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

["Lorenzo Stoakes (ARM)" <ljs@kernel.org>]

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@google.com> wrote:
> >>>
> >>> On Tue, Sep 15, 2026 at 11:00 PM Hao Ge <hao.ge@linux.dev> 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@linux.dev> 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@kernel.org>
> >>>>>> Cc: stable@vger.kernel.org
> >>>>>> Signed-off-by: Hao Ge <hao.ge@linux.dev>
> >>>>>> ---
> >>>>>>  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@linux.dev/ for
context.

>
> > Thanks
> > Best Regards
> > Hao
> >
> >>>>
> >>>> Thanks
> >>>> Best Regards
> >>>> Hao
> >>>>
> >>>>>> +                        */
> >>>>>> +                       if (nr == more_pages)
> >>>>>> +                               vunmap_range(phys_end, phys_end + (nr << PAGE_SHIFT));
> >>>>>>                         /* Clean up and error out */
> >>>>>>                         release_pages(arg, nr);
> >>>>>>                         return -ENOMEM;
> >>>>>> @@ -947,6 +954,7 @@ static void *reserve_module_tags(struct module *mod, unsigned long size,
> >>>>>>                 return ret;
> >>>>>>
> >>>>>>         if (module_tags.size < offset + size) {
> >>>>>> +               unsigned long prev_size = module_tags.size;
> >>>>>>                 int grow_res;
> >>>>>>
> >>>>>>                 module_tags.size = offset + size;
> >>>>>> @@ -961,6 +969,8 @@ static void *reserve_module_tags(struct module *mod, unsigned long size,
> >>>>>>                         shutdown_mem_profiling(true);
> >>>>>>                         pr_err("Failed to allocate memory for allocation tags in the module %s. Memory allocation profiling is disabled!\n",
> >>>>>>                                mod->name);
> >>>>>> +                       release_module_tags(mod, false);
> >>>>>> +                       module_tags.size = prev_size;
> >>>>>
> >>>>> Yes, this one is a valid fix. It fixes 0f9b685626da commit.
> >>>>>
> >>>>>>                         return ERR_PTR(grow_res);
> >>>>>>                 }
> >>>>>>         }
> >>>>>> --
> >>>>>> 2.25.1
> >>>>>>

--
Cheers, Lorenzo