapd_low_wmark_hit_quickly                    0 ->            35  (delta=35)
kswapd_high_wmark_hit_quickly                   0 ->            99  (delta=99)
pageoutrun                                      0 ->           135  (delta=135)

pgmigrate_success                               0 ->        21,317  (delta=21,317)
compact_migrate_scanned                         0 ->        98,848  (delta=98,848)
compact_free_scanned                            0 ->       136,667  (delta=136,667)

swpin_zero                                      0 ->        19,069  (delta=19,069)
swpout_zero                                     0 ->        19,120  (delta=19,120)
swap_ra                                         0 ->            63  (delta=63)
swap_ra_hit                                     0 ->            26  (delta=26)

Happy to do any other tests or pull any other data for you to help.

Thanks,
Matt