linux-kernel - Re: [PATCH v3 1/4] mm: swap: Remove CLUSTER_FLAG_HUGE from swap_cluster

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Message-ID: <861321e8-6b96-4cbf-a61b-d9cbc2dc093e@redhat.com>
Date: Fri, 1 Mar 2024 18:18:59 +0100
From: David Hildenbrand <david@...hat.com>
To: Ryan Roberts <ryan.roberts@....com>, Matthew Wilcox <willy@...radead.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 Huang Ying <ying.huang@...el.com>, Gao Xiang <xiang@...nel.org>,
 Yu Zhao <yuzhao@...gle.com>, Yang Shi <shy828301@...il.com>,
 Michal Hocko <mhocko@...e.com>, Kefeng Wang <wangkefeng.wang@...wei.com>,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v3 1/4] mm: swap: Remove CLUSTER_FLAG_HUGE from
 swap_cluster_info:flags

On 01.03.24 18:14, Ryan Roberts wrote:
> On 01/03/2024 17:00, David Hildenbrand wrote:
>> On 01.03.24 17:44, Ryan Roberts wrote:
>>> On 01/03/2024 16:31, Matthew Wilcox wrote:
>>>> On Fri, Mar 01, 2024 at 04:27:32PM +0000, Ryan Roberts wrote:
>>>>> I've implemented the batching as David suggested, and I'm pretty confident it's
>>>>> correct. The only problem is that during testing I can't provoke the code to
>>>>> take the path. I've been pouring through the code but struggling to figure out
>>>>> under what situation you would expect the swap entry passed to
>>>>> free_swap_and_cache() to still have a cached folio? Does anyone have any idea?
>>>>>
>>>>> This is the original (unbatched) function, after my change, which caused
>>>>> David's
>>>>> concern that we would end up calling __try_to_reclaim_swap() far too much:
>>>>>
>>>>> int free_swap_and_cache(swp_entry_t entry)
>>>>> {
>>>>>      struct swap_info_struct *p;
>>>>>      unsigned char count;
>>>>>
>>>>>      if (non_swap_entry(entry))
>>>>>          return 1;
>>>>>
>>>>>      p = _swap_info_get(entry);
>>>>>      if (p) {
>>>>>          count = __swap_entry_free(p, entry);
>>>>>          if (count == SWAP_HAS_CACHE)
>>>>>              __try_to_reclaim_swap(p, swp_offset(entry),
>>>>>                            TTRS_UNMAPPED | TTRS_FULL);
>>>>>      }
>>>>>      return p != NULL;
>>>>> }
>>>>>
>>>>> The trouble is, whenever its called, count is always 0, so
>>>>> __try_to_reclaim_swap() never gets called.
>>>>>
>>>>> My test case is allocating 1G anon memory, then doing madvise(MADV_PAGEOUT)
>>>>> over
>>>>> it. Then doing either a munmap() or madvise(MADV_FREE), both of which cause
>>>>> this
>>>>> function to be called for every PTE, but count is always 0 after
>>>>> __swap_entry_free() so __try_to_reclaim_swap() is never called. I've tried for
>>>>> order-0 as well as PTE- and PMD-mapped 2M THP.
>>>>
>>>> I think you have to page it back in again, then it will have an entry in
>>>> the swap cache.  Maybe.  I know little about anon memory ;-)
>>>
>>> Ahh, I was under the impression that the original folio is put into the swap
>>> cache at swap out, then (I guess) its removed once the IO is complete? I'm sure
>>> I'm miles out... what exactly is the lifecycle of a folio going through swap out?
>>
>> I thought with most (disk) backends you will add it to the swapcache and leave
>> it there until there is actual memory pressure. Only then, under memory
>> pressure, you'd actually reclaim the folio.
> 
> OK, my problem is that I'm using a VM, whose disk shows up as rotating media, so
> the swap subsystem refuses to swap out THPs to that and they get split. To solve
> that, (and to speed up testing) I moved to the block ram disk, which convinces
> swap to swap-out THPs. But that causes the folios to be removed from the swap
> cache (I assumed because its syncrhonous, but maybe there is a flag somewhere to
> affect that behavior?) And I can't convince QEMU to emulate an SSD to the guest
> under macos. Perhaps the easiest thing is to hack it to ignore the rotating
> media flag.

I'm trying to remember how I triggered it in the past, I thought cow.c 
selftest was able to do that.

What certainly works is taking a reference on the page using vmsplice() 
and then doing the MADV_PAGEOUT. But there has to be a better way :)

I'll dig on Monday!

-- 
Cheers,

David / dhildenb