Re: [PATCH 0/2] mm/zsmalloc: simplify synchronization between zs_page_migrate() and free_zspage()

[Sergey Senozhatsky <senozhatsky@chromium.org>]

On (24/02/27 03:02), Chengming Zhou wrote:
> free_zspage() has to hold locks of all pages, since zs_page_migrate()
> path rely on this page lock to protect the race between zs_free() and
> it, so it can safely get zspage from page->private.
> 
> But this way is not good and simple enough:
> 
> 1. Since zs_free() couldn't be sleepable, it can only trylock pages,
>    or has to kick_deferred_free() to defer that to a work.
> 
> 2. Even in the worker context, async_free_zspage() can't simply
>    lock all pages in lock_zspage(), it's still trylock because of
>    the race between zs_free() and zs_page_migrate(). Please see
>    the commit 2505a981114d ("zsmalloc: fix races between asynchronous
>    zspage free and page migration") for details.
> 
> Actually, all free_zspage() needs is to get zspage from page safely,
> we can use RCU to achieve it easily. Then free_zspage() don't need to
> hold locks of all pages, so don't need the deferred free mechanism
> at all. This patchset implements it and remove all of deferred free
> related code.
> 
> Thanks for review and comments!
> 
> Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
> ---
> Chengming Zhou (2):
>       mm/zsmalloc: don't hold locks of all pages when free_zspage()

That seems to be crashing on me:

[   28.123867] ==================================================================
[   28.125303] BUG: KASAN: null-ptr-deref in obj_malloc+0xa9/0x1f0
[   28.126289] Read of size 8 at addr 0000000000000028 by task mkfs.ext2/432
[   28.127414] 
[   28.127684] CPU: 8 PID: 432 Comm: mkfs.ext2 Tainted: G                 N 6.8.0-rc5+ #309
[   28.129015] Call Trace:
[   28.129442]  <TASK>
[   28.129805]  dump_stack_lvl+0x6f/0xab
[   28.130437]  print_report+0xe0/0x5e0
[   28.131050]  ? _printk+0x59/0x7b
[   28.131602]  ? kasan_report+0x96/0x120
[   28.132233]  ? obj_malloc+0xa9/0x1f0
[   28.132837]  kasan_report+0xe7/0x120
[   28.133441]  ? obj_malloc+0xa9/0x1f0
[   28.134046]  obj_malloc+0xa9/0x1f0
[   28.134633]  zs_malloc+0x22c/0x3e0
[   28.135211]  zram_submit_bio+0x44e/0xee0
[   28.135871]  ? lock_release+0x50c/0x700
[   28.136520]  submit_bio_noacct_nocheck+0x22a/0x650
[   28.137327]  __block_write_full_folio+0x48b/0x710
[   28.138119]  ? __cfi_blkdev_get_block+0x10/0x10
[   28.138885]  ? __cfi_block_write_full_folio+0x10/0x10
[   28.139737]  write_cache_pages+0x83/0xf0
[   28.140397]  ? __cfi_blkdev_get_block+0x10/0x10
[   28.141152]  blkdev_writepages+0x46/0x80
[   28.141810]  do_writepages+0x1be/0x400
[   28.142443]  file_write_and_wait_range+0x104/0x170
[   28.143254]  blkdev_fsync+0x4a/0x70
[   28.143846]  __x64_sys_fsync+0xe9/0x120
[   28.144491]  do_syscall_64+0x8d/0x130
[   28.145106]  entry_SYSCALL_64_after_hwframe+0x46/0x4e