Re: [RFC PATCH 0/2] remove SWAP_MAP_SHMEM

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

On 2024/9/24 23:48, Nhat Pham wrote:
> On Tue, Sep 24, 2024 at 8:08 AM Yosry Ahmed <yosryahmed@google.com> wrote:
>>
>> On Tue, Sep 24, 2024 at 7:32 AM Nhat Pham <nphamcs@gmail.com> wrote:
>>>
>>> On Mon, Sep 23, 2024 at 8:25 PM Baolin Wang
>>> <baolin.wang@linux.alibaba.com> wrote:
>>>>
>>>>
>>>> On 2024/9/24 10:15, Yosry Ahmed wrote:
>>>>> On Mon, Sep 23, 2024 at 6:55 PM Baolin Wang
>>>>> <baolin.wang@linux.alibaba.com> wrote:
>>>>>>
>>>>>>
>>>>>>
>>>>>> On 2024/9/24 07:11, Nhat Pham wrote:
>>>>>>> The SWAP_MAP_SHMEM state was originally introduced in the commit
>>>>>>> aaa468653b4a ("swap_info: note SWAP_MAP_SHMEM"), to quickly determine if a
>>>>>>> swap entry belongs to shmem during swapoff.
>>>>>>>
>>>>>>> However, swapoff has since been rewritten drastically in the commit
>>>>>>> b56a2d8af914 ("mm: rid swapoff of quadratic complexity"). Now
>>>>>>> having swap count == SWAP_MAP_SHMEM value is basically the same as having
>>>>>>> swap count == 1, and swap_shmem_alloc() behaves analogously to
>>>>>>> swap_duplicate()
>>>>>>>
>>>>>>> This RFC proposes the removal of this state and the associated helper to
>>>>>>> simplify the state machine (both mentally and code-wise). We will also
>>>>>>> have an extra state/special value that can be repurposed (for swap entries
>>>>>>> that never gets re-duplicated).
>>>>>>>
>>>>>>> Another motivation (albeit a bit premature at the moment) is the new swap
>>>>>>> abstraction I am currently working on, that would allow for swap/zswap
>>>>>>> decoupling, swapoff optimization, etc. The fewer states and swap API
>>>>>>> functions there are, the simpler the conversion will be.
>>>>>>>
>>>>>>> I am sending this series first as an RFC, just in case I missed something
>>>>>>> or misunderstood this state, or if someone has a swap optimization in mind
>>>>>>> for shmem that would require this special state.
>>>>>>
>>>>>> The idea makes sense to me. I did a quick test with shmem mTHP, and
>>>>>> encountered the following warning which is triggered by
>>>>>> 'VM_WARN_ON(usage == 1 && nr > 1)' in __swap_duplicate().
>>>>>
>>>>> Apparently __swap_duplicate() does not currently handle increasing the
>>>>> swap count for multiple swap entries by 1 (i.e. usage == 1) because it
>>>>> does not handle rolling back count increases when
>>>>> swap_count_continued() fails.
>>>>>
>>>>> I guess this voids my Reviewed-by until we sort this out. Technically
>>>>> swap_count_continued() won't ever be called for shmem because we only
>>>>> ever increment the count by 1, but there is no way to know this in
>>>>> __swap_duplicate() without SWAP_HAS_SHMEM.
>>>
>>> Ah this is my bad. I compiled with CONFIG_THP_SWAP, but forgot to
>>> remove the swapfile check (that's another can of worms, but I need
>>> data before submitting the patch to remove it...)
>>>
>>> One thing we can do is instead of warning here, we can handle it in
>>> the for loop check, where we have access to count - that's the point
>>> of having that for-loop check anyway? :)
>>>
>>> There's a couple of ways to go about it:
>>>
>>> 1. VM_WARN_ON(usage == 1 && nr > 1 && count != 0 );
>>
>> Hmm that should work, although it's a bit complicated tbh.
>>
>>> (or more accurately, (count & ~COUNT_CONTINUED) >= SWAP_MAP_MAX))
>>
>> I think this will make the warning very hard to hit if there's a
>> misuse of __swap_duplicate(). It will only be hit when an entry needs
>> count continuation, which I am not sure is very common. If there's a
>> bug, the warning will potentially catch it too late, if ever.
>>
>> The side effect here is failing to decrement the swap count of some
>> swap entries which will lead to them never being freed, essentially
>> leaking swap capacity slowly over time. I am not sure if there are
>> more detrimental effects.
>>
>>>
>>> 2. Alternatively, instead of warning here, we can simply return
>>> -ENOMEM. Then, at shmem callsite, have a VM_WARN_ON/VM_BUG_ON(), since
>>> this MUST succeed.
>>
>> We still fail to rollback incremented counts though when we return
>> -ENOMEM, right? Maybe I didn't get what you mean.
> 
> My understanding now is that there are two for loops. One for loop
> that checks the entry's states, and one for loop that does the actual
> incrementing work (or state modification).
> 
> We can check in the first for loop, if it is safe to proceed:
> 
> if (!count && !has_cache) {
>      err = -ENOENT;
> } else if (usage == SWAP_HAS_CACHE) {
> if (has_cache)
>      err = -EEXIST;
> } else if ((count & ~COUNT_CONTINUED) > SWAP_MAP_MAX) {
>      err = -EINVAL;
> } else if (usage == 1 && nr > 1 && (count & ~COUNT_CONTINUED) >=
> SWAP_MAP_MAX)) {
>      /* the batched variants currently do not support rollback */
>      err = -ENOMEM;
> }
> 
> At this point, IIUC, we have not done any incrementing, so no rollback
> needed? :)

Right, looks good (although need some cleanup pointed by Yosry).

>>>
>>> Either solutions should follow with careful documentation to make it
>>> clear the expectation/guarantee of the new API.
>>>
>>> Yosry, Baolin, how do you two feel about this? Would something like
>>> this work? I need to test it first, but let me know if I'm missing
>>> something.
>>>
>>> If this does not work, we can do what Baolin is suggesting, and
>>> perhaps maintain the swap_shmem_alloc() helper. It's less than ideal,
>>> but at least we still lose a state...
>>
>> Depending on the complexity tbh, right now removing SWAP_MAP_SHMEM is
>> just a cleanup with small wins, so if it's too complicated to remove
>> it it may not be worth it. I am assuming with your ongoing work, it
>> becomes much more valuable, so maybe if it's too complicated we can
>> defer it until the benefits are realizable?
> 
> I agree :)

One benefit I can mention is that removing 'SWAP_MAP_SHMEM' can help to 
batch free shmem swap entries in __swap_entries_free(), similar to the 
commit bea67dcc5eea ("mm: attempt to batch free swap entries for 
zap_pte_range()") did, which can improve the performance of shmem mTHP 
munmap() function based on my testing.

Without this patch set, I need do following changes to batch free shmem 
swap entries:
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 0cded32414a1..94e28cd60c52 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -163,7 +163,7 @@ static bool swap_is_last_map(struct swap_info_struct 
*si,
         unsigned char *map_end = map + nr_pages;
         unsigned char count = *map;

-       if (swap_count(count) != 1)
+       if (swap_count(count) != 1 && swap_count(count) != SWAP_MAP_SHMEM)
                 return false;

         while (++map < map_end) {
@@ -1503,10 +1503,10 @@ static bool __swap_entries_free(struct 
swap_info_struct *si,
         unsigned int type = swp_type(entry);
         struct swap_cluster_info *ci;
         bool has_cache = false;
-       unsigned char count;
+       unsigned char count = swap_count(data_race(si->swap_map[offset]));
         int i;

-       if (nr <= 1 || swap_count(data_race(si->swap_map[offset])) != 1)
+       if (nr <= 1 || (count != 1 && count != SWAP_MAP_SHMEM))
                 goto fallback;
         /* cross into another cluster */
         if (nr > SWAPFILE_CLUSTER - offset % SWAPFILE_CLUSTER)