Re: [PATCH] zswap: don't warn if none swapcache folio is passed to zswap_load

[Yin, Fengwei]

On 8/11/2023 11:21 AM, Yosry Ahmed wrote:
> On Thu, Aug 10, 2023 at 8:12 PM Yu Zhao <yuzhao@xxxxxxxxxx> wrote:
>>
>> On Thu, Aug 10, 2023 at 5:46 PM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote:
>>>
>>> On Thu, Aug 10, 2023 at 4:44 PM Yu Zhao <yuzhao@xxxxxxxxxx> wrote:
>>>>
>>>> On Thu, Aug 10, 2023 at 5:31 PM Yin, Fengwei <fengwei.yin@xxxxxxxxx> wrote:
>>>>>
>>>>>
>>>>>
>>>>> On 8/11/2023 7:15 AM, Yosry Ahmed wrote:
>>>>>> On Thu, Aug 10, 2023 at 4:09 PM Yin, Fengwei <fengwei.yin@xxxxxxxxx> wrote:
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> On 8/11/2023 2:44 AM, Yu Zhao wrote:
>>>>>>>> On Thu, Aug 10, 2023 at 3:58 AM Yin Fengwei <fengwei.yin@xxxxxxxxx> wrote:
>>>>>>>>>
>>>>>>>>> With mm-unstable branch, if trigger swap activity and it's possible
>>>>>>>>> see following warning:
>>>>>>>>> [  178.093511][  T651] WARNING: CPU: 2 PID: 651 at mm/zswap.c:1387 zswap_load+0x67/0x570
>>>>>>>>> [  178.095155][  T651] Modules linked in:
>>>>>>>>> [  178.096103][  T651] CPU: 2 PID: 651 Comm: gmain Not tainted 6.5.0-rc4-00492-gad3232df3e41 #148
>>>>>>>>> [  178.098372][  T651] Hardware name: QEMU Standard PC (i440FX + PIIX,1996), BIOS 1.14.0-2 04/01/2014
>>>>>>>>> [  178.101114][  T651] RIP: 0010:zswap_load+0x67/0x570
>>>>>>>>> [  178.102359][  T651] Code: a0 78 4b 85 e8 ea db ff ff 48 8b 00 a8 01 0f 84 84 04 00 00 48 89 df e8 d7 db ff ff 48 8b 00 a9 00 00 08 00 0f 85 c4
>>>>>>>>> [  178.106376][  T651] RSP: 0018:ffffc900011b3760 EFLAGS: 00010246
>>>>>>>>> [  178.107675][  T651] RAX: 0017ffffc0080001 RBX: ffffea0004a991c0 RCX:ffffc900011b37dc
>>>>>>>>> [  178.109242][  T651] RDX: 0000000000000000 RSI: 0000000000000001 RDI:ffffea0004a991c0
>>>>>>>>> [  178.110916][  T651] RBP: ffffea0004a991c0 R08: 0000000000000243 R09:00000000c9a1aafc
>>>>>>>>> [  178.112377][  T651] R10: 00000000c9657db3 R11: 000000003c9657db R12:0000000000014b9c
>>>>>>>>> [  178.113698][  T651] R13: ffff88813501e710 R14: ffff88810d591000 R15:0000000000000000
>>>>>>>>> [  178.115008][  T651] FS:  00007fb21a9ff700(0000) GS:ffff88813bc80000(0000) knlGS:0000000000000000
>>>>>>>>> [  178.116423][  T651] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>>>>>>>>> [  178.117421][  T651] CR2: 00005632cbfc81f6 CR3: 0000000131450002 CR4:0000000000370ee0
>>>>>>>>> [  178.118683][  T651] DR0: 0000000000000000 DR1: 0000000000000000 DR2:0000000000000000
>>>>>>>>> [  178.119894][  T651] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7:0000000000000400
>>>>>>>>> [  178.121087][  T651] Call Trace:
>>>>>>>>> [  178.121654][  T651]  <TASK>
>>>>>>>>> [  178.122109][  T651]  ? zswap_load+0x67/0x570
>>>>>>>>> [  178.122658][  T651]  ? __warn+0x81/0x170
>>>>>>>>> [  178.123119][  T651]  ? zswap_load+0x67/0x570
>>>>>>>>> [  178.123608][  T651]  ? report_bug+0x167/0x190
>>>>>>>>> [  178.124150][  T651]  ? handle_bug+0x3c/0x70
>>>>>>>>> [  178.124615][  T651]  ? exc_invalid_op+0x13/0x60
>>>>>>>>> [  178.125192][  T651]  ? asm_exc_invalid_op+0x16/0x20
>>>>>>>>> [  178.125753][  T651]  ? zswap_load+0x67/0x570
>>>>>>>>> [  178.126231][  T651]  ? lock_acquire+0xbb/0x290
>>>>>>>>> [  178.126745][  T651]  ? folio_add_lru+0x40/0x1c0
>>>>>>>>> [  178.127261][  T651]  ? find_held_lock+0x2b/0x80
>>>>>>>>> [  178.127776][  T651]  swap_readpage+0xc7/0x5c0
>>>>>>>>> [  178.128273][  T651]  do_swap_page+0x86d/0xf50
>>>>>>>>> [  178.128770][  T651]  ? __pte_offset_map+0x3e/0x290
>>>>>>>>> [  178.129321][  T651]  ? __pte_offset_map+0x1c4/0x290
>>>>>>>>> [  178.129883][  T651]  __handle_mm_fault+0x6ad/0xca0
>>>>>>>>> [  178.130419][  T651]  handle_mm_fault+0x18b/0x410
>>>>>>>>> [  178.130992][  T651]  do_user_addr_fault+0x1f1/0x820
>>>>>>>>> [  178.132076][  T651]  exc_page_fault+0x63/0x1a0
>>>>>>>>> [  178.132599][  T651]  asm_exc_page_fault+0x22/0x30
>>>>>>>>>
>>>>>>>>> It's possible that swap_readpage() is called with none swapcache folio
>>>>>>>>> in do_swap_page() and trigger this warning. So we shouldn't assume
>>>>>>>>> zswap_load() always takes swapcache folio.
>>>>>>>>
>>>>>>>> Did you use a bdev with QUEUE_FLAG_SYNCHRONOUS? Otherwise it sounds
>>>>>>>> like a bug to me.
>>>>>>> I hit this warning with zram which has QUEUE_FLAG_SYNCHRONOUS set. Thanks.
>>>>>>
>>>>>> Does it make sense to keep the warning and instead change it to check
>>>>>> SWP_SYNCHRONOUS_IO as well? Something like:
>>>>>>
>>>>>> VM_WARN_ON_ONCE(!folio_test_swapcache(folio) &&
>>>>>> !swap_type_to_swap_info(type)->flags && SWP_SYNCHRONOUS_IO);
>>>>>>
>>>>>> Of course this is too ugly, so perhaps we want a helper to check if a
>>>>>> swapfile is synchronous.
>>>>> My understanding was that the WARN here is zswap_load() doesn't expect
>>>>> a folio not in swapcache. With zram, swap_readpage() must accept the
>>>>> folio not in swapcache. So this warn should not be there.
>>>>>
>>>>> But your comment make more sense to me. I will update the patch not
>>>>> to remove this WARN. Thanks.
>>>>
>>>> That can cause another warning.
>>>>
>>>> Please don't overegineer.
>>>
>>> How so?
>>>
>>> Using zswap with zram is a weird combination
>>
>> Not at all -- it can achieve tiering between different compressors:
>> fast but low compression ratio for zswap but the opposite for zram.
> 
> That's definitely an interesting use case, thanks for pointing this out.
> 
> I would prefer creating a helper and using it in both do_swap_fault()
> and zswap_load() in the WARN_ON (with data_race()), but I am not
> against just removing the WARN_ON either. I will leave it up to you
> and Yin :)
OK. I will stick to the current patch.

Regards
Yin, Fengwei

> 
>>
>>> if anything I would
>>> prefer leaving the warning as-is than removing it to be honest.