Re: [Bug 221055] New: Hibernate image write 10x slower since commit 0ff67f990bd4 ("mm, swap: remove swap slot cache")

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

On Fri, 06 Feb 2026 19:51:38 +0000 bugzilla-daemon@kernel.org wrote:

> https://bugzilla.kernel.org/show_bug.cgi?id=221055

Dear Deity, I do wish kernel MM bugzilla would just go away.  Sigh.

Canned response follows:

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


>             Bug ID: 221055
>            Summary: Hibernate image write 10x slower since commit
>                     0ff67f990bd4 ("mm,  swap: remove swap slot cache")
>            Product: Memory Management
>            Version: 2.5
>           Hardware: All
>                 OS: Linux
>             Status: NEW
>           Severity: normal
>           Priority: P3
>          Component: Other
>           Assignee: akpm@linux-foundation.org
>           Reporter: carstengrohmann@gmx.de
>         Regression: No
> 
> === Summary ===
> 
> Hibernate image write 10x slower since commit 0ff67f990bd4 ("mm, 
> swap: remove swap slot cache")

Thanks for all the work you put into this.

> === Description ===
> 
> Since kernel 6.15, writing the hibernate image to swap has become
> approximately 10x slower. The regression was introduced by commit
> 0ff67f990bd45726e0d9e91111d998e7a3595b32 ("mm, swap: remove swap slot
> cache") authored by Kairui Song and merged by Andrew Morton.
> 
> === Performance Data ===
> 
>                  Before (6.14)   After (6.15+)
> Hibernate        28-32 seconds   290-342 seconds
> Effective rate   ~60 MB/s        ~4.5 MB/s
> 
> The swap partition's raw sequential write speed (measured with dd)
> is 255-450 MB/s, confirming the hardware is not the bottleneck.
> 
> === Bisect Result ===
> 
> A full git bisect on the vanilla kernel repository
> (git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git)
> between v6.14 (good) and v6.15 (bad) with 14 iterations
> consistently identified this commit:
> 
>   # first bad commit: [0ff67f990bd45726e0d9e91111d998e7a3595b32]
>   #   mm, swap: remove swap slot cache
> 
> The parent commit 1b7e90020eb7 ("mm, swap: use percpu cluster as
> allocation fast path") is fast (28s), while 0ff67f990bd4 itself
> is slow (307s).
> 
> === Kernel Version Tests ===
> 
>   Kernel              Duration   Result
>   6.12.1-arch1-1      32 sec     OK
>   6.13.1-arch1-1      42 sec     OK
>   6.14.1-arch1-1      32 sec     OK
>   6.15.1-arch1-1     302 sec     SLOW
>   6.18.1-arch1-1     290 sec     SLOW
> 
> === Reproduction ===
> 
> 1. System with swap partition (not swap file)
> 2. Boot kernel >= 6.15
> 3. Run: echo reboot > /sys/power/disk
> 4. Run: echo 1 > /sys/power/pm_print_times
> 5. Run: date; echo disk > /sys/power/state; date
> 6. Compare hibernate duration with kernel <= 6.14
> 
> For best visibility of the issue, use hibernate=nocompress as
> kernel parameter to disable image compression.
> 
> === Hardware ===
> 
> - CPU: Intel Core (Ivy Bridge)
> - RAM: 16 GB
> - SSD: Samsung SSD 830 Series (SATA II, 3.0 Gbps)
> - Swap: 20 GB partition (/dev/sda2)
> - Arch Linux x86_64
> 
> === Kernel Log (6.18, with hibernate=nocompress) ===
> 
>   PM: hibernation: hibernation entry
>   PM: hibernation: Preallocating image memory
>   PM: hibernation: Allocated 2217260 kbytes in 0.39 seconds (5685.28 MB/s)
>   PM: hibernation: Creating image:
>   PM: hibernation: Need to copy 506945 pages
>   PM: hibernation: Wrote 1794356 kbytes in 397.97 seconds (4.51 MB/s)
> 
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