Re: [PATCHv3 06/11] mm/vmscan: Use PG_dropbehind instead of PG_reclaim

[Kairui Song <ryncsn@gmail.com>]

On Thu, Jan 30, 2025 at 6:02 PM Kirill A. Shutemov
<kirill.shutemov@linux.intel.com> wrote:
>
> The recently introduced PG_dropbehind allows for freeing folios
> immediately after writeback. Unlike PG_reclaim, it does not need vmscan
> to be involved to get the folio freed.
>
> Instead of using folio_set_reclaim(), use folio_set_dropbehind() in
> pageout().
>
> It is safe to leave PG_dropbehind on the folio if, for some reason
> (bug?), the folio is not in a writeback state after ->writepage().
> In these cases, the kernel had to clear PG_reclaim as it shared a page
> flag bit with PG_readahead.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Acked-by: David Hildenbrand <david@redhat.com>
> ---
>  mm/vmscan.c | 9 +++------
>  1 file changed, 3 insertions(+), 6 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index bc1826020159..c97adb0fdaa4 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -692,19 +692,16 @@ static pageout_t pageout(struct folio *folio, struct address_space *mapping,
>                 if (shmem_mapping(mapping) && folio_test_large(folio))
>                         wbc.list = folio_list;
>
> -               folio_set_reclaim(folio);
> +               folio_set_dropbehind(folio);
> +
>                 res = mapping->a_ops->writepage(&folio->page, &wbc);
>                 if (res < 0)
>                         handle_write_error(mapping, folio, res);
>                 if (res == AOP_WRITEPAGE_ACTIVATE) {
> -                       folio_clear_reclaim(folio);
> +                       folio_clear_dropbehind(folio);
>                         return PAGE_ACTIVATE;
>                 }
>
> -               if (!folio_test_writeback(folio)) {
> -                       /* synchronous write or broken a_ops? */
> -                       folio_clear_reclaim(folio);
> -               }
>                 trace_mm_vmscan_write_folio(folio);
>                 node_stat_add_folio(folio, NR_VMSCAN_WRITE);
>                 return PAGE_SUCCESS;
> --
> 2.47.2
>

Hi, I'm seeing following panic with SWAP after this commit:

[   29.672319] Oops: general protection fault, probably for
non-canonical address 0xffff88909a3be3: 0000 [#1] PREEMPT SMP NOPTI
[   29.675503] CPU: 82 UID: 0 PID: 5145 Comm: tar Kdump: loaded Not
tainted 6.13.0.ptch-g1fe9ea48ec98 #917
[   29.677508] Hardware name: Red Hat KVM/RHEL-AV, BIOS 0.0.0 02/06/2015
[   29.678886] RIP: 0010:__lock_acquire+0x20/0x15d0
[   29.679891] Code: 90 90 90 90 90 90 90 90 90 90 41 57 41 56 41 55
41 54 55 53 48 83 ec 30 8b 2d 10 ac f3 01 44 8b ac 24 88 00 00 00 85
ed 74 64 <48> 8b 07 49 89 ff 48 3d 20 1d bf 83 74 56 8b 1d 8c f5 b1 01
41 89
[   29.683852] RSP: 0018:ffffc9000bea3148 EFLAGS: 00010002
[   29.684980] RAX: ffff8890874b2940 RBX: 0000000000000200 RCX: 0000000000000000
[   29.686510] RDX: 0000000000000000 RSI: 0000000000000000 RDI: 00ffff88909a3be3
[   29.688031] RBP: 0000000000000001 R08: 0000000000000001 R09: 0000000000000000
[   29.689561] R10: 0000000000000000 R11: 0000000000000020 R12: 00ffff88909a3be3
[   29.691087] R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
[   29.692613] FS:  00007fa05c2824c0(0000) GS:ffff88a03fa80000(0000)
knlGS:0000000000000000
[   29.694339] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   29.695581] CR2: 000055f9abb7fc7d CR3: 00000010932f2002 CR4: 0000000000770eb0
[   29.697109] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[   29.698637] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[   29.700161] PKRU: 55555554
[   29.700759] Call Trace:
[   29.701296]  <TASK>
[   29.701770]  ? __die_body+0x1e/0x60
[   29.702540]  ? die_addr+0x3c/0x60
[   29.703267]  ? exc_general_protection+0x18f/0x3c0
[   29.704290]  ? asm_exc_general_protection+0x26/0x30
[   29.705345]  ? __lock_acquire+0x20/0x15d0
[   29.706215]  ? lockdep_hardirqs_on_prepare+0xda/0x190
[   29.707304]  ? asm_sysvec_apic_timer_interrupt+0x1a/0x20
[   29.708452]  lock_acquire+0xbf/0x2e0
[   29.709229]  ? folio_unmap_invalidate+0x12f/0x220
[   29.710257]  ? __folio_end_writeback+0x15d/0x430
[   29.711260]  ? __folio_end_writeback+0x116/0x430
[   29.712261]  _raw_spin_lock+0x30/0x40
[   29.713064]  ? folio_unmap_invalidate+0x12f/0x220
[   29.714076]  folio_unmap_invalidate+0x12f/0x220
[   29.715058]  folio_end_writeback+0xdf/0x190
[   29.715967]  swap_writepage_bdev_sync+0x1e0/0x450
[   29.716994]  ? __pfx_submit_bio_wait_endio+0x10/0x10
[   29.718074]  swap_writepage+0x46b/0x6b0
[   29.718917]  pageout+0x14b/0x360
[   29.719628]  shrink_folio_list+0x67d/0xec0
[   29.720519]  ? mark_held_locks+0x48/0x80
[   29.721375]  evict_folios+0x2a7/0x9e0
[   29.722179]  try_to_shrink_lruvec+0x19a/0x270
[   29.723130]  lru_gen_shrink_lruvec+0x70/0xc0
[   29.724060]  ? __lock_acquire+0x558/0x15d0
[   29.724954]  shrink_lruvec+0x57/0x780
[   29.725754]  ? find_held_lock+0x2d/0xa0
[   29.726588]  ? rcu_read_unlock+0x17/0x60
[   29.727449]  shrink_node+0x2ad/0x930
[   29.728229]  do_try_to_free_pages+0xbd/0x4e0
[   29.729160]  try_to_free_mem_cgroup_pages+0x123/0x2c0
[   29.730252]  try_charge_memcg+0x222/0x660
[   29.731128]  charge_memcg+0x3c/0x80
[   29.731888]  __mem_cgroup_charge+0x30/0x70
[   29.732776]  shmem_alloc_and_add_folio+0x1a5/0x480
[   29.733818]  ? filemap_get_entry+0x155/0x390
[   29.734748]  shmem_get_folio_gfp+0x28c/0x6c0
[   29.735680]  shmem_write_begin+0x5a/0xc0
[   29.736535]  generic_perform_write+0x12a/0x2e0
[   29.737503]  shmem_file_write_iter+0x86/0x90
[   29.738428]  vfs_write+0x364/0x530
[   29.739180]  ksys_write+0x6c/0xe0
[   29.739906]  do_syscall_64+0x66/0x140
[   29.740713]  entry_SYSCALL_64_after_hwframe+0x76/0x7e
[   29.741800] RIP: 0033:0x7fa05c439984
[   29.742584] Code: c7 00 16 00 00 00 b8 ff ff ff ff c3 66 2e 0f 1f
84 00 00 00 00 00 f3 0f 1e fa 80 3d c5 06 0e 00 00 74 13 b8 01 00 00
00 0f 05 <48> 3d 00 f0 ff ff 77 54 c3 0f 1f 00 55 48 89 e5 48 83 ec 20
48 89
[   29.746542] RSP: 002b:00007ffece7720f8 EFLAGS: 00000202 ORIG_RAX:
0000000000000001
[   29.748157] RAX: ffffffffffffffda RBX: 0000000000002800 RCX: 00007fa05c439984
[   29.749682] RDX: 0000000000002800 RSI: 000055f9cfa08000 RDI: 0000000000000004
[   29.751216] RBP: 00007ffece772140 R08: 0000000000002800 R09: 0000000000000007
[   29.752743] R10: 0000000000000180 R11: 0000000000000202 R12: 000055f9cfa08000
[   29.754262] R13: 0000000000000004 R14: 0000000000002800 R15: 00000000000009af
[   29.755797]  </TASK>
[   29.756285] Modules linked in: zram virtiofs

I'm testing with PROVE_LOCKING on. It seems folio_unmap_invalidate is
called for swapcache folio and it doesn't work well, following PATCH
on top of mm-unstable seems fix it well:

diff --git a/mm/filemap.c b/mm/filemap.c
index 4fe551037bf7..98493443d120 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1605,8 +1605,9 @@ static void folio_end_reclaim_write(struct folio *folio)
         * invalidation in that case.
         */
        if (in_task() && folio_trylock(folio)) {
-               if (folio->mapping)
-                       folio_unmap_invalidate(folio->mapping, folio, 0);
+               struct address_space *mapping = folio_mapping(folio);
+               if (mapping)
+                       folio_unmap_invalidate(mapping, folio, 0);
                folio_unlock(folio);
        }
 }
diff --git a/mm/truncate.c b/mm/truncate.c
index e922ceb66c44..4f3e34c52d8b 100644
--- a/mm/truncate.c
+++ b/mm/truncate.c
@@ -565,23 +565,29 @@ int folio_unmap_invalidate(struct address_space
*mapping, struct folio *folio,
        if (!filemap_release_folio(folio, gfp))
                return -EBUSY;

-       spin_lock(&mapping->host->i_lock);
+       if (!folio_test_swapcache(folio)) {
+               spin_lock(&mapping->host->i_lock);
+               BUG_ON(folio_has_private(folio));
+       }
+
        xa_lock_irq(&mapping->i_pages);
        if (folio_test_dirty(folio))
                goto failed;

-       BUG_ON(folio_has_private(folio));
        __filemap_remove_folio(folio, NULL);
        xa_unlock_irq(&mapping->i_pages);
        if (mapping_shrinkable(mapping))
                inode_add_lru(mapping->host);
-       spin_unlock(&mapping->host->i_lock);
+
+       if (!folio_test_swapcache(folio))
+               spin_unlock(&mapping->host->i_lock);

        filemap_free_folio(mapping, folio);
        return 1;
 failed:
        xa_unlock_irq(&mapping->i_pages);
-       spin_unlock(&mapping->host->i_lock);
+       if (!folio_test_swapcache(folio))
+               spin_unlock(&mapping->host->i_lock);
        return -EBUSY;
 }