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

[Kairui Song <ryncsn@gmail.com>]

On Fri, Feb 06, 2026 at 12:11:51PM +0800, Andrew Morton wrote:
> 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.

Thanks for letting me know!

> 
> > === 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

Oh, I took a look and I think I know the problem now, the new added
fast path in that series is only used by allocation from vmscan,
for hibernation it always use the slow path, as I didn't expect the
slow path is below hibernation's requirement.

Or maybe, it not the allocation path that is slow, but because
cluster rotation in slow path splatted the allocation to every
cluster makes the IO much slower.

Both are not too hard to fix. Is that's SSD only issue? And
should we fix this in every LTS or just mainline? It could
be a bit trouble some if every LTS needs a fix.