Re: [PATCH -next v6] mm: vmscan: retry folios written back while isolated for traditional LRU

[Chen Ridong <chenridong@huaweicloud.com>]

On 2024/12/24 12:19, Yu Zhao wrote:
> On Mon, Dec 23, 2024 at 1:30 AM Chen Ridong <chenridong@huaweicloud.com> wrote:
>>
>> From: Chen Ridong <chenridong@huawei.com>
>>
>> The page reclaim isolates a batch of folios from the tail of one of the
>> LRU lists and works on those folios one by one.  For a suitable
>> swap-backed folio, if the swap device is async, it queues that folio for
>> writeback.  After the page reclaim finishes an entire batch, it puts back
>> the folios it queued for writeback to the head of the original LRU list.
>>
>> In the meantime, the page writeback flushes the queued folios also by
>> batches.  Its batching logic is independent from that of the page reclaim.
>> For each of the folios it writes back, the page writeback calls
>> folio_rotate_reclaimable() which tries to rotate a folio to the tail.
>>
>> folio_rotate_reclaimable() only works for a folio after the page reclaim
>> has put it back.  If an async swap device is fast enough, the page
>> writeback can finish with that folio while the page reclaim is still
>> working on the rest of the batch containing it.  In this case, that folio
>> will remain at the head and the page reclaim will not retry it before
>> reaching there.
> 
> For starters, copying & pasting others' commit messages as your own is
> plagiarism. You need to quote them.

Hi, Yu, Thank you for reminding me, I did not mean any plagiarism. I am
a beginner, and I do not know much about that.

I wrote the message in my v1 and v2 to describe what issue I was fixing,
which is wordy. What you wrote is much clearer in the commit
359a5e1416ca, so I pasted it. I am sorry. Should resend a new patch to
modify the message?

I have sent all patch versions to you, and I don't know whether you have
noticed. How I wish you could point it out at the first I pasted it, so
I wouldn't have made this mistake again and again.

>> The commit 359a5e1416ca ("mm: multi-gen LRU: retry folios written back
>> while isolated") only fixed the issue for mglru. However, this issue
>> also exists in the traditional active/inactive LRU.
> 
> You need to prove it with some numbers.

Do you mean I should prove it with some info in my message? Actually, I
encountered this in the traditional active/inactive LRU, I did not know
you had fixed for mglru until Barry told me. I offered how to reproduce
this with Link.

>> This issue will be
>> worse if THP is split, which makes the list longer and needs longer time
>> to finish a batch of folios reclaim.
>>
>> This issue should be fixed in the same way for the traditional LRU.
>> Therefore, the common logic was extracted to the 'find_folios_written_back'
>> function firstly, which is then reused in the 'shrink_inactive_list'
>> function. Finally, retry reclaiming those folios that may have missed the
>> rotation for traditional LRU.
>>
>> Link: https://lore.kernel.org/linux-kernel/20241010081802.290893-1-chenridong@huaweicloud.com/
>> Link: https://lore.kernel.org/linux-kernel/CAGsJ_4zqL8ZHNRZ44o_CC69kE7DBVXvbZfvmQxMGiFqRxqHQdA@mail.gmail.com/
>> Signed-off-by: Chen Ridong <chenridong@huawei.com>
>> Reviewed-by: Barry Song <baohua@kernel.org>
>> ---
>>
>> v5->v6:
>>  - fix compile error(implicit declaration of function 'lru_gen_distance')
>>    when CONFIG_LRU_GEN is disable.
> 
> Did you build-test it this time? I don't think LRU_REFS_FLAGS is
> defined when CONFIG_LRU_GEN=y.
> 

Yes, I tested. I didn't test when CONFIG_LRU_GEN=n in patch v5.
I tested with CONFIG_LRU_GEN=n and CONFIG_LRU_GEN=y in patch v6. I am
using the next.

Best regards,
Ridong

> 
> 
>>  - rename 'is_retried' to is_retrying suggested by Barry Song.
>>
>> v5: https://lore.kernel.org/linux-kernel/CAGsJ_4x3Aj7wieK1FQKQC4Vbz5N+1dExs=Q70KQt-whS1dMxpw@mail.gmail.com/
>>
>>  include/linux/mm_inline.h |   5 ++
>>  mm/vmscan.c               | 108 ++++++++++++++++++++++++--------------
>>  2 files changed, 75 insertions(+), 38 deletions(-)