Re: [PATCH v7 2/2] mm: support large folios swap-in for sync io devices

[Usama Arif <usamaarif642@gmail.com>]

On 03/09/2024 23:05, Yosry Ahmed wrote:
> On Tue, Sep 3, 2024 at 2:36 PM Barry Song <21cnbao@gmail.com> wrote:
>>
>> On Wed, Sep 4, 2024 at 8:08 AM Andrew Morton <akpm@linux-foundation.org> wrote:
>>>
>>> On Tue, 3 Sep 2024 11:38:37 -0700 Yosry Ahmed <yosryahmed@google.com> wrote:
>>>
>>>>> [   39.157954] RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000007
>>>>> [   39.158288] R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000001
>>>>> [   39.158634] R13: 0000000000002b9a R14: 0000000000000000 R15: 00007ffd619d5518
>>>>> [   39.158998]  </TASK>
>>>>> [   39.159226] ---[ end trace 0000000000000000 ]---
>>>>>
>>>>> After reverting this or Usama's "mm: store zero pages to be swapped
>>>>> out in a bitmap", the problem is gone. I think these two patches may
>>>>> have some conflict that needs to be resolved.
>>>>
>>>> Yup. I saw this conflict coming and specifically asked for this
>>>> warning to be added in Usama's patch to catch it [1]. It served its
>>>> purpose.
>>>>
>>>> Usama's patch does not handle large folio swapin, because at the time
>>>> it was written we didn't have it. We expected Usama's series to land
>>>> sooner than this one, so the warning was to make sure that this series
>>>> handles large folio swapin in the zeromap code. Now that they are both
>>>> in mm-unstable, we are gonna have to figure this out.
>>>>
>>>> I suspect Usama's patches are closer to land so it's better to handle
>>>> this in this series, but I will leave it up to Usama and
>>>> Chuanhua/Barry to figure this out :)
>>
>> I believe handling this in swap-in might violate layer separation.
>> `swap_read_folio()` should be a reliable API to call, regardless of
>> whether `zeromap` is present. Therefore, the fix should likely be
>> within `zeromap` but not this `swap-in`. I’ll take a look at this with
>> Usama :-)
> 
> I meant handling it within this series to avoid blocking Usama
> patches, not within this code. Thanks for taking a look, I am sure you
> and Usama will figure out the best way forward :)

Hi Barry and Yosry,

Is the best (and quickest) way forward to have a v8 of this with
https://lore.kernel.org/all/20240904055522.2376-1-21cnbao@gmail.com/
as the first patch, and using swap_zeromap_entries_count in alloc_swap_folio
in this support large folios swap-in patch?

Thanks,
Usama