Re: [PATCH v2 0/9] support large folio swap-out and swap-in for shmem

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 2024/6/19 16:16, Hugh Dickins wrote:
> On Wed, 19 Jun 2024, Baolin Wang wrote:
>> On 2024/6/19 04:05, Andrew Morton wrote:
>>> On Tue, 18 Jun 2024 14:54:12 +0800 Baolin Wang
>>> <baolin.wang@linux.alibaba.com> wrote:
>>>
>>>> Shmem will support large folio allocation [1] [2] to get a better
>>>> performance,
>>>> however, the memory reclaim still splits the precious large folios when
>>>> trying
>>>> to swap-out shmem, which may lead to the memory fragmentation issue and can
>>>> not
>>>> take advantage of the large folio for shmeme.
>>>>
>>>> Moreover, the swap code already supports for swapping out large folio
>>>> without
>>>> split, and large folio swap-in[3] series is queued into mm-unstable branch.
>>>> Hence this patch set also supports the large folio swap-out and swap-in for
>>>> shmem.
>>>
>>> I'll add this to mm-unstable for some exposure, but I wonder how much
>>> testing it will have recieved by the time the next merge window opens?
>>
>> Thanks Andrew. I am fine with this series going to 6.12 if you are concerned
>> about insufficient testing (and let's also wait for Hugh's comments). Since we
>> (Daniel and I) have some follow-up patches that will rely on this swap series,
>> hope this series can be tested as extensively as possible to ensure its
>> stability in the mm branch.
> 
> Thanks for giving it the exposure, Andrew, but please drop it from
> mm-unstable until the next cycle. I'd been about to write to say I
> wouldn't be trying it until next cycle, when your mm-commits came in:
> so I thought I ought at least to give mm-everything-2024-06-18 a try.
> 
> Baolin may have fixed stuff, but he (or the interaction with other mm
> work) has broken stuff too: I couldn't get as far with it as with the
> previous version. Just "cp -a" of a kernel source tree into a tmpfs
> huge=always size=<bigenough> failed with lots of ENOSPCs, and when
> "rm -rf"ed lots of WARN_ON(inode->i_blocks) from shmem_evict_inode();
> and on second attempt, then a VM_BUG_ON_FOLIO(!folio_contains) from
> find_lock_entries().

Thanks Hugh for giving it a try. I also can reproduce the 
WARN_ON(inode->i_blocks) issue with today's mm-unstable branch (sadly, 
this issue didn't occur in the older mm-unstable branch), and now I have 
a fix in my local tree. But I will not send out a V3 so quickly, and I 
agree we can drop this series from mm-unstable branch until next cycle.

> Or maybe that VM_BUG_ON_FOLIO() was unrelated, but a symptom of the bug

I did not encounter the VM_BUG_ON_FOLIO() issue, but let me try your 
testing case...

> I'm trying to chase even when this series is reverted: some kind of page
> double usage, manifesting as miscellaneous "Bad page"s and VM_BUG_ONs,
> mostly from page reclaim or from exit_mmap(). I'm still getting a feel
> for it, maybe it occurs soon enough for a reliable bisection, maybe not.
> 
> (While writing, a run with mm-unstable cut off at 2a9964cc5d27,
> drop KSM_KMEM_CACHE(), instead of reverting just Baolin's latest,
> has not yet hit any problem: too early to tell but promising.)
> 
> And before 2024-06-18, I was working on mm-everything-2024-06-15 minus
> Chris Li's mTHP swap series: which worked fairly well, until it locked
> up with __try_to_reclaim_swap()'s filemap_get_folio() spinning around
> on a page with 0 refcount, while a page table lock is held which one
> by one the other CPUs come to want for reclaim. On two machines.
> 
> None of these problems seen on Stephen's last next-2024-06-13.
> I had wanted to see if mm-everything-2024-06-18 fixed that lockup,
> but with the new problems I cannot tell (or it could all be the same
> problem: but if so, odd that it manifests consistently differently).
> 
> There are way too many mTHP shmem and swap patch series floating
> around at the moment, in mm and in fs, for me to cope with:
> everyone, please slow down and test more.

Sure. I will continue to do more testing. Thanks.