Re: [PATCH v10 2/6] alloc_tag: clean up the populate failure path

[Uladzislau Rezki]

On Fri, Sep 18, 2026 at 05:35:02PM +0100, Lorenzo Stoakes (ARM) wrote:
> Looping in Ulad who is best placed to respond on this.
> 
> Ulad could you look at the below? Thanks! :)
> 
> On Thu, Sep 17, 2026 at 10:32:48AM +0800, Hao Ge wrote:
> > Hi Suren、David、Lorenzo
> >
> > On 2026/9/17 09:12, Hao Ge wrote:
> > > Hi Suren
> > >
> > > On 2026/9/17 03:07, Suren Baghdasaryan wrote:
> > >> On Wed, Sep 16, 2026 at 9:31 AM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
> > >>>
> > >>> On Tue, Sep 15, 2026 at 11:00 PM Hao Ge <hao.ge@xxxxxxxxx> wrote:
> > >>>>
> > >>>> Hi Suren
> > >>>>
> > >>>> Thanks for you review.
> > >>>>
> > >>>> On 2026/9/16 05:09, Suren Baghdasaryan wrote:
> > >>>>> On Mon, Sep 14, 2026 at 11:59 PM Hao Ge <hao.ge@xxxxxxxxx> wrote:
> > >>>>>>
> > >>>>>> The reservation is already stored in the maple tree when
> > >>>>>> vm_module_tags_populate() fails. A failed load never unloads the
> > >>>>>> module, so nothing releases the entry. Release it and roll
> > >>>>>> module_tags.size back. Without the rollback a concurrent load that
> > >>>>>> already passed needs_section_mem() can reuse the freed gap, skip
> > >>>>>> vm_module_tags_populate() and write to unmapped memory.
> > >>>>>>
> > >>>>>> vmap_pages_range() may have installed some PTEs before failing. A
> > >>>>>> retry to populate the same range would BUG on them, so undo them,
> > >>>>>> but only if vmap actually ran.
> > >>>>>>
> > >>>>>> Fixes: 4835f747d3ed ("alloc_tag: support for page allocation tag compression")
> > >>>>>> Fixes: 0f9b685626da ("alloc_tag: populate memory for module tags as needed")
> > >>>>>
> > >>>>> If a patch fixes two other patches, it likely needs to be split so
> > >>>>> that each part fixes one patch only. But read on please. I'm not sure
> > >>>>> if one of the fixes is correct.
> > >>>>>
> > >>>>
> > >>>> Kept them in one patch because they only make sense together.
> > >>>>
> > >>>> After a failed populate the reservation is released and the
> > >>>> size rolled back, so the next load takes the same gap and calls
> > >>>> vm_module_tags_populate() again, and that retry would trip over
> > >>>> the PTEs the failed vmap left behind.
> > >>>>
> > >>>> Without the rollback a load that already passed needs_section_mem()
> > >>>> just reuses the gap, skips populate and writes to unmapped memory.
> > >>>> So splitting them leaves each half broken.
> > >>>
> > >>> Yeah, that's why I think vmap_pages_range() cleanup should be a
> > >>> completely separate patch with vmap_pages_range() doing cleanup
> > >>> itself.
> > >>>
> > >
> > > I think I get where you're coming from now.
> > > Thanks for taking the time to explain.
> > >
> > >>>>
> > >>>>>> Reported-by: Sashiko <sashiko-bot@xxxxxxxxxx>
> > >>>>>> Cc: stable@xxxxxxxxxxxxxxx
> > >>>>>> Signed-off-by: Hao Ge <hao.ge@xxxxxxxxx>
> > >>>>>> ---
> > >>>>>>  mm/alloc_tag.c | 10 ++++++++++
> > >>>>>>  1 file changed, 10 insertions(+)
> > >>>>>>
> > >>>>>> diff --git a/mm/alloc_tag.c b/mm/alloc_tag.c
> > >>>>>> index 2070e682fe10..95ddf5b743d0 100644
> > >>>>>> --- a/mm/alloc_tag.c
> > >>>>>> +++ b/mm/alloc_tag.c
> > >>>>>> @@ -804,6 +804,13 @@ static int vm_module_tags_populate(void)
> > >>>>>>                                      next_page, PAGE_SHIFT) < 0) {
> > >>>>>>                         release_pages_arg arg = { .pages = next_page };
> > >>>>>>
> > >>>>>> +                       /*
> > >>>>>> +                        * vmap_pages_range() only runs once all pages were
> > >>>>>> +                        * allocated, and it may have installed some mappings
> > >>>>>> +                        * before failing. Undo them.
> > >>>>>
> > >>>>> If vmap_pages_range() failed to map the range, why should we need to
> > >>>>> undo it? If it indeed leaves some partial mapping when failing then I
> > >>>>> would argue that vmap_pages_range() should be undoing these partial
> > >>>>> mappings itself before returning the error.
> > >>>>>
> > >>>> Yeah it does - none of the paths under vmap_pages_range() undo themselves on error.
> > >>>
> > >>> Hmm. I wonder if this behavior is intentional or requires a fix. The
> > >>> only possible reason I can think of is performance but I can't imagine
> > >>> a failure to map is a performance-critical case.
> > >>>
> > >>> +David Hildenbrand (Red Hat), +Lorenzo Stoakes (Oracle) what do you
> > >>
> > >> Huh, my gmail thinks you are still at your previous companies :)
> > >>
> > >
> > > I manually fixed it on the mailing list for this thread.
> > >
> > >>> guys think? Shouldn't vmap_pages_range() undo its possible partial
> > >>> mapping when it fails?
> > >>>
> > >>>
> > >>>> pcpu_map_pages does perform cleanup on its own as well. There may be other similar sites.
> > >>>> https://elixir.bootlin.com/linux/v7.3-rc3/source/mm/percpu-vm.c#L255
> > >>>> so I did the same here.
> > >>>>
> > >>>> If we really want vmap_pages_range() to handle this cleanup internally, I believe that would
> > >>>> be a relatively large change, and we would need to consider many more cases. So I think we keep
> > >>>> the current behaviour for now.
> > >>>
> > >>> Yeah, I understand it would require a deeper cut but if that's the
> > >>> right way to handle this we might as well do that. Let's see what MM
> > >>> folks think.
> > >>>
> > >
> > > Sounds good to me. Let's wait and hear what the MM folks say.
> > >
> >
> > I just looked into the call sites for __vmap_pages_range.
> > And I found there's a bug in one vmalloc caller [1].
> > When gfp_mask has __GFP_NOFAIL, the code keeps retrying
> > __vmap_pages_range. If there are leftover PTE mappings,
> > this can trigger a BUG(); [2]
> > https://elixir.bootlin.com/linux/v7.3-rc3/source/mm/vmalloc.c#L3946 [1]
> > https://elixir.bootlin.com/linux/v7.3-rc3/source/mm/vmalloc.c#L121  [2]
> 
> I think best person to look at this is Ulad as vmalloc is his baby :)
> 
> Ulad, what do you think?
> 
> Thread is at
> https://lore.kernel.org/all/20260915070001.113559-3-hao.ge@xxxxxxxxx/ for
> context.
> 
Uh.. I missed this. Let me check!

--
Uladzislau Rezki