Re: [PATCH] mm/memory_hotplug: fix call folio_test_large with tail page in do_migrate_range

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

On Tue, 1 Apr 2025 18:44:06 +0200 David Hildenbrand <david@redhat.com> wrote:

> On 29.03.25 00:37, Andrew Morton wrote:
> > On Tue, 25 Mar 2025 20:05:50 +0100 David Hildenbrand <david@redhat.com> wrote:
> > 
> >> On 25.03.25 04:02, Jinjiang Tu wrote:
> >>>
> >>>> Moving that will not make it able to skip the large frozen
> >>>> (refcount==0, e.g., free hugetlb) folio in the continue/put_folio case
> >>>> above. Hmmmm ..
> >>> For free hugetlb, pfn is increased by 1 in each loop. This leads to skip
> >>> free hugetlb slower.
> >>
> >> Yes. But now I realize that we have the same issue with free buddy pages
> >> already (folio_try_get of each individual page :( ).
> >>
> >>>>
> >>>> We could similarly to dumping folios, snapshot them, so we can read
> >>>> stable data.
> >>> extract the code in __dump_page()? But snapshot may lead to
> >>> do_migrate_range() slower too.
> >>
> >> There is a patch series on the list to do that, but it might take a
> >> while to clean that up. Ideally, we'd also jump over free buddy pages.
> >> In the future we might have better ways to do that.
> >>
> >> I don't consider this change here really important, but if all it
> >> affects is free hugetlb folios, it's not really worth it to have this
> >> code around.
> >>
> >> Acked-by: David Hildenbrand <david@redhat.com>
> >>
> >> But: I suspect 8135d8926c08 is not the introducing commit. Please re-verify.
> >>
> >> We should not CC stable.
> > 
> 
> Sorry for the late reply.
> 
> > Why shouldn't we cc:stable?  A userspace-triggerable BUG?
> 
> My reasoning was that it's a VM_BUG_ON_PAGE, and nothing is actually 
> broken on !CONFIG_DEBUG_VM systems ... combined with hugetlb allocation 
> and memory offlining usually being rare and usually privileged operations.
> 
> It would be a different story if the VM_BUG_ON_PAGE revealed an actual 
> issue; but this is a well documented race.
> 
> One might argue that unplugging a DIMM could automatically trigger it 
> (rare operation ..), and as we discovered in the meantime, THPs are also 
> affected. But again, CONFIG_VM_DEBUG ... which even Fedora stopped setting.
> 
> So I don't think CC stable is warranted here, but I wouldn't object if 
> there are good reasons to do it: like assuming that there are 
> CONFIG_VM_DEBUG stable users out there that rely on these features.

Oh.  I thought that quite a few distros use CONFIG_DEBUG_VM.

I think I shall add the cc:stable - the patch is pretty simple and will
be well tested by the time it hits -stable trees and probably someone
will benefit from it.