Re: [Bug 220605] New: Very slow access on HDD when a swap partition is used on this HDD on kernel 6.16

[Andrew Morton <akpm@linux-foundation.org>]

(switched to email.  Please respond via emailed reply-to-all, not via the
bugzilla web interface).

On Sat, 27 Sep 2025 08:07:59 +0000 bugzilla-daemon@kernel.org wrote:

> https://bugzilla.kernel.org/show_bug.cgi?id=220605
> 
>             Bug ID: 220605
>            Summary: Very slow access on HDD when a swap partition is used
>                     on this HDD on kernel 6.16
>            Product: Memory Management
>            Version: 2.5
>           Hardware: Intel
>                 OS: Linux
>             Status: NEW
>           Severity: high
>           Priority: P3
>          Component: Page Allocator
>           Assignee: akpm@linux-foundation.org
>           Reporter: pierre.juhen@orange.fr
>         Regression: No
> 
> Hello everybody,
> 
> I have a strange behavior on my system (Fedora 42 up to date => kernel 6.16.8).

You certainly do.  I wonder what we did to cause this.

I'll cc the MM development list.  Please prefer to report issues over
email this way - we don't use bugzilla and it's rather annoying that MM
bugzilla even exists.


> I have many disks but but my two swap partitions are on 2 HDD, sda and sdb,
> which are identical (SEAGATE Barracuda 2To). Swap partition on sda has a higher
> priority than the one on sdb.
> 
> I don't use zram.
> 
> When a swap partion is used, the disk on wich it seats is very slow.
> 
> $ sudo hdparm -t /dev/sda
> /dev/sda:
>  Timing buffered disk reads:  24 MB in  3.72 seconds =   6.46 MB/sec
> 
> $ sudo hdparm -t /dev/sdb
> /dev/sdb:
>  Timing buffered disk reads: 540 MB in  3.16 seconds = 170.85 MB/sec
> pierre@pierre:~$ 
> 
> Sometimes, speed gets down to less than 2 MB/s.
> 
> Lets turn swap off => swapoff -a 
> 
> $ sudo hdparm -t /dev/sdb
> /dev/sdb:
>  Timing buffered disk reads: 462 MB in  3.00 seconds = 153.75 MB/sec
> 
> $ sudo hdparm -t /dev/sda
> 
> /dev/sda:
>  Timing buffered disk reads: 474 MB in  3.00 seconds = 157.84 MB/sec
> pierre@pierre:~$ 
> 
> Now speed is normal.
> 
> Lets put swap on again => sudo swapon -a 
> 
> Speed is still normal :
> 
> $ sudo hdparm -t /dev/sda
> /dev/sda:
>  Timing buffered disk reads: 612 MB in  3.01 seconds = 203.56 MB/sec
> $ sudo hdparm -t /dev/sdb
> /dev/sdb:
>  Timing buffered disk reads: 570 MB in  3.00 seconds = 189.95 MB/sec
> pierre@pierre:~$ 
> 
> But swap is not used 
> 
> MiB Mem :  31952,0 total,  27091,7 free,   3057,6 used,   2343,0 buff/cache     
> MiB Swap:  36000,0 total,  36000,0 free,      0,0 used.  28894,4 avail Mem 
> 
> 
> So I suspect that, when the swap is used, there is some sort of lock that slows
> the whole disk that contains the swap partition that is being used.
> 
> Logically, the system becomes very slow.
> 
> There is nothing in the journal that seems related to this problem.
> 
> Since the virtual memory has been modified in 6.16, I suspect a nasty bug.
> 
> This affects 6.16.5 to 6.16.8.
> 
> This appeared "recently", probably with 6.16
> 
> Thanks,
> 
> Regards,
> 
> Pierre
> 
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