Re: [PATCH] mm/page_alloc: make percpu_pagelist_high_fraction reads lock-free

[David Hildenbrand (Red Hat)]

On 12/3/25 10:23, Gregory Price wrote:
> On Wed, Dec 03, 2025 at 10:08:55AM +0100, David Hildenbrand (Red Hat) wrote:
> > On 12/3/25 10:02, Gregory Price wrote:
> > > My transient failure (although i'm not sure it was actually transient, i
> > > killed it and retried after a few minutes and it succeeded immediately)
> > > was on a ZONE_MOVABLE block.
> >
> > Okay, so that one should not bail out. Longterm pinnins must never end up on
> > such memory, and if it happens, we have to identify why and fix it.
> >
> > We have this known problem of "stream of short-term pinnings" that can
> > temporarily turn memory effectively unmovable. Juan will talk about that at
> > LPC [1].
>
> Nice, fun, good topic. Looking forward to Japan n_n
>
> > We have another set of problematic cases (vmsplice(), fuse) but I would
> > assume that these are not the cases you are hitting.
>
> We do use fuse, but this system was relatively quiet when i tried this.
>
> We do have some proactive reclaim / demotion going on, but i don't think
> it was that (see below).
>
> > > Kind of suggested to me there was some bad condition the resolved once I
> > > took a second to release the lock and try again.
> >
> > Hard to tell I'm afraid. Do you still have the dump_folio() calls we print
> > when migration fails?
>
> What luck, I do! :D

:)

> And i just noticed it's the same page over and over
>
> [ 3404.119270] migrating pfn c06f176 failed ret:1
> [ 3404.129152] page: refcount:4 mapcount:0 mapping:0000000061ca20ba index:0xad28e5b pfn:0xc06f176
> [ 3404.148284] memcg:ffff88842e855000
> [ 3404.155834] aops:btree_aops ino:1

Small folio. Not GUP-pinned (FOLL_PIN, otherwise our refcount would be 
>= 1024.

It could be ordinary GUP (FOLL_GET) e.g., from vmsplice or some older 
O_DIRECT user that was not converted to FOLL_PIN yet. But maybe it's 
just btrfs / something else that temporarily holds a folio reference.


Given that this is from 6.13 ... hard to tell :)

> [ 3404.163193] flags: 0x17ffff066c00420c(referenced|uptodate|workingset|private|node=1|zone=3|lastcpupid=0x1ffff)

Neither dirty nor under writeback.

--
Cheers

David