Re: [REGRESSION][BISECTED] Unexpected OOM instead of reclaiming inactive file pages

[Damien Le Moal <dlemoal@kernel.org>]

On 8/12/25 5:42 AM, Oleksandr Natalenko wrote:
> Hello.
> 
> On pondělí 11. srpna 2025 18:06:16, středoevropský letní čas David Rientjes wrote:
>> On Mon, 11 Aug 2025, Oleksandr Natalenko wrote:
>>
>>> Hello Damien.
>>>
>>> I'm fairly confident that the following commit
>>>
>>> 459779d04ae8d block: Improve read ahead size for rotational devices
>>>
>>> caused a regression in my test bench.
>>>
>>> I'm running v6.17-rc1 in a small QEMU VM with virtio-scsi disk. It has got 1 GiB of RAM, so I can saturate it easily causing reclaiming mechanism to kick in.
>>>
>>> If MGLRU is enabled:
>>>
>>> $ echo 1000 | sudo tee /sys/kernel/mm/lru_gen/min_ttl_ms
>>>
>>> then, once page cache builds up, an OOM happens without reclaiming inactive file pages: [1]. Note that inactive_file:506952kB, I'd expect these to be reclaimed instead, like how it happens with v6.16.
>>>
>>> If MGLRU is disabled:
>>>
>>> $ echo 0 | sudo tee /sys/kernel/mm/lru_gen/min_ttl_ms
>>>
>>> then OOM doesn't occur, and things seem to work as usual.
>>>
>>> If MGLRU is enabled, and 459779d04ae8d is reverted on top of v6.17-rc1, the OOM doesn't happen either.
>>>
>>> Could you please check this?
>>>
>>
>> This looks to be an MGLRU policy decision rather than a readahead 
>> regression, correct?
>>
>> Mem-Info:
>> active_anon:388 inactive_anon:5382 isolated_anon:0
>>  active_file:9638 inactive_file:126738 isolated_file:0
>>
>> Setting min_ttl_ms to 1000 is preserving the working set and triggering 
>> the oom kill is the only alternative to free memory in that configuration.  
>> The oom kill is being triggered by kswapd for this purpose.
>>
>> So additional readahead would certainly increase that working set.  This 
>> looks working as intended.
> 
> OK, this makes sense indeed, thanks for the explanation. But is inactive_file explosion expected and justified?
> 
> Without revert:
> 
> $ echo 3 | sudo tee /proc/sys/vm/drop_caches; free -m; sudo journalctl -kb >/dev/null; free -m
> 3
>                total        used        free      shared  buff/cache   available
> Mem:             690         179         536           3          57         510
> Swap:           1379          12        1367
> /* OOM happens here */
>                total        used        free      shared  buff/cache   available
> Mem:             690         177          52           3         561         513
> Swap:           1379          17        1362 
> 
> With revert:
> 
> $ echo 3 | sudo tee /proc/sys/vm/drop_caches; free -m; sudo journalctl -kb >/dev/null; free -m
> 3
>                total        used        free      shared  buff/cache   available
> Mem:             690         214         498           4          64         476
> Swap:           1379           0        1379
> /* no OOM */
>                total        used        free      shared  buff/cache   available
> Mem:             690         209         462           4         119         481
> Swap:           1379           0        1379
> 
> The journal folder size is:
> 
> $ sudo du -hs /var/log/journal
> 575M    /var/log/journal
> 
> It looks like this readahead change causes far more data to be read than actually needed?

For your drive as seen by the VM, what is the value of
/sys/block/sdX/queue/optimal_io_size ?

I guess it is "0", as I see on my VM.
So before 459779d04ae8d, the block device read_ahead_kb was 128KB only, and
459779d04ae8d switched it to be 2 times the max_sectors_kb, so 8MB. This change
significantly improves file buffered read performance on HDDs, and HDDs only.

This means that your VM device is probably being reported as a rotational one
(/sys/block/sdX/queue/rotational is 1), which is normal if you attached an
actual HDD. If you are using a qcow2 image for that disk, then having
rotational==1 is questionable...

The other issue is the device driver for the device reporting 0 for the optimal
IO size, which normally happens only for SATA drives. I see the same with
virtio-scsi, which is also questionable given that the maximum IO size with it
is fairly limited. So virtio-scsi may need some tweaking.

The other thing to question, I think, is setting read_ahead_kb using the
optimal_io_size limit (io_opt), which can be *very large*. For most SCSI
devices, it is 16MB, so you will see a read_ahead_kb of 32 MB. But for SCSI
devices, optimal_io_size indicates a *maximum* IO size beyond which performance
may degrade. So using any value lower than this, but still reasonably large,
would be better in general I think. Note that lim->io_opt for RAID arrays
actually indicates the stripe size, so generally a lot smaller than the
component drives io_opt. And this use changes the meaning of that queue limit,
which makes things even more confusing and finding an adequate default harder.

-- 
Damien Le Moal
Western Digital Research