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Date: Fri, 7 Jun 2024 21:03:44 +0200
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Subject: Re: [PATCH] mm: zswap: add VM_BUG_ON() if large folio swapin is
 attempted
To: Yosry Ahmed <yosryahmed@google.com>
Cc: Barry Song <21cnbao@gmail.com>, Andrew Morton
 <akpm@linux-foundation.org>, Johannes Weiner <hannes@cmpxchg.org>,
 Nhat Pham <nphamcs@gmail.com>, Chengming Zhou <chengming.zhou@linux.dev>,
 Baolin Wang <baolin.wang@linux.alibaba.com>, Chris Li <chrisl@kernel.org>,
 Ryan Roberts <ryan.roberts@arm.com>, Matthew Wilcox <willy@infradead.org>,
 linux-mm@kvack.org, linux-kernel@vger.kernel.org
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From: David Hildenbrand <david@redhat.com>
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On 07.06.24 20:58, Yosry Ahmed wrote:
> On Fri, Jun 7, 2024 at 11:52 AM David Hildenbrand <david@redhat.com> wrote:
>>
>>>> I have no strong opinion on this one, but likely a VM_WARN_ON would also
>>>> be sufficient to find such issues early during testing. No need to crash
>>>> the machine.
>>>
>>> I thought VM_BUG_ON() was less frowned-upon than BUG_ON(), but after
>>> some digging I found your patches to checkpatch and Linus clearly
>>> stating that it isn't.
>>
>> :) yes.
>>
>> VM_BUG_ON is not particularly helpful IMHO. If you want something to be
>> found early during testing, VM_WARN_ON is good enough.
>>
>> Ever since Fedora stopped enabling CONFIG_DEBUG_VM, VM_* friends are
>> primarily reported during early/development testing only. But maybe some
>> distro out there still sets it.
>>
>>>
>>> How about something like the following (untested), it is the minimal
>>> recovery we can do but should work for a lot of cases, and does
>>> nothing beyond a warning if we can swapin the large folio from disk:
>>>
>>> diff --git a/mm/page_io.c b/mm/page_io.c
>>> index f1a9cfab6e748..8f441dd8e109f 100644
>>> --- a/mm/page_io.c
>>> +++ b/mm/page_io.c
>>> @@ -517,7 +517,6 @@ void swap_read_folio(struct folio *folio, struct
>>> swap_iocb **plug)
>>>           delayacct_swapin_start();
>>>
>>>           if (zswap_load(folio)) {
>>> -               folio_mark_uptodate(folio);
>>>                   folio_unlock(folio);
>>>           } else if (data_race(sis->flags & SWP_FS_OPS)) {
>>>                   swap_read_folio_fs(folio, plug);
>>> diff --git a/mm/zswap.c b/mm/zswap.c
>>> index 6007252429bb2..cc04db6bb217e 100644
>>> --- a/mm/zswap.c
>>> +++ b/mm/zswap.c
>>> @@ -1557,6 +1557,22 @@ bool zswap_load(struct folio *folio)
>>>
>>>           VM_WARN_ON_ONCE(!folio_test_locked(folio));
>>>
>>> +       /*
>>> +        * Large folios should not be swapped in while zswap is being used, as
>>> +        * they are not properly handled.
>>> +        *
>>> +        * If any of the subpages are in zswap, reading from disk would result
>>> +        * in data corruption, so return true without marking the folio uptodate
>>> +        * so that an IO error is emitted (e.g. do_swap_page() will sigfault).
>>> +        *
>>> +        * Otherwise, return false and read the folio from disk.
>>> +        */
>>> +       if (WARN_ON_ONCE(folio_test_large(folio))) {
>>> +               if (xa_find(tree, &offset, offset +
>>> folio_nr_pages(folio) - 1, 0))
>>> +                       return true;
>>> +               return false;
>>> +       }
>>> +
>>>           /*
>>>            * When reading into the swapcache, invalidate our entry. The
>>>            * swapcache can be the authoritative owner of the page and
>>> @@ -1593,7 +1609,7 @@ bool zswap_load(struct folio *folio)
>>>                   zswap_entry_free(entry);
>>>                   folio_mark_dirty(folio);
>>>           }
>>> -
>>> +       folio_mark_uptodate(folio);
>>>           return true;
>>>    }
>>>
>>> One problem is that even if zswap was never enabled, the warning will
>>> be emitted just if CONFIG_ZSWAP is on. Perhaps we need a variable or
>>> static key if zswap was "ever" enabled.
>>
>> We should use WARN_ON_ONCE() only for things that cannot happen. So if
>> there are cases where this could be triggered today, it would be
>> problematic -- especially if it can be triggered from unprivileged user
>> space. But if we're concerned of other code messing up our invariant in
>> the future (e.g., enabling large folios without taking proper care about
>> zswap etc), we're good to add it.
> 
> Right now I can't see any paths allocating large folios for swapin, so
> I think it cannot happen. Once someone tries adding it, the warning
> will fire if CONFIG_ZSWAP is used, even if zswap is disabled.
> 
> At this point we will have several options:
> - Make large folios swapin depend on !CONFIG_ZSWAP for now.
> - Keep track if zswap was ever enabled and make the warning
> conditional on it. We should also always fallback to order-0 if zswap
> was ever enabled.
> - Properly handle large folio swapin with zswap.
> 
> Does this sound reasonable to you?

Yes, probably easiest is 1) -> 3) or if we don't want to glue it to 
config options 2) -> 3).

-- 
Cheers,

David / dhildenb